Invertebrate Glossary

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Acoela("not" "hollow"): an Order of marine turbellarian flatworms characterized by absence of gut and excretory organs. The name refers to the absence of a gut and NOT to the absence of a coelom, even though they are also acoelomate in that they do lack a coelom (see next entry)
Acoelomate ("not" "coelom"): refers to the condition, as seen in flatworms, in which a coelom is absent. The term is applied to triploblasts and not to diploblasts (cnidarians), even though the latter are also "acoelomate". The reason for this is that owing to the lack of mesoderm in diploblasts, a coelom is not possible
Acontium ("javelin or dart"): tubular extensions off the ventral edges of the internal mesenteries in sea anemones which are well-endowed with nematocysts and which function to tangle and disable ingested prey. The acontia generally reside in the gastrovascular cavity but, in some species, they can be protruded out of holes in the body column where they act in defense
Adaptive radiation: refers to the evolutionary spread of different species (from a single common ancestor) into different habitats. Animals most successful at this have been arthropods (especially insects), gastropods, and nematodes
Alternation of generation: a term sometimes used to describe the alternation of sexual (medusa) and asexual (polyp) stages in cnidarians such as hydroids and jellyfish. It is not a useful term for cnidarians because, unlike in mosses and ferns where the expression originated, the two stages in cnidarians often occur simultaneously
Ambulacral groove ("ambulare": to walk): a groove running down the oral surface of an echinoderm arm, containing the tube feet
Ammonia a metabolic waste product resulting from the deamination of amino acids. Ammonia is highly toxic and must be eliminated. It is highly soluble in water (as ammonium ion) and highly diffusible; hence, is the commonest form of nitrogenous excretion in aquatic animals
Amoebocyte ("amoibe": change; "kyto": cell): in a sponge is a type of cell which functions in food transport and storage, and in riddance of solid wastes. Amoebocytes have the ability to transform into all other cell types of the sponge. Specialized amoebocytes contain pigments and impart colours to sponges
Amphiblastula ("amphi": on either side): A specialized larval stage of calcareous sponges (Cl. Calcarea) consisting of a hollow ball of cells, half of which are small flagellated cells for locomotion, the other half, larger cells. After a short free-living life in the sea, the larva settles to the bottom and undergoes cellular movements which lead to the creation of an inner lining of flagellated cells (choanocytes) covered by a protective layer of pinacocyte cells. Note that there is no gastrulation, a feature which separates sponges from all other animals.

View Amphiblastula Animation

Analogous: similar in function or appearance but not in origin. Thus, Malpighian tubules of insects and green glands of crustaceans are analogous in that both are involved in excretion, but have evolved differently; hence, are not homologous
Annelida ("ring"): a Phylum of segmented worms including tubeworms, sandworms, earthworms, and leeches
Anthozoa ("flower" "animal"): a Class of Phylum Cnidiaria which includes sea anemones, corals, and gorgonians
Aposematic ("away" "signal"): colours, smells, or structures on or from an organism that act as defensive signals. They convey to a potential predator the message that the organism is unpalatable. Colours are usually bright, and their pattern, hue, and intensity may be specially "tailored" to the sensory capabilities of the predator. An example is the bright orange and black colouration on milkweed bugs which are toxic to bird and other predators owing to the sequestration of toxins from their diet
Apterygote ("without" "wings" "gotta"): name given to the type of development exhibited by primitive wingless insects such as collembolids (Order Collembola). Development in apterygotes involves a series of instars (moult stages) which are similar in body proportions but differ in size (thus, young stages are perfect miniatures of older stages). Contrast with exopterygota and endopterygota
Aquiferous system ("aqua": water): describes the system unique to sponges in which water flows through canals and chambers, pumped by choanocytes
Archenteron ("ancient" "gut"): formed during gastrulation and open to the external environment, this is the tube that runs through the developing embryo. It consists of endoderm and will develop into the digestive system of the organism. Because it is topographically outside of the body, it is not considered to be a "body cavity" as such. The term "body cavity" should be restricted to the blastocoel and its derivatives (pseudocoel, hemocoel, and coelom)
Archeocyte ("ancient" "cell"): in a sponge refers to a type of cell that can differentiate into all other types. A specific example is the modified amoebocyte that is contained in a gemmule and which, on hatching, can form all the diverse cell types in the new sponge. In a general sense, amoebocytes can also be called archeocytes, becauses they can differentiate into all other cell types in the sponge
Archeogastropoda ("ancient" "gastropod"): an Order of the SubClass Prosobranchia in Class Gastropoda including the limpets and abalone. These are among the most plesimorphic gastropods
Arthropoda ("jointed" "foot): a Phylum including horseshoe crabs, insects, millipedes, centipedes, mites, spiders, ticks, and crabs
Ascon or asconid or asconoid: the simplest (and most primitive) form of sponges, consisting of a vase-shape with the inner chamber, or spongocoel, being lined by choanocytes or feeding cells
Ascidiacea ("little bag" "characterized by"): a Class of the SubPhylum Urochordata of Phylum Chordata which includes the sea squirts
Asteroidea ("star"): a Class of Echinodermata which includes the seastars
Batesian mimicry: a type of mimicry in which otherwise palatable insects evolve colour patterns and/or shape and/or behaviour which mimic a toxic unpalatable form. The mimic thus gains a secondarily-derived immunity from the predator. Batesian mimicry is found mainly in lepidopterans (butterflies, moths), dipterans (flies), and hymenopterans (bees, ants), but may also be found (rarely) in other invertebrates. Contrast with Mullerian mimicry
Benthic ("benthos": deep): refers to organisms that live on the bottom of any body of water. Epibenthic describes organisms that live just above the bottom
Bilateral symmetry: see Symmetry
Biogenetic law ("bios": life; "genetikos": origin): an idea that "ontogeny recapitulates phylogeny", or that embryology or early development of an organism repeats the evolutionary history that gave rise to the organism. Torsion was thought by Walter Garstang (evolutionist, poet) to be a kind of flashback or speeded-up repeat of what may have occurred in gastropod evolution to bring the posterior-facing mantle cavity to the front of the animal
Bipolar ("two" "pole or end"): er direction when referring to the nerve cells of triploblastic animals the term indicates that the neurones have different ends (axon and dendrite) and passage of action potentials is in one direction only. Contrast with certain neurones of (diploblastic) cnidarians which are apolar, in which action potentials can pass in either direction
Biradial symmetry: see Symmetry
Biramous ("two" "branch"): an appendage-type in crustaceans consisting of a basal portion with two separate branches. Because this type of appendage is present in primitive present-day crustaceans, it is believed to represent the form of ancestral appendage from which all other specialized appendage types evolved in crustaceans. In advanced crustaceans, such as crayfish or crabs, the appendage-type most closely resembling this ancestral form is the pleopod or swimmeret on the abdomen.

2 views: one of uniramous appendage, one of biramous

cross-section through abdomen of lobster to show biramous pleopods (swimmerettes).

