IImago The final instar, or adult stage, of an insect life cycle.
Incident light In microscopy, light striking the object from above the stage.
Incomplete cleavage Cleavage furrows do not completely cut through the egg mass; meroblastic.
Incurrent canal Tubular invagination of the sponge pinacoderm that leads into the flagellated chambers.
Incurrent pore Small opening on the surface of sponges that leads into an incurrent canal. Ostium.
Indeterminate cleavage Fate of the blastomeres is fixed relatively late in development. Regulative development.
Indirect deposit feeding (selective deposit feeding) Use of appendages, tentacles, or cilia to select organic particles for ingestion.
Indirect development Having a larval stage(s) between egg and adult.
Inequilateral anterior and posterior ends of a bivalve valve are dissimilar.
Inequivalve The two valves of a clam of different sizes.
Infauna Animals that live within bottom sediments.
Infraciliary system The entire assemblage of ciliary basal bodies, or kinetosomes, and the fibers that link them together in the cell cortex of ciliates.
Infusoriform larva The final free swimming larval stage of rhombozoans.
Ingression Mode of gastrulation in which cells of the blastula wall proliferate cells into the blastocoel.
Insertion One of the two attached ends of a muscle. Of the two, the insertion is usually distal and moves when the muscle contracts.
Instar Each of the several stages between successive ecdysozoan molts.
Integument The outer layers of the body wall. Usually comprising the epidermis and underlying connective tissue (dermis) plus any secreted cuticle or exoskeleton.
Intercellular junction Membrane specialization that binds cells together, promotes communication between cells, or helps to regulate transport across an epithelium.
Intermediate host The host for larval stages of a parasite.
Interstitial cell A small, rounded totipotent cnidarian cell, sandwiched between cells of the epidermis and gastrodermis.
Interstitial fauna Animals that live in the spaces between sand grains.
Intertidal The coastline between the low and high tide levels, also known as the littoral zone.
Intrinsic Confined within a structure; not extending to neighboring structures.
Introvert Eversible part of the bryozoan, priapulid, or sipunculan body.
Invagination Infolding. In gastrulation, this refers to a type of morphogenetic movement in which the cells of the vegetal hemisphere fold into the interior to form the archenteron.
Isolecithal egg Egg in which the yolk is uniformly distributed. Homolecithal.
Isomyarian Anterior and posterior adductor muscles approximately equal in size..
JJellyfish A cnidarian medusa.
KKairomone A substance released by a predator to which the prey may respond defensively.
Kenozooid A bryozoan heterozooid modified to form a stolon.
Kinetodesma (pl. Kinetodesmata) A fine striated fiber that connects kinetosomes (basal bodies) of ciliates.
Kinetoplast Conspicuous mass of DNA that is situated within the single, large, elongated mitochondrion of kinetoplastid (trypanosome) protozoans.
Kinetosome A ciliary or flagellar basal body.
Kinety (pl. Kineties) One row of cilia, kinetosomes, and kinetodesmata of ciliates.
LLacunar canal system Acanthocephalan circulatory system within the syncytial epidermis.Metamerism Segmentation;division of the body into a linear series of similar modules.
Lacustrine Pertaining to lakes.
Lamella (pl. Lamellae) A sheet or flat plate of tissue. In bivalves, each of the gill surfaces.
Languet One of several folds of tissue along the dorsal pharyngeal wall of some ascidians which together convey food to the esophagus. A discontinuous dorsal lamina.
Lappet Lobe formed by the margin of the scalloped scyphozoan bell. Movable flaps that can expose or cover the ambulacral groove of crinoids.
Larva (pl. Larvae) An independent, motile, developmental stage that does not resemble the adult.
Larviparous Eggs brooded internally within the female that are later released as larvae.
ateral canal Part of the echinoderm water vascular system that joins the radial canal and tube feet.
Laurer’s canal Short, inconspicuous canal that extends from the seminal receptacle of trematodes to the dorsal surface, where it may open at a minute pore.
Lecithotrophic brooding Viviparous development in which the embryo is nourished by yolk.
Lecithotrophic larva A nonfeeding larva that utilizes yolk as a source of nutrition.
Lemniscus (pl. Lemnisci) Fluid filled invaginations of unknown function in acanthocephalans.
Leuconoid Refers to a type of sponge body organization built around flagellated chambers and an extensive system of canals.
Ligament sac A connective tissue sac in the acanthocephalan hemocoel containing the gonads.
