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 (1^st and 2^nd 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 3^rd instar, larger in the 4^th and 5^th, 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 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