uniramous insect leg

Bivalvia ("two" "valves"): a Class of Phylum Mollusca which includes clams, mussels, oysters, and scallops. The protective shell is in two parts
Blastocoel ("bud" "hollow"): a cavity within the blastula. Following gastrulation, the blastocoel may remain fluid-filled but otherwise undifferentiated (nematodes and rotifers), become filled with a proteinaceous gelatinous matrix (poriferans and cnidarians), become filled with a solid cell mass known as mesenchyme (platyhelminthes and nemertines), or become lined with mesoderm to form a cavity known as the coelom. (the coelom may be fluid-filled as in annelids and echinoderms, or just be a space as in vertebrates). See gastrulation.
Blastopore ("bud" "opening"): an opening into the primitive gut (archenteron) of the gastrula which will later become the mouth in Protostomes and the anus in Deuterostomes
Blastula ("bud" "little"): a developmental stage in animals occurring at about the 128-512 cell stage (7th-9th division). The blastula consists of one layer of cells and, where little yolk is present, is hollow (the cavity is the blastocoel). It usually is characterized by having a hemisphere of small cells (micromeres) and a hemisphere of large cells (macromeres). The blastual stage is reached a few hours or a day following fertilization and in all animals save sponges develops into the the gastrula (see gastrulation)
Blind gut: describes a situation, as found in cnidarians (sea jellies, sea anemones) and platyhelminthes (flatworms) where there is one opening only into the gut. Thus, the mouth functions for both ingestion and egestion
Body cavity: generally refers to cavities which are remnants of the embryological blastocoel, such as pseudocoels, hemocoels, or coeloms, but sometimes refers (in the absence of the above) to the gastrovascular cavity or gut of cnidarians. In triploblasts, however, it does NOT refer to the gut
Calcarea: a class of marine sponges characterized by calcareous spicules and no spongin
Carapace ("carapacho": covering or shield): in Crustacea is the protective dorsal shield or plate that covers the animal
Cecum ("blind"): an expansible space in the foreguts of certain molluscs and arthropods used for food storage after ingestion but before digestion
Cephalopoda ("head" "foot"): a Class of Phylum Mollusca including squids, octopuses, cuttlefish, and pearly nautilus
Cephalization ("head" "process of"): the appearance of a head end in evolution of animals. Cephalization accompanies bilateral symmetry as an emerging pattern in invertebrate evolution
Cephalochordata ("head" "string"): a subPhylum of Phylum Chordata which includes primitive fish-like chordates known as sand-lances ("amphioxus")
Cestoda: a Class of parasitic flatworms known as tapeworms
Chemotactile ("chemical" "touch"): refers to a sensory device which is both chemical and touch sensitive
Chilopoda ("thousand" "foot"): a Class of SubPhylum Uniramia of Phylum Arthropoda which includes the centipedes
Chitin ("tunic" or "mantle"): a complex (contains protein) carbohydrate material consisting of linearly arranged N-acetyhl-glucosamine units. It acts as a strengthening and/or skeletal support in various invertebrates. In the cuticle of arthropods it forms an exoskeleton
Chiton ("tunic" or "mantle"): a member of Class Polyplacophora of Phylum Mollusca, characterized by having 8 shell plates and a broad foot. Chitons are slow-moving marine herbivores. They are known as "sea beef" in the Caribbean
Chlorogogue cells ("green/yellow"): special cells lining the outside of an earthworm’s gut involved in energy transformations and energy storage
Choanocyte ("choan": collar; "cyte": cell): unique collar-shaped cells in sponges responsible for generating water flow. The resemblance of the choanocytes to choanoflagellate protozoa provides compelling evidence for the earliest stages of sponge evolution
Chordata ("string"): a Phylum containing sea squirts, sand-lances, and vertebrates. Common chordate features include a notochord, dorsal tubular nerve cord, pharyngeal gill slits, and tail. Of chordates, only sea squirts (SubPhylum Urochordata) are considered "invertebrates"
Chromatophore ("colour" "bear or carry"): a specialized cell-type in molluscs and arthropods which contain pigments. Expansion and contraction (mediated by nerves in cephalopods and by neurohormones in arthropods) causes colour change
Chrysalis: see Pupa
Circus ("ring or circle"): one of a pair of appendages extending from the last abdominal segment in insects, sensitive to vibrations (pl. circi)
Cladistics ("clados": branch ): a method of classifying organisms based on shared ancestral and derived characters. The resulting arrangement reflects the phylogenetic relationships between the organisms. A "clade" includes the ancestral group and all other groups or lineages that arose from it. A "cladogram" is a visual representation of the phylogenetic branching of the different groups based on cladistics analysis
Cloaca ("sewer"): a shared common opening that may include the terminal openings of reproductive, digestive, and excretory systems
Closed circulatory system a blood circulatory system in which the circulating fluid flows in vellels or tubes connected to each other by capillaries. Such systems in invertebrates are limited to annelids, cephalopods, and to a lesser extent nemerteans
Cnidaria ("char. by nettles"): a phylum of invertebrates including hydroids, Hydra, sea jellies, sea anemones, gorgonians, and corals
Cnidoblast ("nettle" "bud"): a cell-type found in both epi- and gastrodermal layers of cnidarians which produces the stinging "cells" or nematocysts
Cnidocyte ("nettle" "cell" ): identical to cnidoblast
Cnidosac ("nettle" "sac"): this refers to a special sac at the tips of the cerata (dorsal projections) on certain shell-less snails (nudibranchs), the cells of which contain undischarged nematocysts sequestered from the cnidarian prey of the snails. The mechanism by which the nematocysts are ingested, transported internally, and stored in an undischarged state in the cells of the cnidosacs is unknown, but it seems clear that their function is for defense. On stimulation of the snail, the cnidosac contracts, the cells in the cnidosacs rupture, and the nematocysts are extruded into the seawater. On contact with seawater the nematocysts discharge
Coelenterata ("koilos": hollow): this is an old name for the Phylum Cnidaria and, in the form "coelenteron", is still used to describe the incomplete gut cavity of the Cnidarians
Coelom ("koiloma": cavity ): a body cavity created when the embryological blastocoel becomes lined by tissue of mesodermal origin. The resulting condition is known as "coelomate". Amongst the major phyla a full-body coelom is present in annelids, echinoderms, and vertebrates. The coelom may be fluid-filled as in annelids and echinoderms, or not, as in vertebrates. The coelom functions as a space for organs, allows for expansion of the gut, and, if fluid-filled, may be involved in nutrient and energy transport, and in locomotion. In annelids and vertebrates, internal organs are supported from the inner body wall in membranous mesodermal tissue known as peritonea (sing., peritoneum) and hang into the coelomic space. Other phyla such as molluscs and arthropods are also coelomate, but the coelom is restricted to tubules of the circulatory, excretory, and reproductive systems (molluscs) or just to the excretory and reproductive systems (arthropods)
Coelomocytes ("koiloma": cavity; "cyte": cell): cells, often functioning in defensive phagocytosis, found in coelomic fluid
Coenosarc ("koinos": common; "sarc": meat): living connections between adjacent polyps in hydroid colonies representing extensions of the gastrovascular cavity of each polyp and allowing for transfer of energy and nutrient matter
Coiling: refers to the planar or spiral growth of gastropod shells. Coiling probably preceded torsion in gastropod evolution
Collagen ("kolla": glue; "gen": to produce): a flexible and tough fibrous protein found in various invertebrates; e.g., a principal component of spongin in sponges
Commensal ("cum": with; "mensa": table): a type of symbiosis ("living together") where one member of the coexisting pair of organisms benefits while the other is not affected. Such relationships exist in theory only, as it would be impossible to say whether the one partner is truly "unaffected" by the presence of the other
Complete gut or "complete digestive tract" or "through gut" refers to a gut system which has both mouth and anus. The evolutionary implication is that such a system may be linearly specialised for food processing (storing, grinding, digesting)
Complete metamorphosis see metamorphosis
Compound eye a type of eye found in Arthropoda characterised by multiple optical units known as "ommatidia". Although each ommatidium generates its own image, it is thought that the ultimate image perceived by an arthropod is an integrated single image
Convergent evolution when distinct and different evolutionary lineages evolve similar structures or features from different ancestors. Such features are analogous. An example would be the camera-type eye found in vertebrates and cephalopods
Corpora allata: neurosecretory regions of the brain of insects involved in producing a peptide hormone known as juvenile hormone. In the presence of juvenile hormone, moulting is to another larval stage; when production decreases, moulting is to the pupal stage; when no juvenile hormone is produced, metamorphosis ensues to the adult stage
Corpora cardiaca: neurosecretory regions of the brain of insects involved in producing a peptide hormone which stimulates the prothoracic gland to produce a moulting hormone (a steroid known as ecdysone) which initiates the moult sequence
Crinoidea ("lily"): a Class of Phylum Echinodermata which includes the sea lilies and feather stars
Crop a portion of the anterior part of the digestive tract used to store food prior to its digestion. Crops tend to be associated with "chancey" food supplies (female mosquitoes and cephalopods) and with hard-to-digest food supplies that may need overnight processing (locusts)
Crustacea ("crust or shell"): a SubPhylum of Phylum Arthropoda including the crabs, crayfish, shrimps, prawns, isopods, lobsters, and amphipods
Cryptobiosis ("hidden" "life"): a dormant or suspended state of animation that allows organisms to survive severe environmental conditions such as low temperatures or drying. In nematodes it refers to the fact that when the eggs are dried they remain viable for a long period, perhaps many decades, yet exhibit no measurable life signs
Crystalline style a transluscent rod-like structure found in some molluscs, notably bivalves, containing enzymatic proteins used in digestion. The style is produced in a sac and extends into the stomach. It is rotated by cilia and grinds against a special roughened part of the stomach wall known as the "gastric shield" to release its content of enzymes
Ctenidium ("comb"): another name for the gills of molluscs
Cuticle ("skin"): a protective sheathing secreted by the epidermis, usually proteinaceous. Found in nematodes, annelids, and arthropods (in the latter it has a complex structure and is known as the exoskeleton). In nematodes and arthropods, the cuticle is moulted to allow growth to occur
Cuvierian tubules special defensive organs in holothuroids described by Georges Cuvier (1769-1832), the well-known and influential French physiologist. The tubules are sticky (after release) and toxic to vertebrates, and are expelled from the cloaca when the sea cucumber is threatened
Cypris larva:. It is non-feeding a pre-settlement larval stage in barnacles succeeding from the nauplius stage. The cypris is characterized by two shell-valves and a pair of chemotactile-sensitive antennae which it uses to identify a suitable spot to settle and attach