Littoral In the sea; synonymous with intertidal. In lakes; the nearshore zone in which light sufficient to support rooted vegetation reaches the bottom.
Lobopodium (pl. Lobopodia) A pseudopodium that is rather wide with rounded or blunt tips, is commonly tubular, and is composed of both ectoplasm and endoplasm.
Longitudinal cord Ridge that entends the length of the body created by the inward expansion of the epidermis in nematodes and some gastrotrichs.
Lophophoral organ An area on the phoronid lophophore where spermatophores are formed.
Lophophore A circular or horseshoe shaped fold of the mesosomal body wall encircling the mouth, bearing hollow ciliated tentacles, and excluding the anus.
Lorica In rotifers; an intracytoplasmic skeleton.
Luciferase Enzyme that catalyzes the bioluminescence reaction.
Luciferin The substrate of luciferin capable of bioluminescence.
Lunule One of the large, elongated notches or openings in the bodies of some clypeasteroids (sand dollars).
Lyriform organ Group of slit sense organs found on some arachnids.
MMacerate To soften and separate the parts of a solid object.
Macromere One of several large blastomeres located in the yolky vegetal hemisphere of early embryos.
Macronucleus (pl. Macronuclei) Large, usually polyploid, ciliate nucleus concerned with the synthesis of RNA, as well as DNA, and therefore directly responsible for the phenotype of the cell.
Macrophagous Collecting and ingesting large food particles.
Madreporite Pore or sieve plate of the echinoderm water vascular system that connects the stone canal to the exterior seawater (most echinoderms) or to the perivisceral coelomic fluid (crinoids and holothuroids).
Malpighian tubule Excretory system consisting of a blind, tubular, contractile, excretory evagination of the arthropod midgut.
Manca larva A peracarid postlarva that has all appendages except the eighth thoracopods.
Mangrove A small tropical tree or large shrub adapted for living in the intertidal zone.
Mantle A body wall fold that secretes a shell, as in molluscs, barnacles, and brachiopods. The body wall beneath the ascidian tunic.
Mantle cavity Protective chamber created by the overhang of a mantle; pallial cavity.
Manubrium (pl. Manubria) ••Tubelike extension, bearing the mouth, that hangs down from the center of the subumbrella of cnidarian medusae. Hypostome of hydroid polyps.
Marsupium (pl. Marsupia) Brood pouch outside the body.
Mastax The cuticular pharyngeal jaw apparatus of a rotifer.
Mastigoneme One of the many fine, lateral branches of some flagella.
Mastigont system Complex formed by groups of flagella and several microtubular and fibrillar organelles.
Matrotrophic brooding Viviparous development in which the embryo is nourished by the mother.
Medulla Central part of the heliozoan cell that is composed of dense endoplasm, containing one to many nuclei and the bases of the axial rods.
Medusa (pl. Medusae) Form of cnidarian that has a well developed, gelatinous mesoglea and is generally free swimming.
Megalops Crab postlarva with a large abdomen and full complement of appendages.
Megasclere A large spicule forming one of the chief supporting elements in the skeleton of sponges.
Mehlis’s gland Conspicuous unicellular gland cells associated with the reproductive system of trematodes which play a role in egg capsule formation.
Meiofauna Small metazoans that pass through a 1 mm sieve but are retained by a mesh of 42 µm; usually referring to those living in small confined spaces.
Membranelle Type of ciliary organelle derived from two or three short rows of cilia, all of which adhere to form a more or less triangular or fan shaped plate that beats as a unit.
Meroblastic Cleavage furrows do not completely cut through the egg mass.
Meroplankton Plankters that spend only part of the life cycle in the plankton.
Merozoites Individuals produced by multiple fission of sporozoan trophozoites.
Mesenchyme A network of loosely associated, often motile, embryonic cells, that are usually, but not always, of mesodermal origin. The term is still applied to adult connective tissues of some groups of animals.
Mesentery (pl. Mesenteries) A longitudinal sheet of tissue that divides the body cavity of bilaterally symmetrical animals.
Mesentoblast Blastomere associated with spirally cleaving zygotes that contains an unidentified cytoplasmic factor which causes the cell and its progeny to form mesoderm.
Mesoderm Embryonic germ layer that forms the tissues situated between ectoderm and endoderm.
Mesoglea Connective tissue layer between the epidermis and gastrodermis of cnidarians and ctenophores.
