Choanocytes

A flagellated feeding cell found in sponges. Also called a collar cell, it has a collar-like ring that traps food particles around the base of its flagellum.

Mesohyl

A gelatinous region between the two layers of cells of a sponge.

Amoebocytes

An amoeba-like cell that moves by pseudopodia and is found in most animals. Depending on the species, it may digest and distribute food, dispose of wastes, form skeletal fibers, fight infections, or change into other cell types.

Hermophrodites

An individual that functions as both male and female in sexual reproduction by producing both sperm and eggs.

Gastrovascular cavity

A central cavity with a single opening in the body of certain animals, including cnidarians and flatworms, that functions in both the digestion and distribution of nutrients.

Polyps

The sessile variant of the cnidarian body plan. The alternate form is the medusa.

Medusa

The floating, flattened, mouth-down version of the cnidarian body plan. The alternate form is the polyp.

Cnidocytes

A specialized cell unique to the phylum Cnidaria; contains a capsule-like organelle housing a coiled thread that, when discharged, explodes outward and functions in prey capture or defense.

Nematocysts

In a cnidocyte of a cnidarian, a capsule-like organelle containing a coiled thread that when discharged can penetrate the body wall of the prey.

Protonephridium

An excretory system, such as the flame bulb system of flatworms, consisting of a network of tubules lacking internal openings.

Planarians

A free-living flatworm found in ponds and streams.

Alimentary canal

A complete digestive tract, consisting of a tube running between a mouth and an anus.

Parthogenesis

A form of asexual reproduction in which females produce offspring from unfertilized eggs.

Ectoprocts

A sessile, colonial lophophorate; also called a bryozoan.

Exoskeleton

A hard encasement on the surface of an animal, such as the shell of a mollusc or the cuticle of an arthropod, that provides protection and points of attachment for muscles.

Brachiopods

A marine lophophorate with a shell divided into dorsal and ventral halves; also called a lamp shell.

Foot

(1) The portion of a bryophyte sporophyte that gathers sugars, amino acids, water, and minerals from the parent gametophyte via transfer cells. (2) One of the three main parts of a mollusc; a muscular structure usually used for movement.

Visceral mass

One of the three main parts of a mollusc; a fold of tissue that drapes over the mollusc’s visceral mass and may secrete a shell. See also foot, visceral mass.

Mantle

One of the three main parts of a mollusc; a fold of tissue that drapes over the mollusc’s visceral mass and may secrete a shell.

Mantle cavity

A water-filled chamber that houses the gills, anus, and excretory pores of a mollusc.

Radula

A straplike scraping organ used by many molluscs during feeding.

Torsion

In gastropods, a developmental process in which the visceral mass rotates up to 180°, causing the animal’s anus and mantle cavity to be positioned above its head.

Ammonites

A member of a group of shelled cephalopods that were important marine predators for hundreds of millions of years until their extinction at the end of the Cretaceous period (65.5 million years ago).

Cuticle

(1) A waxy covering on the surface of stems and leaves that prevents desiccation in terrestrial plants. (2) The exoskeleton of an arthropod, consisting of layers of protein and chitin that are variously modified for different functions. (3) A tough coat that covers the body of a nematode.

Molting

A process in ecdysozoans in which the exoskeleton is shed at intervals, allowing growth by the production of a larger exoskeleton.

Arthropods

A segmented ecdysozoan with a hard exoskeleton and jointed appendages. Familiar examples include insects, spiders, millipedes, and crabs.

Open circulatory system

A circulatory system in which fluid called hemolymph bathes the tissues and organs directly and there is no distinction between the circulating fluid and the interstitial fluid.

Chelicerates

An arthropod that has chelicerae and a body divided into a cephalothorax and an abdomen. Living one include sea spiders, horseshoe crabs, scorpions, ticks, and spiders.

Myriapods

A terrestrial arthropod with many body segments and one or two pairs of legs per segment. Millipedes and centipedes are the two major groups of living myriapods.

Hexapods

An insect or closely related wingless, six-legged arthropod.

Crustaceans

A member of a subphylum of mostly aquatic arthropods that includes lobsters, crayfishes, crabs, shrimps, and barnacles.

Chelicerae

One of a pair of clawlike feeding appendages characteristic of chelicerates.