("daktylos": finger; "zoon": animal):

special polyps in colonial hydrozoans functioning in defense. The nematocysts in the dactylozoids are especially toxic
Demospongiae: a Class of sponges characterized by the presence of both spongin and siliceous spicules
Dermal papula ("skin" "pimple"): a protusion from the skin of seastars, consisting of inner and outer linings of mesoderm and epidermis, respectively, within which coelomic fluid circulates. These "skin gills" (also called dermal branchiae) are mostly involved in gas exchange, but may facilitate excretion
Determinate development: a type of developmental pattern in animals in which the fates of cells are established early in embryological development, even as early as the 4-cell stage. Contrast with indeterminate development, in which cell fates are established much later. Note that the time-lines differentiating these two types of development are arbitrary and vary with the animal groups in question. Protostomes (annelids, arthropods, molluscs) are generally characterized by having determinate development, while deuterostomes (echinoderms, chordates) are characterized by having indeterminate development
Detorsion: a process occurring during the larval stages of opisthobranch molluscs in which the larva "untwists" by 90 or 180 degrees. In the former, a ctenidium (gill) may be retained and a rudimentary shell, but the mantle cavity opens to the right side; in the latter, the ctenidia, shell, and mantle cavity are lost and new gills evolve at the back of the animal
Deuterostomia ("second" "mouth"): name given to a group of animal phyla (echinoderms, chaetognaths, chordates) in which the blastopore forms the anus in the embryo and a new opening develops to form the mouth. Contrast with Protostomia (annelids, arthropods, molluscs) in which the blastopore forms the mouth. Other deuterostome characteristics are radial cleavage, indeterminate development, and formation of a coelom by enterocoely
Dioecious ("two" "house"): indicates that sexes are separate in a species. Contrast with monoecious
Diploblastic ("two" "buds"): refers to a two-tissue layer state as found in cnidarians (jellyfish, anemones). The cell layers are endoderm and ectoderm and are separated by proteinaceous gel-like layer known as mesoglea
Diplopoda ("two" "foot"): a Class of SubPhylum Uniramia of Phylum Arthropoda which includes the millipedes. The name diplopoda refers to two pairs of legs per segment


the periodic moulting or ecdysis of the exoskeleton of an arthropod
Ecdysone: a steroid hormone produced in insects which stimulates moulting or ecdysis
Echinodermata ("spiny" "skin"): a Phylum of invertebrates which includes seastars, sea urchins, sea cucumbers, sea lilies, and brittle stars
Echinoidea ("hedgehog"): a Class of Phylum Echinodermata which includes sea urchins, heart urchins, and sand dollars
Ectoderm ("outside" "skin"): an embryological cell layer created by the process of gastrulation which forms the epidermis or outer covering (skin) of the adult. Other adult derivatives of ectoderm include bristles, sensory cells, gills, exoskeleton, shell, as well as the nervous system and its derivatives (eyes, eyespots, balance receptors)
Endocuticle ("within" "thin cuticle" ): the innermost, unhardened, layer of the arthropod procuticle. The endocuticle is digested and re-used to build new cuticle during the moult cycle
Endoderm ("inner" "skin"): an embryological cell layer, created by the process of gastrulation, which lines the primordial gut. Its fate is to form the gastrodermis or gut lining in the adult, as well as all digestive cells or gland cells associated with the gastrodermis
Endopterygote ("inside" "wings" "gotta"): insects such as butterflies, flies, fleas, bees, and beetles in which wings are not present externally in the larval stages but are developed during pupation or metamorphosis. Endopterygote development is also known as "complete metamorphosis" or "holometabolous". Contrast with exopterygota and apterygota
Endoskeleton ("within" "skeleton") a skeletal support, such as bones in a vertebrate, which lies within the body. Contrast this with an exoskeleton, like that of an insect or snail, which lies outside of the body. Endoskeletons, such as bones or ossicles of a starfish, are produced from mesoderm, while exoskeletons are produced from the epidermis
Endostyle ("within" "stake, column") a mucus-secreting gland in tunicates involved in feeding. Iodine-derivatives in the mucus suggest a phylogenetic affinity with the vertebrate thyroid gland
Enterocoely: refers to a developmental process whereby the coelom originates as outpocketings from the gut. This type of coelom development is found in deuterostomes (echinoderms, chordates).