Mesohyl Sponge connective tissue. Lies beneath the pinacoderm and consists of a gelatinous proteinaceous matrix containing skeletal material and ameboid cells.
Mesopelagic Subdivision of the pelagic zone, 200-1000 m.
Mesothelium (pl. Mesothelia) Single, nonstratified epithelium lining the coelom.
Metacercaria (pl. Metacercariae) Encysted final stage of digenean development.
Metachrony Wave pattern that results from the sequential coordinated action of cilia or flagella over the surface of a cell or organism.
Metamere A body segment or somite.
Metamorphosis (pl. Metamorphoses) ••Transformation from a larva into an adult.
Metanauplius (pl. Metanauplii) One of several instars following the crustacean nauplius.
Metanephridial system Excretory system composed of a vascular ultrafiltration site, a coelomic space, and a metanephridium tubule.
Metanephridium (pl. Metanephridia) An excretory tubule that opens into the coelom by a ciliated funnel and to the exterior by a nephridiopore.
Metatroch A second girdle of cilia that develops posterior to the prototroch of a trochophore.
Micromere One of many small blastomeres located in the animal hemisphere of the cleaving zygote.
Micronucleus (pl. Micronuclei) ••Small, usually diploid, ciliate nucleus concerned primarily with the synthesis of DNA. It undergoes meiosis before functioning in sexual reproduction.
Microphagous Specialized for feeding on small food particles.
Micropyle An opening in the eggshell or resting stage from which the primordium eventually emerges.
Microsclere A tiny sponge spicule.
Microtrich Type of microvillus found on the tegument of tapeworms.
Microtubule organizing center (MTOC) A region around basal bodies and centrioles that controls the organized assembly of microtubules.
Mictic egg Type of fertilized rotifer egg that is thin shelled, haploid, and can be fertilized.
Miracidium (pl. Miracidia) Ciliated, free swimming, first larva of digenean trematodes.
Molt To shed the old cuticle as a new cuticle is being secreted. Ecdysis.
Monocondylic Articulated by one movable hinge (condyle).
Monolayered epithelium Consisting of a single layer of cells resting on a basal lamina (= simple epithelium).
Monomyarian Bivalve condition in which the anterior adductor muscle is lost.
Monopectinate Refers to a gill in which the filaments occur on only one side of the axis.
Monophyletic group All species descended from a common ancestor.
Monospecific A taxon consisting of a single species.
Monotocardian Heart with one atrium.
Monotypic A taxon consisting of a single species.
Mosaic development Embryonic fate determination in which cell fate is determined early in development and is the result of the action of specific factors that are unevenly distributed, like pieces of a mosaic, in the cytoplasm of the uncleaved egg.
Mucocysts Mucigenic bodies that are arranged in rows, similar to ciliate trichocysts, and discharge a mucoid material.
Mucus Animal secretion utilized in a variety of ways as an adhesive, protective cover, or lubricant.
Mutualism A symbiotic relationship in which both species benefit.
Myocyte Type of sponge mesohyl cell which displays some similarities to a smooth muscle cell in shape and contractility. A muscle cell.
Myoepithelial cell A muscle cell that is part of an epithelium.
Myogenic Originating in a muscle cell.
Myoneme A bundle of contractile filaments that lies in the pellicle of some protozoans.
NNacre The smooth, lustrous, usually innermost, shell layer of some molluscs; mother of pearl.
Natatory Adapted for swimming.
Naupliar eye Median crustacean eye composed of three or four ocelli.
Nauplius Earliest hatching stage and basic crustacean larva; has three pairs of appendages.
Neap tides Tides occurring on quarter moons characterized by modest tidal amplitudes.
Nectophore Mouthless, pulsating swimming bell of siphonophores.
Nematocyst Stinging cnida of cnidarians.
Nematodesma (pl. Nematodesmata) One of several microtubular rods that line and support the wall of the ciliate cytopharynx and assist in the inward transport of food vacuoles.
Nematogen Adult dicyemid.
Neoblast A totipotent cell that is important in wound healing and regeneration.
Nephridium (pl. Nephridia) An excretory tubule usually opening to the exterior via a nephridiopore. See protonephridium, metanephridium.
Nephrocyte A large phagocytic cell, alone or in clusters, in the hemocoel of many arthropods.
Nephromyces A unicellular fungus that occurs in the renal sacs or the pericardium of some ascidians.
Nephrostome An open ciliated funnel at the inner, coelomic, end of a metanephridium.