Eurypterids

An extinct carnivorous chelicerate; also called a water scorpion.

Arachnids

A member of a major arthropod group, the chelicerates. Arachnids include spiders, scorpions, ticks, and mites.

Book lungs

An organ of gas exchange in spiders, consisting of stacked plates contained in an internal chamber.

Mandibles

One of a pair of jaw-like feeding appendages found in myriapods, hexapods, and crustaceans.

Incomplete metamorphosis

A type of development in certain insects, such as grasshoppers, in which the young (called nymphs) resemble adults but are smaller and have different body proportions. The nymph goes through a series of molts, each time looking more like an adult, until it reaches full size.

Complete metamorphosis

The transformation of a larva into an adult that looks very different, and often functions very differently in its environment, than the larva.

Isopods

A member of one of the largest groups of crustaceans, which includes terrestrial, freshwater, and marine species. Among the terrestrial ones are the pill bugs, or wood lice.

Decapods

A member of the group of crustaceans that includes lobsters, crayfishes, crabs, and shrimps.

Copepods

Any of a group of small crustaceans that are important members of marine and freshwater plankton communities.

Echinoderms

A slow-moving or sessile marine deuterostome with a water vascular system and, in larvae, bilateral symmetry. They include sea stars, brittle stars, sea urchins, feather stars, and sea cucumbers.

Water vascular system

A network of hydraulic canals unique to echinoderms that branches into extensions called tube feet, which function in locomotion and feeding.

Tube feet

One of numerous extensions of an echinoderm’s water vascular system. Tube feet function in locomotion and feeding.

Phylum Porifera (Metazoa)

Sponges. Lack true tissues; have choanocytes (collar cells—flagellated cells that ingest bacteria and tiny food particles)

Phylum Cnidaria (Metazoa --> Eumetazoa)

Hydras, jellies, sea anemones, corals. Unique stinging structures (nematocysts) housed in specialized cells (cnidocytes); diploblastic; radially symmetrical; gastrovascular cavity (digestive compartment with a single opening)

Phylum Platyhelminthes (Metazoa --> Eumetazoa --> Bilateria --> Lophotrochozoa)

Flatworms. Dorsoventrally flattened, unsegmented acoelomates; gastrovascular cavity or no digestive tract

Phylum Rotifera (Metazoa --> Eumetazoa --> Bilateria --> Lophotrochozoa)

Rotifers. Pseudocoelomates with alimentary canal (digestive tube with mouth and anus); jaws (trophi) in pharynx; head with ciliated crown

Phylum Lophophorates (Metazoa --> Eumetazoa --> Bilateria --> Lophotrochozoa)

Ectoprocta, Brachiopoda. Coelomates with lophophores (feeding structures bearing ciliated tentacles)

Phylum Mollusca (Metazoa --> Eumetazoa --> Bilateria --> Lophotrochozoa)

Clams, snails, squids. Coelomates with three main body parts (muscular foot, visceral mass, mantle); coelom reduced; most have hard shell made of calcium carbonate.

Phylum Annelida (Metazoa --> Eumetazoa --> Bilateria --> Lophotrochozoa)

Segmented worms. Coelomates with segmented body wall and internal organs (except digestive tract, which is unsegmented)

Phylum Nematoda (Metazoa --> Eumetazoa --> Bilateria --> Ecdysozoa)

Roundworms. Cylindrical, unsegmented pseudocoelomates with tapered ends; no circulatory system; undergo ecdysis

Phylum Athropoda (Metazoa --> Eumetazoa --> Bilateria --> Ecdysozoa)

Crustaceans, insects, spiders. Coelomates with segmented body, jointed appendages, and exoskeleton made of protein and chitin

Phylum Echinodermata (Metazoa --> Eumetazoa --> Bilateria --> Deuterostomia)

Sea stars, sea urchins. Coelomates with bilaterally symmetrical larvae and five-part body organization as adults; unique water vascular system; endoskeleton

Phylum Chordata (Metazoa --> Eumetazoa --> Bilateria --> Deuterostomia)

Lancelets, tunicates, vertebrates. Coelomates with notochord; dorsal, hollow nerve cord; pharyngeal slits; post-anal tail.