View animation of enterocoely

Ephyra ("ephyra": sea nymph): juvenile medusa of jellyfish (scyphozoans) produced by asexual budding from the scyphistoma polyp
Epibranchial spaces ("above" "gill"): are spaces located above the gills in bivalves in which are carried the posteriorly moving water streams involved in feeding and gas exchange
Epicuticle ("upon": "thin skin"): outermost part of the arthropod cuticle. It contains waxes in certain terrestrial arthropods, including spiders and insects, to minimize drying
Epidermis ("upon" "skin"): tissue forming the outer covering or skin of an organism, derived from ectoderm
Epitheliomuscular cell: a contractile cell found in the epidermis of cnidarians, such as Hydra. It is characterized by an extended base which, in concert with the extended bases of adjoining epitheliomuscular cells, forms a tissue which contracts in the longitudinal or lengthwise direction. In some hydrozoans, such as Physalia (Portuguese Man-of-war), this tissue can contract over 30 times in length
Excretion ("throw out"): discharge from the body of waste products of metabolism (not to be confused with elimination of undigested food matter from the mouth or anus which are known as egestion and defecation, respectively). If a substance has been biochemically modified in the body and then eliminated because it is not useful, it is an excretion. The most common excretory products in animals are nitrogenous, such as ammonia, urea, and uric acid
Exocuticle ("outside" "thin skin"): outermost layer of the arthropod procuticle consisting of proteins and chitin. This layer is hardened or sclerotized with phenolic compounds that chemically link amino acids in the proteins within the chitin. The exocuticle is too hard to be digested and re-used during the moult cycle
Exopterygota ("outside" "wings" "gotta"): insects such as grasshoppers, bugs, and dragonflies in which the wings are visible as developing buds during the later nymphal stages. Additionally, parts of the body, especially the head, differ proportionally in size in the instars. Because the transformation to adult stage is more gradual, exopterygotes do not go through a pupal stage as do endopterygotes (butterflies, flies, fleas, bees, and beetles). Development of exopterygotes is also known as "gradual metamorphosis" or "hemimetabolous". Contrast with endopterygota and apterygota
Exoskeleton: skeletons such as in arthropods and other invertebrates which are outside of the body. In arthropods, the exoskeleton is secreted by the epithelium and is also known as the cuticle. It is comprised of many overlapping layers of chitin which make a strong protective "box" in which the animal resides. In insects and some spiders, the outermost layers of cuticle are impregnated with wax for waterproofing; in crustaceans, the inner layers are impregnated with calcium salts for strength. Exoskeletons may need to be moulted for growth to occur (as in arthropods)
Extant ("exstare": to stand forth): organisms still alive today; i.e., not extinct
Extinct ("extinguere": extinguish): organisms that existed in the past but are no longer living
4d cell: see mesentoblast
Flame cell: a unique cell found in the protonephridia of certain flatworms (e.g., Planaria) which uses beating cilia to drive intercellular fluid through a filtering grid in the cell and then into an excretory tubule for release at the exterior via nephridiopores
Ganglion ("ganglion": knot): a mass of nerve cells acting as a controlling centre for passage of nerve impulses
Gastrodermis ("gut" "skin"): tissue lining the gut of an adult animal
Gastropod ("stomach" "foot"): a Class of Phylum Mollusc which includes limpets, snails, abalone, slugs, nudibranchs, and sea hares
Gastrovascular cavity ("stomach" "vessel" "cavity"): a name given to the gut of a cnidarian. It has only a single opening, the mouth, and an anus is absent (abbr: GVC)
Gastrozooid ("stomach" "animal"): in hydrozoans this is the feeding polyp. Contrast this type of polyp with gonozooids which are involved in reproduction
Gastrula ("stomach" "little"): a developmental stage in animals succeeding the blastula stage which possesses the rudiments of stomach and skin and skin derivatives. The gastrula stage occurs within a few days of fertilization and is free-living for only a few hours. See gastrulation
Gastrulation ("stomach" "little" "process"): the process of forming a layered embryo from the blastula. The layers consist of the ectoderm and endoderm. The endoderm lines a newly created cavity known as the archenteron which will later form the gut. An opening, the blastopore, will form either the mouth or the anus, depending on the group. Gastrulation occurs in several ways, but all involve movements of cells, often with large cells or macromeres being enclosed by small cells, or micromeres. Gastrulation takes only a few minutes or hours to complete. One common type of gastrulation, invagination, is shown here. DIAGRAM
Gemmule ("bud"): an obligatory life phase in most (or all) freshwater sponges and some intertidal marine sponges. It consists of a double cover containing protective spicules, enclosing a mass of specialized amoebocytes. Gemmules are produced in late summer/autumn and function in over-wintering after the parent sponge dies. In spring, the amoebocytes stream out of openings known as micropyles and differentiate into the various cell types of the adult
Glutinant ("glue"): a type of nematocyst found in cnidarians such as Hydra which have a long sticky thread. In Hydra, glutinants are used mainly in locomotion and, for this reason, have a low discharge threshold on the physical (touch) side. Contrast with volvents
Gnathobase ("jaw"): in arthropods, such as horseshoe crabs, the bases of the appendages are modified into grinding surfaces used to break up food before it is passed to the mouth
Gonocoel theory: a theory of coelom origin involving expansion of the gonadal spaces using nemertines as intermediaries between flatworms and annelids
Gonopore ("egg" "opening"): an opening on the outside of an animal from which the reproductive products are discharged
Gonozooid ("seed" "animal"): a specialized poly-type in hydrozoans which functions in the asexual production of medusae. Contrast with gastrozooid
Gravid ("gravis": heavy): an animal filled with eggs
HAM: refers to Hypothetical Ancestral Mollusc, an inference of what a prototypal mollusc would have looked like
Hemal system: a system of tubes and ducts which morphologically parallels that of the water vascular system in echinoderms, but lies just orally to it. Its function is not known, but it seems clear that it does not perform roles typically associated with a blood-vascular system in other animals. Possible functions ascribed to it are internal defense and endocrine, but direct evidence for either is lacking
Hemimetabolous ("half" "change"): insects which go through "gradual" metamorphosis, in which 5 instar stages (nymphs) are passed through to the adult. Wings develop externally and usually first appear in the 3rd instar stage. There is no pupal stage. Instars resemble the adult and frequent the same habitats; hence, there is potential competition between nymphal and adult stages. Contrast with holometabolous development
Hemocoel ("blood" "cavity"): a space representing the remant of the embryonic blastocoel which is filled with a fluid that in many ways resembles blood in form and function, but which is known as hemolymph. Hemocoels are features of molluscs and arthropods
Hemolymph ("blood" "water"): the fluid contained within the hemocoel (as in molluscs and arthropods with open circulatory systems) with functions similar to blood
Hexactinellida ("six" "rays"): a Class of Phylum Porifera characterized by having 6-rayed siliceous spicules. Hexactinellids live mostly in deep ocean habitats
Hirudinea ("leech"): a Class of Phylum Annelida which includes the leeches
Holometabolous ("whole" "change"): insects which pass through several larval stages and then a pupal stage during development (also known as "complete metamorphosis"). The larvae (caterpillars, grubs, maggots) are unlike the adults in form and behaviour; hence, competition is absent between the larval and adult stages. Examples of holometabolous insects are butterflies, flies, fleas, bees, and beetles
Homologous: similar in structural form and development, indicating common ancestry. Thus, the similarity of the double ventral nerve cords of arthropods and annelids, while found in seemingly disparate groups, is suggestive of homologous origins
Hydraulics: many invertebrates use the pressure of moving internal fluids in locomotion. This differs from the notion of an hydrostatic skeleton which, by definition, is more or less stationary. Such moving fluids are employed in gastropod and bivalve burrowing through extension and expansion of the foot by hemolymph, and in echinoderm locomotion and feeding through extension and contraction of the tubefeet by coelomic fluid
Hydrostatic skeleton ("water" "standing" "skeleton"): a primitive type of supporting system found in nematodes and other worms, that consists of a volume of fluid surrounded by contractile elements. Differential contraction of these elements, usually muscles, deforms the fluid sac and creates shape-changes useful in locomotion. Because the fluid sac is usually under pressure (hydrostatic pressure), it is rigid and provides structural support
Hydromedusa ("serpent" "jellyfish"): free-living sexual life-stage of hydrozoans, comparable to the sea jelly (scyphomedusa) stage of scyphozoans. Contrast with the asexual sessile polyp
Hydrozoa ("water" "animal"): a Class of Phylum Cnidaria which includes hydroids, Hydra, stinging "coral", and the Portuguese Man-of-War
Hygroscopic ("water" "affinity for"): indicates an affinity for water, such as the flocculent proteinaceous substance found in nematocysts of cnidarians that is involved in their discharge
Hyperosmotic ("above" "push"): a solution with higher osmotic content than another. A crayfish is described as being "hyperosmotic" with its environment because its hemolymph is more concentrated than the freshwater in which it lives
Hypoosmotic ("under" "push"): a solution with lower osmotic content than another. A crab living in a lagoon from which water is evaporating making the seawater more concentrated is described as being "hypoosmotic" with its environment because its hemolymph is less concentrated than the water in which it lives
Incomplete gut a digestive system, as found in cnidarians and platyhelminthes, in which there is only a mouth opening. Thus, both ingested food and egested food must pass through the same opening
Incomplete metamorphosis see hemimetabolous development