Neritic zone The water above the continental shelves.
Nerve net Plexus.
Neurogenic Originating with a neuron.
Neuropil A concentration of axons and synapses in a ganglion.
Neuropodium (pl. Neuropodia) The ventral branch of a polychaete parapodium.
Niche An organism's role in its ecosystem.
Nocturnal Circadian behavior characterized by activity at night.
Non-selective deposit feeding See direct deposit feeding.
Notopodium (pl. Notopodia) The dorsal branch of a polychaete parapodium.
Nuchal organ One of a pair of ciliated chemosensory pits or slits that are often eversible and are situated in the head region of most polychaetes.
Nutritive muscle cell A muscle cell in the cnidarian gastrodermis that usually bears a cilium and is responsible for intracellular digestion.
OObturaculum (pl. Obturacula) One of two elongated, medially fused structures which arise anteriorly from the head of vestimentiferan pogonophores and bear and support the gills.
Occluding junction Sealing junction between cells.
Oceanic zone The division of the pelagic realm seaward of the continental shelf.
Ocellus (pl. Ocelli) A simple eye.
Odontophore A muscular and cartilaginous mass in the buccal cavity of many molluscs, it supports the radula.
Oligomery Division of the body into three linear regions, characteristic of many deuterostome animals. Tricoelomate.
Oncomiracidium (pl. Oncomiracidia) The ciliated larva of monogeneans.
Oncosphere Encapsulated first stage in the life cycle of certain tapeworms that bears six hooks and cilia. Typically referred to as a coracidium when released into the water.
Ontogeny The development of the individual from fertilization to death.
Oostegite A large medial platelike process of a thoracic coxae that contributes to a marsupium.
Ootype Small, centrally positioned sac within the female reproductive system of most parasitic platyhelminths.
Oozooid The zooid developing from the fertilized egg of urochordates.
Operculum (pl. Opercula) A lid or covering of an opening or chamber.
Ophiopluteus (pl. Ophioplutei) Planktotrophic larva of many species of ophiuroids.
Opisthodetic In bivalves; the hinge ligament situated posterior to the umbo.
Opisthognathus Posteriorly directed position of insect head.
Opisthosoma The posterior end of the pogonophore body which is composed of numerous (up to 95) segments. The posterior chelicerate tagma, also called the abdomen.
Oral arm One of the four, often frilly extensions of the scyphozoan manubrium.
Oral ciliature Cilia that are associated with the mouth region of ciliates.
Oral disc Area around the mouth of an anthozoan polyp which bears eight to several hundred hollow tentacles.
Oral shield One of a series of large plates that frame the ophiuroid mouth and also form a chewing apparatus with five triangular, interradial jaws at the center.
Oral sucker Organ that surrounds the trematode mouth, prevents dislodgement and aids in feeding.
Organ of Tömösvary Hygroreceptive or chemoreceptive organs on the tracheate head.
Origin One of the two attached ends of a muscle. Of the two, the origin is usually proximal and remains stationary when the muscle contracts.
Osculum (pl. Oscula) The excurrent opening of the water circulation system of the sponge.
Osmoconformation Internal osmolarity is allowed to vary with external osmolarity.
Osmoregulation The maintenance of an internal osmolarity unlike the external.
Ossicle An internal skeletal piece, commonly calcareous as in echinoderms.
Ostium (pl. Ostia) A small incurrent opening or pore on the surface of a body, gill, or heart.
Ovigerous Bearing eggs externally.
Ovigerous leg The third appendage of pycnogonids, used by the male to brood the fertilized eggs.
Oviparous Egg laying.
PPaedogenesis Achievement of sexual maturity as a larva, without attaining adult morphology.
Pallial line The line of mantle attachment impressed on the inner surface of the shell as a scar.
Palmella Nonflagellated stage of flagellated protozoans.
Papula (pl. Papulae) Finger like, respiratory evagination of the aboral body wall of some asteroids.
Paramylon Photosynthetic storage product of euglenoids.
Paraphyletic A taxon containing some, but not all, of the descendants of an ancestor.
Parapodium (pl. Parapodia) ••Lateral, fleshy, paddle like appendage on polychaete annelids.
Parasitism A symbiotic relationship in which one species (parasite) benefits from the relationship and the other species (host) is harmed but usually not killed.
Parasitoidism A prolonged intimate symbiosis in which one member eventually kills the other.
Paratomy The phenomenon of linear budding in some turbellarians and annelids.
