Dactylozooid

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

special polyps in colonial hydrozoans functioning in defense. The nematocysts in the dactylozoids are especially toxic

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

Ecdysis

("escape"):

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 6^th 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)