Indirect eye: a type of light-perceiving or image-resolving eye in which light does not impinge directly onto sensory cells; rather, the light is first reflected from a special pigment layer at the back of the eye. The freshwater flatworm Planaria has an indirect type of eye and is capable of light-perception only. Contrast with a direct type of eye in which the light impinges directly onto the light-sensitive retinal cells, as found in molluscs and arthropods
Indirect development where an egg hatches to a larva, with no resemblance to the adult which it will become
Industrial melanism: describes the circumstance of peppered moths (Biston spp.) in Great Britain and North America, normally camouflaged from bird predators when they rested on lichen-covered tree-trunks during the day, but slowly evolving to a blacker form during the Industrial Revolution when soot gradually blackened the tree-bark. With the "greening" of these industrial areas, mostly accomplished by the banning of coal-burning, the black phenotype presumably became visually dominant to birds and selection favoured once again the peppered form. To illustrate, the incidence of black morphs of B. betularia near Liverpool was 94% in 1960 and 19% in 1994. Industrial melanism has always been considered a "touchstone" example of natural selection and still is, despite some evidence which suggests that the pollutants may have themselves been causing the moths to become black
Inquilinism a type of symbiosis in which one organism lives on or in another without any harm befalling the host. Because of the uncertainty over the last feature, inquilinisms (and commensalisms) grade into parasitisms, where harm to the host is presumed
Instar: a moult stage during development of an arthropod. Also known as a nymph
Interstitial cell: a generic name for a cell-type found in both epi- and gastrodermis of cnidarians. These cells are "totipotent"; that is, they have the ability to differentiate into any other cell type in the cnidarian
Intracellular digestion where final digestion of food particles occurs inside of cells within vacuoles. This is usually preceded by phagocytosis. Intracellular digestion is the only type of digestion found in sponges, but it occurs to varying extents throughout the cnidarians and platyhelminths
Larva: post-gastrula young stage, most often free-living, which passes through a metamorphosis to the adult form. Larvae function in marine invertebrates principally for distribution of the species. The vast majority live in and feed from the plankton. In insects, a larva is found only in endopterygote insects such as butterflies (the larva is a caterpillar), flies (maggot), fleas, bees, and beetles (grub). Their function is chiefly to build energy for the subsequent transformation to the adult stage. Because the transformation from a crawling caterpillar stage or an encased honeybee larval stage to a flying adult stage is so enormous, such insects enter a special metamorphic stage known as a pupa or chrysalis
Leucon or leuconid or leuconoid: most complex form of sponges in which the flagellated chambers are most highly ramified. All hexactinellid and demosponges are leuconid as adults. This means that over 90% of all sponges have this form. Contrast with ascon and sycon forms
Madreporite: a sieve-like structure located externally in most echinoderms, connected to the ring canal of the water vascular system via the stone canal. Although it is tempting to think that the madreporite is a conduit for seawater to enter the water vascular system, there is no solid evidence for this. One idea is that the madreporite, so intimately positioned with the axial gland of the hemal system, may function in some way in reproduction, perhaps in enabling synchronous release of gametes from the different sexes. However, neither of these ideas is compatible with the internal positioning of the madreporites of holothuroids (sea cucumbers)
Malpighian tubules: a system of blind-ending tubules originating from the junction of midgut and hindgut in insects, suspended in the hemolyph, and involved in excretion of uric acid; described initially by Marcello Malphigi (1628-1694)
Mantle ("mantle or cloak"): a fleshy protective outer tissue of molluscs which secretes the shell and encloses a space known as the mantle cavity
Mantle cavity: a space in a mollusc enclosed by mantle tissue which houses the gills (ctenidia) and receives discharge from the anus, kidney, and gonads. Note that the mantle cavity, while often protected by a shell, is topographically outside of the body and, thus, is NOT a "body cavity". Body cavities are pseudocoels, hemocoels, or coeloms (note also that the gut cavity is not considered to be a "body cavity" because it, too, is topographically outside of the body)
Marsupium ("bag or pouch"): a special region, in or on the body, in which eggs are incubated to hatching. Marsupium-containing invertebrates include isopods, amphipods, barnacles, sea squirts, and brittle stars
Medusa ("jellyfish"): the sexual stage in hydrozoans and scyphozoans. In each case the medusae are produced asexually from polyps known as gonozooids in Class Hydrozoa, and scyphistomas in Class Scyphozoa
Megalops ("large" "lops"): final larval stage of a crab, succeeding from the zoeal stage. This stage is crab-like in appearance and spends most of its time clinging to floating vegetation and pieces of wood, waiting to drift into shallow-enough water to jump off and colonize the sea bottom
Melanism: a type of coloration in insects involving the deposition of melanin pigment, which is black
Mesenchyme ("middle" "juice"): a type of cellular tissue found in flatworms between the epi- and gastrodermis layers. Mesenchyme acts to support various organs, provides a somewhat deformable medium on which the muscles can act for locomotion, and also functions in energy storage. Mesenchyme is a complex tissue because it may function more "passively", as described above for flatworms, while in other phyla it can function more "actively", in which case it behaves more like a kind of mesoderm in fashioning organs and so on. Mesenchyme derives from both ectoderm and endoderm and these different origins may play a role in how it ultimately functions. In Chordates, and especially in anatomical references to mammals, mesenchyme is often used synonymously with the term mesoderm
Mesentoblast ("middle" "bud"): this special cell, deriving from the 3D cell during the 6th cleavage in protostomes (annelids, molluscs, arthropods), is also known as the 4d cell and produces all the mesoderm. Because the 3D is a presumptive endodermal cell, we consider mesoderm in protostomes to be derived from endoderm (syn. entoderm). See schizocoely
Mesoderm ("middle" "skin"): a special type of cell layer with the ability to form organ systems such as circulatory, reproductive, kidney, and muscle. It originates in one of two ways: from a special cell called the 4d cell (the mesoblast cell) in protostomes (annelids, molluscs, and arthropods); or from the endoderm (syn. entoderm) in deuterstomes (echinoderms and chordates )
Mesodermal skeleton: a skeleton, consisting of ossicles or bones, or the like, produced from mesoderm. This type of skeleton is found in deuterostomes (echinoderms and chordates) and contrasts with the epidermally produced skeletons of most other invertebrates (protostomes: annelids, arthropods, molluscs)
Mesoglea ("middle" "glue"): a proteinaceous substance located between the epidermis and gastrodermis of cnidarians. It is a thin layer in hydrozoans and anthozoans, but is thick ("jelly"-like) in scyphozoans. The mesoglea occupies a space remnant from the embryonic blastocoel
Mesohyl ("middle" "layer"): this is a gelatinous proteinaceous matrix which separates the choanocyte and pinacocyte layers in sponges. The space it occupies is derived from the embryonic blastocoel
Metabolic waste these are the biochemical waste products of metabolism and include ammonia, carbon dioxide, and water
Metameric or metamerism: see segmentation
Metamorphosis ("later" "form"): process of transformation from the larval to adult life phase. It involves not only morphological change (as from a crawling form such as a maggot to an adult flying form in insects), but also extensive physiological and behavioural changes
Metanephridium ("later" "kidney"): an excretory organ as in an annelid consisting of a ciliated funnel, or nephrostome, that collects coelomic fluid and passes it through a tubule where absorption occurs to an outside excretory opening, or nephridiopore; more generally, any "advanced" kidney
Micromere ("small" "unit"): refers to the smaller of the two types of cells found in a blastula that will eventually develop into ectoderm; contrast with the larger type of cell, or macromere, that will eventually develop into endoderm (syn. entoderm)
Monoecious ("one" "house"): indicates that both sexes in a species are contained in one individual (hermaphrodite)
Monophyletic ("one" "history"): (syn. "clade") refers to a group of species that includes a common ancestor and all of its descendants. Contrast with paraphyletic and polyphyletic
Monoplacophora ("one" "plate"): a small and primitive group of single-shelled molluscs, characterized by a degree of segmentation
Morphostasis ("form" "static"): refers to relative stasis in morphological characteristics (also referred to as "character stasis"). Unchanging morphological features are inferred to result from external factors, such as stabilizing selection, and internal factors, such as the intrinsic contraints on integrated character complexes
Moult (also known as ecdysis: "escape or slip out"): process of casting off the exoskeleton during growth in arthropods, mainly, but also found in onychophorans. It involves resorption of useful materials from the old exoskeleton, secretion of a new soft one, splitting of and withdrawal from the old one, and inflation (water or air) of the new one, followed by hardening to give room for growth
Mullerian mimicry: a type of mimicry found in toxic insects in which defensive (aposematic) colours and sometimes shape evolve to a common type or form. An example is the common-ness of black, orange, and yellow coloration in unpalatable insects and other arthropods. In simple terms, this presumably makes it easier for a predator to remember which colours, out of a myriad of possible colours, signify toxicity
Mutualism ("mutual or together"): a type of symbiosis characterized by both partners benefitting from the relationship (e.g., clownfish in anemones, gobies and blind shrimp; corals and plant-cells)
Myriopoda ("many" "foot"): a collective name, non-taxonomic, applied to chilopods (centipedes) and diplopods (millipedes)