Parenchyma Connective tissue compartment between the body wall musculature and gut of platyhelminths.
Parenchymula A sponge larva that lacks an internal cavity and bears flagellated cells over all of its outer surface except, often, the posterior pole. Parenchymella.
Parthenogenesis Unisexual reproduction with unfertilized eggs and no contribution by males.
Parturial molt The molt that results in the appearance of complete, functional oöstegites.
Patch reef A small circular or irregular reef that rises from the floor of a lagoon behind a barrier reef or within an atoll.
Paurometabolous Insect development in which nymphs closely resemble adults but lack wings and are sexually immature. There is no radical metamorphosis.
Paxilla (pl. Paxillae) An echinoderm ossicle crowned with small, movable spines.
Pectinate Having teeth or side branches arranged like a comb.
Pectine A comb-like sensory appendages unique to scorpions.
Pedal disc In some sea anemones, a flattened disc at the aboral end of the column for attachment.
Pedal laceration Method of asexual reproduction in some anemones in which parts of the pedal disc are left behind as the animal moves.
Pedicellaria (pl. Pedicellariae) A small, specialized jawlike appendage of asteroids and echinoids which is used for protection and feeding.
Pedicle Muscular, flexible stalk that attaches articulate brachiopods to the substratum.
Pedipalp The second chelicerate appendage, it is modified for a variety of functions.
Peduncle Muscular, flexible attachment stalk of goose barnacles.
Pelagic The water of the open ocean, including the neritic and oceanic zones. Also, organisms living in the water independent of the bottom.
Pelagosphera Secondary planktotrophic larva of sipunculans.
Pellicle Protozoan “body wall” composed of cell membrane, cytoskeleton, and other organelles.
Pellucid Clear, transparent.
Penetration anchor An anchor that holds one part of a burrowing animal’s body in place as another part penetrates and advances into the sediment.
Peniculus (pl. Peniculi) A modified membranelle that is greatly lengthened and thus tends to be similar to an undulating membrane in function.
Pentactula Metamorphosing stage of holothuroid development that bears five primary tentacles.
Pentamerous Divided into five parts, characteristic of the body of echinoderms.
Periostracum The outer proteinaceous layer of a molluscan shell, composed conchiolin.
Periproct The membranous area, often bearing ossicles, around the anus of echinoids.
Perisarc A supporting, nonliving chitinous cuticle secreted by the epidermis surrounding most hydroids.
Peristalsis A wave of muscular contraction moving along a body or internal tube or vessel. Peristome Buccal cavity of ciliates. The membranous area around the mouth of some echinoderms, i.e., sea urchins.
Peristomium The first true segment, immediately posterior to the prostomium, of an annelid. Usually lacks locomotory appendages.
Peritoneum The innermost, noncontractile layer of a stratified coelomic epithelium; separates the coelomic fluid from the musculature.
Petaloid One of five petal shaped areas on the aboral surface of irregular urchins that bear specialized respiratory podia.
Phagocytosis The engulfment of large particles, such as bacteria and protozoans, by evagination of the cell surface.
Pharynx (pl. Pharynges) An anterior gut region, often heavily muscularized.
Phorozooid A locomotory zooid of doliolids that has a short posterior spur upon which buds differentiate into gonozooids.
Photocyte Specialized cell within which light is produced.
Photophore A light producing organ.
Photosynthate The organic carbon fixed by the photosynthetic pathway.
Phyllobranch Having flat, leaflike gills.
Phyllode Each of five oral ambulacral areas of irregular echinoids that contains specialized podia for obtaining food particles.
Phyllopod Flattened, leaflike appendage.
Phytoflagellate A photosynthetic flagellate.
Phytophagous Plant eating.
Phytoplankton Microscopic algae suspended in the water column of lakes and seas.
Pilidium A free swimming and planktotrophic larva of many heteronemerteans which is characterized by an apical tuft of cilia and is somewhat helmet shaped.
Pinacocyte One of the epithelial like flattened cells which together make up the sponge pinacoderm.
Pinnate Having side branches, like a feather.
Pinnule Side branch of an appendage, i.e., on octocoral tentacles, crinoid arms.
Pinocytosis A nonspecific form of endocytosis in which the rate of uptake is in simple proportion to the external concentration of the material being absorbed.
Planispiral All whorls of a coiled molluscan shell lying in a single plane.
Plankton Organisms suspended in the water column and unable to move independently of water current because of small size or insufficient motility.