Nauplius larva ("nauplios":

shellfish ):

a larval stage of crustaceans (free-living only in primitive crustaceans such as copepods and cirripedes), characterized by 3 pairs of biramous  appendages (1st and 2nd antennae, and mandibles). In barnacles, the nauplius feeding stage is succeeded by a nonfeeding cypris larva whose function is to select a suitable settlement and attachment spot. In higher crustaceans, such as crabs, the nauplius stage is passed through in the egg and the first free-living larva is the zoea
Nematocyst ("thread" "sac"): a stinging device in cnidarians. These microscopic units are commonly referred to as "stinging cells", but really are sub-cellular organelles produced in specialized epithelial cells called cnidoblasts
Nematoda or Nemata ("thread"): a Phylum of worms known as roundworms
Nemertea ("thread"): a Phylum of unsegmented worms known as ribbon worms. Although nemertines possess many features in common with flatworms, they uniquely possess complete guts and circulatory systems
Neogastropoda ("new" "gastropod"): an Order of SubClass Prosobranchia in Phylum Gastropoda including whelks. These are the most advanced type of gastropod
Neorhabdocoela ("new" "rod" "hollow"): an Order of turbellarian flatworms characterized by having a straight gut. NOTE that "coel" here refers to the gut and not to a "coelom"
Nephridiopore ("kidney" "opening"): refers to the opening on the outside of an animal from which are released waste products in the form of urine
Nephridium ("kidney"): general name for a kidney. Functions in ridding the body of unwanted metabolic byproducts, such as ammonia, urea, and uric acid
Nephrostome ("kidney" "mouth") an internal opening into the kidney. In annelids, the nephrostome through action of beating cilia forces coelomic fluid into the kidney duct. Useful substances such as sugars, ions, and water are selectively resorbed in the duct, and waste products such as ammonia are released as urine from nephridiopore on the outside of the body
Neuropodium ("nerve" "foot"): the lowermost of the two divisions of a polychaete’s parapodium
Notochord("back" "string"): an ancestral feature of the Ph. Chordata. It is a stiff dorsal cartilaginous rod providing support for the body
Notopodium ("back" "foot"): the uppermost of the two divisions of a polychaete’s parapodium
Nutritive-muscle cell: a contractile cell found in the gastrodermis of cnidarians, such as Hydra. It is characterized by an extendedbase which, in concert with the extended bases of adjoining nutritive-muscle cells, forms a tissue which contracts in the circular direction. When these interact with the complementary epitheliomuscular cells of the epidermis, locomotory and other movements are enabled by contraction and expansion of the body
Nymph: name given to the post-hatching developmental stages of exopterygote insects such as grasshoppers, bugs, and dragonflies. There are usually 5 nymphal stages. The fifth-instar nymph moults into the adult stage. It is in the later nymphal stages that the wings appear, initally as buds in the 3rd instar, larger in the 4th and 5th, then mature in the adult
Ocellus: an eyespot, usually consisting of a few dozens or hundreds of light-sensitive cells. Incapable of image-resolution
Oligochaeta ("few" "bristles"): a Class of Phylum Annelida which includes earthworms
Onychophora ("claw" "bearing"): a small but unique Phylum of wormlike invertebrates known as velvet worms. Onychophorans are of special interest in phylogeny because of many features possessed in common with annelids and arthropods
Ontogeny Recapitulates Phylogeny ("development repeats evolution"): the idea that one can see the semblance of evolutionary stages in the embryological development of animals. Thus, features of embryology offer a "flash-back" to past ancestral events. In this view, torsion, which occurs relatively rapidly in the developing veliger larva of gastropods, could represent a speeded-up view of what happened gradually during evolution over tens or hundreds of thousands of years. The presence of gill-slits and a rudimentary tail in human embryos was thought by armchair-philosophers in Victorian times similarly to be evolutionary recapitulations. See also Biogenetic Law
Open circulatory system: a situation, as in most molluscs and all arthropods, where the circulatory system is incomplete, i.e., lacking in arterioles, venules, and capillary systems, and where heart and aortas open into large spaces known as hemocoels. Contrast with the closed circulatory systems of annelids, nemertines, and cephalopods
Operculum ("cover or lid"): a protective plate found in marine gastropods and some tube-dwelling polychaetes which closes off the shell or tube after the soft parts of the body have been withdrawn
Ophiuroidea ("serpent"): a Class of Phylum Echinodermata which includes the brittle or serpent stars and the basket stars
Opisthobranchia ("behind" "gill"): a SubClass of Class Gastropoda of Phylum Mollusca which includes the nudibranchs and sea hares. In these forms, owing to detorsion, the gills have either been lost (as in nudibranchs: in some, new gas-exchange devices may be present around the anus) or displaced 90 degrees to the right (sea hares)
Osmoregulation ("pushing"): regulation of water and salt balance in the body, involving active transport of ions usually via the kidney, but also via gills, rectum and, in some, the gut
Osphradium ("osphra": smell): a sensory structure, probably associated with assessment of water quality, located in the mantle cavity near the ctenidium in certain gastropod molluscs
Parapodium ("beside" "foot"): the external appendages of polychaetes used in swimming and crawling. Also, in certain opisthobranch molluscs, the name refers to two fleshy flaps or mantle extensions used in swimming (e.g., sea hares)
Paraphyletic ("beside" "history"): refers to an artificial group of species that includes a common ancestor and only some of it's descendants (e.g., "Reptilia" and "Invertebrata"). Contrast with polyphyletic and monophyletic
Parasitism ("eat at another’s table"): a relationship between two organisms that live together, one usually on or in the other, in which one partner benefits while the other is harmed
Parenchymula larva: a specialized larva of most marine sponges characterized by a solid mass of cells with flagellated propulsive cells at the back. This differs from the amphiblastula larvae of calcareous sponges
Parthenogenesis ("virgin" "birth"): a form of asexual reproduction where offspring, generally female, develop from unfertilized eggs
Pectine ("comb"): ventral appendages of a scorpion, comb-like in shape thought to be mechanoreceptive (sensing vibration) and possibly chemoreceptive
Pedicellaria ("little foot"): jaw-like structures found on the skin of asteroids (seastars) and echinoids (sea urchins) which operate to bite at potential predators and to crush or slice organisms which may settle on the skin surface
Pelagic ("pelagos": sea) description of oceanic habitat located between the ocean surface and sea bottom
Pentamerous symmetry: refers to the five-fold symmetry unique to echinoderms, such as seastars and sea urchins. Known also as "pentaradiate"
Periostracum ("around" "shell"): the outermost of the 3 layers of a mollusc shell. It consists of a protective protein known as "chonchin"
Peristalsis ("around" "contraction"): refers to alternate waves of contraction in the muscular walls of a tube, such as a gut or oviduct, which function to move the contents along the tube
Perivisceral coelom ("around" "internal organs" "space"): the coelomic space surrounding the main internal organ systems in an animal such as an annelid, echinoderm, or vertebrate
Phagocyte ("eat" "cell"): specialized cells which can take in bits of food and incorporate them into vacuoles for digestion. Such cells are the operative digestive entities when we talk about "intracellular" digestion in poriferans (sponges), cnidarians (sea anemones, sea jellies), and platyhelminths (flatworms)
Pharyngeal gill slits ("throat"): lateral openings in the wall of the pharynx that allows water entering the mouth to exit from the pharynx. In fishes it facilitates gas exchange. In Urochordata (most notably, sea squirts), the slits allow passage of water out of the pharynx at the same time as they trap food material in mucous strands
Pheromone a chemical substance released from one individual of a species which affects another individual of the same species. Pheromones may carry alarm, reproductive, or food-location signals and are most well-known in insects
Photophore ("light" "carry or bear"): special light-producing organs in the skin of deep-sea cephalopods and crustaceans; may be coloured
Phyllopodia ("leaf" "foot"): refers to the leaf-like shape of the appendages of certain primitive crustaceans (e.g., Artemia, the brine shrimp). Their undifferentiated biramous (two-part) structure is reminiscent of what we believe to have been the shape of appendages of ancestral proto-crustaceans
Phylogeny ("tribe" "genesis or origin"): the evolutionary history of a species or more inclusive taxon
Pinacocyte ("plank" "cell"): a flattened type of cell that covers the surface of a sponge. Collectively they form a cell layer known as the "pinacoderm"
Planktonic ("planktos": wandering ): descriptive of an organism's lifestyle, floating and drifting in water. "Phytoplankton" refers to plant cells; "zooplankton" to animals, generally small in size with good buoyancy
Planospiral coiling ("flat" "spiral"): a type of shell coiling, found in some cephalopods and in freshwater snails, in which the coils lie against one another in a single plane, as a hose coiled on the ground
Planula ("flat"): the soid, ciliated larval stage of cnidarians characterized by two cell layers: a ciliated epidermis and a presumptive gastrodermis which will later form the gut lining
Platyhelminthes ("flat" "worm"): the Phylum of flatworms
Plesiomorphy ("ancestral" "character"): an ancestral character state as inferred from the most parsimonious distribution of character states on a specific cladogram
Pleuron ("side"): the soft membrane in uniramians (insects, millipedes, centipedes) joining tergal and sternal plates. It is through the pleuron that the spiracles penetrate.
Polychaeta ("many" "bristles"): a Class of Phylum Annelida which includes marine segmented worms known as tubeworms and sandworms
Polycladida ("many" "branch"): an Order of turbellarian flatworms characterized by having a many-branched gut. These worms are mostly marine and are often brightly-colored
Polymorphism ("many" "shapes"): refers to the existence in some animals, such as hydrozoans, of different phenotypic expressions of the same genotype. Thus, in hydrozoans there are polyp and medusa, and gastrozooids, gonozooids, and other types of polyps, all with identical genotypes (i.e., different forms of the same individual. Inherent in the concept of polymorphism as it applies to hydrozoans is that the different morphs perform different functions. In insects other types of polymorphisms occur, such as in butterflies where different individuals (thus, different genotypes) appear morphologically identical, and in social insects such as honeybees, ants, and termites where different castes of male, queen, and worker (again, with different genotypes) are created by the presence or absence of certain substances provided to the immature stages by other members of the colony (e.g., royal jelly in the case of bees to produce a queen rather than a worker).
Polyp ("many footed "): a sessile, asexual stage in a cnidarian life cycle. Polyps are often polymorphic, some being involved in feeding (gastrozooid), some in reproduction (gonozooid), and others in defense (dactylozooid) and water flow (siphonozooid)
Polyphyletic ("many" "tribes"): refers to an artificial grouping of species that arose from separate evolutionary lineages -- i.e., the group excludes the most recent common acncestor of all the species in the group (e.g., "Protista" and "Achhelminthes" or "pseudocoelomates"). Contrast with monophyletic and paraphyletic
Polyplacophora ("many" "plates"): a Class of Phylum Mollusca which includes the chitons
Porifera: ("pore" "bearer"): the Phylum of sponges
Porocyte: ("pore" "cell"): a cell in asconid sponges bearing a hole through its center allowing water to enter the sponge
Power-mass ratio: the smaller the size, the greater is the surface area relative to volume. When applied to flying insects, this expression tells you that a smaller-sized insect has relatively greater surface area available for gas exchange and general metabolic diffusion; hence, efficiency is relatively greater than in a larger-sized insect
Prismatic: the middle, chalky, layer of a molluscan shell
Proboscis: ("snout"): a tubular feeding structure that extends from the animal's body and carries the mouth at its tip. In many forms it is eversible, that is, can be pushed out and retracted (e.g., some worms, snails)
Procuticle: ("before" "skin"): the innermost of two layers of the arthropod cuticle (the outer layer is the epicuticle). In Insecta it is divided into two layers: the endocuticle and the exocuticle
Proglottid: ("before" "tongue"): serially repeating segment-like components making up the tapeworm's body that contain the reproductive organs. New proglottids are added at the scolex, or attachment end of the worm. Despite their appearance, tapeworms are not considered to be metameric
Prosobranchia ("before" "gill"): a SubClass of Class Gastropoda of Phylum Mollusca which includes marine snails
Prothoracic gland: a steroid-producing gland in the anterior thorax region of insects which produces a hormone, ecdysone, which initiates the moulting sequence
Protonephridium ("first" "kidney"): refers to the "flame-cell"- type of excretory system as found in flatworms - most notably in freshwater flatworms. Owing to their general absence in marine flatworms, it is thought that protonephridia function mainly in regulation of water content. Contrast with metanephridia or "later kidney", which are kidneys found in "higher" animals and which function both in the release of metabolic waste products and in the regulation of water content in the body
Protostomia ("first" "mouth"): name given to a group of animals including annelids, arthropods, and molluscs. The name refers to the fact that in the embryo the blastopore develops into the mouth. Contrast with Deuterostomia (echinoderms, chordates) in which the blastopore forms the anus, and a new opening breaks through in the embryo to form the mouth
Pseudocoelomate ("false" "hollow"): refers to the state of body construction in animals like nematodes and rotifers, where the embryonic blastocoel is retained as a largely fluid-filled space. The space functions in nematodes as a hydrostatic skeleton, permits expansion of the gut and, in the absence of a circulatory system, functions to move nutrient and energy materials throughout the body. While there is nothing "false" about the cavity, the name refers to the fact that since it remains unlined by mesoderm, it is not a coelom
Pulmonata ("lung"): a SubClass of Class Gastropoda of Phylum Mollusca which includes the terrestrial snails and slugs. In pulmonates, the gills have been lost and the mantle cavity converted into a form of "lung" for gas exchange
Pupa: metamorphic stage of endopterygote insects such as butterflies, flies, fleas, bees, and beetles during which the larval form transforms to the adult. Also known as chrysalis


Radial cleavage: a type of cleavage pattern found in deuterostomes (echinoderms, chordates) in which the daughter cells cleave off without rotation. Contrast with spiral cleavage of protostomes (annelids, molluscs, arthropods)

View animation of Radial Cleavage

Radial symmetry: see Symmetry
Radula ("scraper"): a specialized toothed device used for feeding in molluscs (not found in Bivalvia)
Recapitulation ("to repeat"): see Ontogeny Recapitulates Phylogeny
Respiratory tree ("respiratio": breathing ): unique paired and branched structures originating from the cloaca of sea cucumbers, suspended in the coelom and involved in gas exchange
Rhabdites or rhabdoids ("rod"): these are rod-like organelles found in some flatworms (e.g., Planaria) produced either by gland cells in the mesenchyme (in which case they are known as rhabdites) or by epidermal cells in the skin (rhabdoids). When released they rupture and create mucus which may function in defense or perhaps in locomotion
Rhopalium ("club"): a specialized sensory device found in sea jellies (scyphozoans) combining light- and balance-perception. There are usually 8 of these arranged symmetrically around the margin of the bell
Scaphopoda ("hatchet" "foot"): a small Class of Phylum Mollusca known as tusk shells
Schizocoely: a theory of coelom origin in which splits appear in the mesoderm which later enlarge. This type of coelom development is seen in protostomes (annelids, molluscs, arthropods). Contrast with enterocoely as found in deuterostomes (echinoderms, chordates)

View animation of Schizocoely

Sclerite ("scleros": hard): hard plates, as part of the arthropod cuticle. Axillary sclerites are found at the base of the wings in some or all Insecta, and their articulation allows the wings to be folded back (not in primitive insects such as dragonflies) and their position in flight adjusted during wing-beats
Sclerocyte ("hard" "cell"): specialized cells that produce the spicules in sponges
Sclerotize ("hard" "to make "): the hardening of protein by creation of covalent cross-linking between adjacent polypeptide chains. Phenols are involved in the process. In chitin the cross-linkings strengthen the polymer chains and create a tough, not easily broken down or digestible, substance
Scolex ("worm" or "grub"): the unique anterior attachment organ of tapeworms, endowed with suckers and often hooks. Proglottids develop from behind the scolex
Scutum ("shield"): the ventral protective plates in uniramians. These plates articulate with one other by means of soft, flexible membranes. They articulate with the dorsal tergal plates by means of a similar flexible membrane, the pleuron
Scyphistoma ("cup" "mouth"): an asexually reproducing polyp stage in scyphozoans (jellyfish) that buds off juvenile medusae known as ephyrae in a process known as strobilation
Scyphomedusa ("cup" "Medusa: creature in Gr mythology"): the sexually-reproducing medusa stage of scyphozoans (jellyfish). Medusa was one of the 3 Gorgons in Greek mythology. She apparently had snakes for hair and turned anyone who looked at her into stone
Scyphozoa ("cup" "animal"): a Class of Phylum Cnidaria which includes jellyfish
Segmentation: where the body is differentiated along the longitudinal axis into a series of units or segments, each containiing parts of organ systems. In an "idealized" form, each segment contains: muscle, nephridia, nerves, blood vessels, external appendages, and a portion of the gut
Seminal receptacle ("seed" "container"): part of the female reproductive system where sperm received from a male during mating is stored for later use to fertilize the eggs
Seminal vesicle ("seed" "bladder"): part of the male reproductive system where sperm is stored prior to mating
Septum ("wall"): a sheet of tissue that separates two compartments or cavities, as in annelids
Seta: a bristle as on annelids or arthropods
Siphonoglyph ("tube or pipe" "carving or engraving"): in sea anemones these are symmetrically paired openings on either side of the mouth which are apparently used for passage of water when the mouth is closed. Some anemones have only a single siphonoglyph
Size-strength ratio: relative strength in a tubular system, as that of a flying insect, is greatest when diameters are small. Hence, smaller insects are relatively stronger than larger ones
Spermatheca ("seed" "case or cover"): part of the female reproductive system in insects where sperm received from the male during mating is stored for later use in fertilizing the eggs (see also seminal receptacle)
Spermatophore ("seed" "to carry"): a packet of sperm passed to the female or inserted into the femal reproductive tract by the male (as in cephalopods, spiders, crustaceans)
Spicule ("small spike"): any needle-like structure protective or skeletal structure found in invertebrates; in sponges, the needle-like structures produced by sclerocytes used for support and protection
Spiracles: ("air hole"): outside openings of tracheal or tracheal-like gas-exchange systems found in arthropods such as insects, myriapods, a few isopods, and in onychophorans. Only in insects can the spiracles be closed
Spiral cleavage: one of two main types of cleavage patterns in animals in which successive division products shift back and forth, clockwise and counterclockwise. Spiral cleavage is found in the group of animals known as protostomes: annelids, arthropods, molluscs