Planktotrophic larva A planktonic larva that feeds on other planktonic organisms.
Planula (pl. Planulae) A cnidarian larva that is elongated and radially symmetrical but with anterior and posterior ends.
Plasmodium (pl. Plasmodia) Amoeboid syncytial mass.
Pleopod The anterior abdominal appendages of malacostracans.
Plerocercoid The final stage in the life cycle of certain tapeworms.
Plesiomorphic Refers to an evolutionarily primitive state of a homolog.
Pleurite (pl. Pleura) Either of the two primary, lateral, exoskeletal plates of each segment of an arthropod; also pleuron.
Plicate Folded or ridged.
Podocyst A foot extension of some pulmonate embryos for excretion and absorption.
Podocyte Cell with branching interdigitating toelike processes, usually over the surface of a blood vessel. An adaptation for ultrafiltration.
Polyembryony Development of multiple embryos from a single cell mass.
Polymorphism Two or more individuals or zooids of a species modified for different functions.
Polyp Form of cnidarian that has a thin layer of mesoglea and is generally sessile.
Polyphyletic A taxon that includes the descendants of more than one ancestor.
Polypide The innermost parts of a bryozoan zooid, including the introvert, lophophore, and viscera but not the body wall or zooecium.
Polypide regression Degeneration and replacement of bryozoan polypide from the cystid.
Porocyte A sponge cell that surrounds an opening which extends from the external surface to the spongocoel.
Preoral pit The developmental precursor of the wheel organ and Hatschek’s groove that opens on the left side of the head of larval cephalochordates.
Pressure drag The difference in pressure at the front end (higher pressure) of a forward moving organism as compared to the rear end (lower pressure).
Pretrochal region Apical plate, prototroch, and mouth region of a trochophore larva.
Primary host See definitive host.
Proboscis (pl. Proboscides) Any tubular process of the head or anterior part of the gut, usually used in feeding and often extensible.
Proboscis apparatus The complex, eversible, prey capturing organ of nemerteans.
Proboscis pore The opening of the proboscis apparatus at or near the anterior tip of a nemertean.
Procercoid Developmental stage of certain tapeworms between oncosphere and plerocercoid.
Proctodeum Invaginated embryonic ectoderm joining the anus with the endodermal midgut. Procuticle Thick, inner layer of the arthropod exoskeleton.
Proglottid One of the linearly arranged segment like sections that make up the strobila of a tapeworm.
Prognathus Anteriorly directed position of insect head.
Prograde Propagating in the direction in which the animal is moving, ie posterior to anterior (= direct propagation).
Pronate Rotation of the leading edge down.
Propodium (pl. Propodia) The front of a gastropod foot which acts like a plough and anchor.
Prosopyle Internal opening of a sponge through which water flows from the incurrent canal into a radial canal or flagellated chambers.
Protandry Type of hermaphroditism in which the individual is first a male and then a female.
Protoconch The shell of the veliger which may remain at the apex of the adult shell.
Protogyny Type of consecutive hermaphroditism in which the individual is first female then male.
Protonephridium (pl. Protonephridia) A ciliated excretory tubule capped internally by one or more terminal cells specialized for ultrafiltration.
Protopod The basal part of a crustacean appendage, consisting of the combined coxa and basis.
Protostome Member of a major branch of the Animal Kingdom, in which the blastopore contributes to the formation of the mouth.
Prototroch Preoral ring of cilia of a trochophore larva.
Protozoea Third larval stage of a decapod (shrimp); after the metanauplius and before the zoea.
Proximal Close to the origin, center, or midline.
Pseudocoel Fluid filled body cavity that occupies the connective tissue compartment. Differs from the hemocoel only in the absence of a heart.
Pseudofeces In filter feeders such as bivalves, material removed from the water flow, aggregated, and rejected before entering the gut.
Pseudolamellibranch gill Bivalve gill with filaments bound together with small tissue junctions.
Pseudopodium (pl. Pseudopodia) A flowing extension of a cell.
Ptychocyst A cnida that discharges a thread used to weave a tube.
Pygidium (pl. Pygidia) The terminal, nonsegmental part of the body of a segmented animal. Typically bears the anus. Telson.
Pupa (pl. Pupae) In holometabolous insects, the stage between the last larval instar and the adult.
Pyramid Large calcareous plate that composes Aristotle’s lantern; shaped somewhat like an arrowhead with the point projected toward the mouth.