View animation of Spiral Cleavage

Spongin: a resiliant protein fibre that makes up the "spongy" material in demosponges
Spongocoel ("sponge" "hollow"): the space within a sponge that receives the exhalent water after it exits from the flagellated (choanocyte) chambers. Because sponges do not gastrulate, there is no endoderm and, thus, no gastrodermis and, thus, this space is NOT a gut
Statocyst ("standing" "bag"): a sensory device providing information on balance. It usually consists of a small space lined by sensory cells and containing a solid concretion (as a sand grain or bit of calcium carbonate)
Stellate ganglion ("star"): a large ganglionic cluster embedded on either side of the mantle wall in cephalopods with nerves radiating to all parts of the mantle. The stellate ganglia are involved in jet-propulsive locomotion. Nerves from the stellate ganglia increase in diameter with length; thus, since speed of nerve impulses is directly related to diameter of the neurones, impulses tend to arrive at all parts of the mantle simultaneously. This increases the effectiveness of the jet in locomotion
Sternum ("breast or chest"): in arthropods, the plate on the ventral surface of each body segment
Stolon ("shoot" or "branch"): in colonial tunicates and cnidarians, a tissue connection between zooids or polyps allowing transfer of nutrients and energy
Strobila ("twisted" or a "cone"): the stage in the life cycle of a scyphozoan also known as an ephyra. Strobilation is the process of asexual division of the scyphistoma polyp into ephyrae
Suspension feeding a mode of feeding where particulate food matter suspended in the surrounding water is removed; also known as filter-feeding
Sycon: an intermediate morphological type of sponge with the flagellated chambers occupying inpocketings off the spongocoel
Symbiosis ("to live together"): a general term for two organisms living together. Mutualism, commensalism, parasitism, and inquilinism are all types of symbioses
Symmetry: is the arrangement of parts into geometrical designs. Invertebrates have spherical, radial, biradial, or bilateral symmetry. Spherical symmetry is obvious and is found in a few groups of protists to one degree or another. Radial symmetry is found in hydroid polyps or sea jellies, which have a single longitudinal axis about which the body parts are arranged. Here, the body can be divided into an infinite number of symmetrical halves by single hypothetical cuts along the longitudinal axis. If some structures are off-centre, such as siphonoglyphs in a sea anemone, then the symmetry is biradial because symmetrical halves can be produced in only two ways: by a cut down the longitudinal and transverse axes, and by a cut down the longitudinal and sagittal axes. Any other cut produces asymmetrical halves. Note: 1) the designation of transverse and sagittal axes is arbitrary (one can be the other) and 2) the off-centre structures are themselves arranged symmetrically. If structures are off-centre and not arranged symmetrically, then like halves can be produced by only a single cut through the longitudinal and sagittal axes. This type of symmetry, bilateral, is a characteristic of all "higher" animals.

Symplesiomorphy("shared" "ancestral" "character'): a shared ancestral character state as inferred from the most parsimonious distribution of character states on a specific cladogram. Contrast with synapomorphy
Synchrony-asynchrony in wing-beats: this refers to whether wing-beats in insects are in synchrony or not with nervous stimulation of the flight muscles. If muscles attach directly to the wing bases, then the wing-beat is likely to be synchronous; that is, elevator and depressor muscles will separately be stimulated by a nerve impulse (or set of impulses), each stimulus causing the wing to move up or down. This is a primitive condition, only found in odonatans (dragonflies, mayflies). Most insects have a combination of muscle/nerve innervation and attachments. Midges, some of which can beat their wings up to 1000 times per second, have an asynchronous/indirect system which allows a certain number of oscillations (wingbeats) to occur from a single nerve impulse; otherwise, there would not be time enough to regenerate action potentials sufficiently fast enough to run the system
Synapomorphy ("shared" "derived" "character'): a shared derived character state as inferred from the most parsimonious distribution of character states on a specific cladogram. Contrast with symplesiomorphy
Tadpole larva: the larval type of tunicates (sea squirts). Only the tadpole larva has the 4 chordate characteristics of notochord, dorsal tubular nerve cord, pharyngeal gill slits, and tail. All but the gill slits are lost during metamorphosis to the adult sea squirt. Tadpole larvaae function mainly in substrate selection and not so much in despersion, as their free-living lives are generally quite short
Tanning ("tanning"): a process in which fibrous proteins are strengthened by chemical cross-linking, usually involving the formation of bonds between sulphur-containing amino acids in adjacent protein strands
Tegument ("to cover"): the protective outer covering of parasitic flatworms, including flukes and tapeworms, consisting of a syncytial (cell nuclei not separated by cell boundaries; hence, multinucleate protoplasm) outer layer connected to cells lying deep within the worm's mesenchyme
Tergum ("back"): the dorsal or uppermost exoskeletal plate in uniramians such as insects, millipedes, and centipedes. The tergal plates articulate with one another by means of soft, flexible membranes and with the ventral scutal plates by means of a similar flexible membrane, the pleuron
Test ("shell"): usually an outer, nonliving shell or case surrounding an organism; in echinoids (sea urchins), however, the test is actually an endoskeleton produced from mesoderm
Tetrodotoxin: a neurotoxic poison produced in the anterior salivary glands of cephalopods used in conjuction with the beak for immobilizing and killing prey
Torsion ("twist"): a twisting of the head/velum 180 degrees on the foot during early larval life in gastropods. This brings the mantle cavity to the front and creates a problem of fouling of the head with anal and kidney discharges, but provides a protective space represented by the mantle cavity into which the head/velum and foot can be withdrawn. A protective plate or operculum attached to the foot then plugs the hole
Totipotent ("whole or all" "potency"): with reference to a cell: one that can change into any one of a variety of specialized cells in the organism (e.g., amoebocyte of a sponge or lymphocyte of a tunicate)
Trachea ("trough"): refers in invertebrates to the branching system of air-tubes found in arthropods such as insects, spiders, and myriapods (centipedes, millipedes), in onychophorans and in some terrestrial isopods. At their finest diameter, the tubes are known as tracheoles, are fluid-filled, and penetrate into the tissue cells to allow direct exchange of respiratory gases
Trematoda: a Class of parasitic Platyhelminthes known as liver flukes
Triaxon ("three" "axis"): a type of sponge spicule with 3 axes. Since this type of spicule has 6 rays, it is also known as hexactinellid, and is the spicule type characterizing the Class of sponges known as Hexactinellida
Trichinosis ("hair"): the state of being parasitized by the nematode Trichinella spiralis
Tricladida ("three" "branch"): an Order of turbellarian flatworms characterized by having a 3-branched gut; e.g., Planaria. Triclads are freshwater, marine, and terrestrial
Trilobitomorpha: a SubPhylum of Phylum Arthropoda which includes the extinct trilobites
Triploblastic ("three" "buds"): refers to the 3 cell-layer state of animals which have endoderm (syn. entoderm), ectoderm, and mesoderm. All bilaterian animals are triploblastic (cnidarians are diploblastic)
Trochophore larva ("wheel" "bearing"): primitive larval type of marine molluscs characterized by a circlet of propulsive and food-gathering cilia. This stage precedes the veliger larval stage in classes of molluscs where the latter is present (scaphopods, gastropods, bivalves). A trochophore larva is also present in polychaetes, thus suggesting homologous origins of molluscs and annelids
Tube foot fluid-filled tubular structures that are part of the water-vascular system of echinoderms. These soft hydraulic structures can be extended and retracted, and function in feeding, locomotion, anchoring, and gas exchange
Tunicin ("tunic or cloak"): a type of exoskeleton functioning in support and protection in tunicates (sea squirts), consisting of a combination of protein and mucopolysaccharides (tough and indigestible)
Turbellaria: a Class of free-living Platyhelminthes, including colorful marine forms and the freshwater-inhabiting Planaria
Tympanum ("drum"): a taut membrane located at the junction of the thorax and abdomen in insects, used for sound perception
Typhlosole ("blind" "pipe"): a lobulated ridge of tissue extending into the lumen of the gut of earthworms which presumably functions to increase the digestive and absorptive surface area of the gut
Uniramia ("one" "branch"): a SubPhylum of Phylum Arthropoda which includes the millipedes, centipedes, and insects. The name uniramia refers to the fact that the appendages are unbranched, or uniramous
Uric acid: an excretory chemical containing amine groups (NH2) used in insects and terrestrial gastropods as a means to rid the body of unwanted ammonia produced as a byproduct of metabolism. Uric acid is insoluble and in insects is excreted as harmless crystals which are voided from the body along with the feces
Urochordata: a SubPhylum of Phylum Chordata which includes the sea squirts, mucus-house builders, and appendicularians
Veliger larva ("veil or sail"): advanced larval type of marine molluscs characterized by a ciliated lobe (or lobes) known as the velum which functions in propulsion and food-gathering
Velum ("veil" or "covering"): in hydromedusae is a thin flap of tissue around the inner surface of the bell, perhaps functioning venturi-like to increase the velocity of the locomotory jet of water; in the veliger larva of molluscs is a double-lobed ciliated structure used to locomote and filter-feed
Vermiform ("wormlike"): wormlike in appearance; in locusts, the egg hatches to a "vermiform larva" that wiggles its way to the soil surface - it then splits out of the vermiform cuticle and becomes the 1st instar stage
Visceral mass ("viscera": internal organs): one of the 3 main parts of a mollucan body: head, foot, and visceral mass. It is the fleshy part that contains the main organ systems
Volvent ("roll or turn"): a type of nematocyst found in Hydra which coils tightly on discharge, thus tangling a prey. Volvents are non-toxic and, owing to their special function, have a low discharge threshold on the chemical (prey-perception) side
Water-vascular system: system of water ducts and tubefeet in echinoderms (seastars, sea urchins, sea cucumbers) used in locomotion, anchoring, gas exchange, and feeding
Zoea larva("animal" "larva"): the first free-living larval stage of crabs which transforms into a crab-like megalops stage prior to settling from the plankton. Zoeae are carnivorous, feeding on other zooplankters


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