Abdomen
The region of the body that contains much of the digestive tract and sometimes part of the reproductive system; in insects, the region behind the thorax.
Acoelomate
A body plan of bilaterally symmetrical animals that lack a body cavity between the gut and the body wall.
Anterior
Indicating the head end of an animal.
Archenteron
The central endoderm-lined cavity of an embryo at the gastrula stage, which forms the primitive gut.
bilateral symmetry
The body plan of animals in which the body can be divided into mirror image right and left halves by a plane passing through the midline of the body.
Bilateria
A branch of the Eumetazoa, comprising animals with bilateral body symmetry.
Blastopore
The opening at one end of the archenteron in the gastrula that gives rise to the mouth in protostomes and the anus in deuterostomes.
Choanocyte
One of the inner layer of flagellated cells lining the body cavity of a sponge.
circulatory system
An organ system consisting of a fluid, a heart, and vessels for moving important molecules, and often cells, from one tissue to another.
Cleavage
Mitotic cell divisions of the zygote that produce a blastula from a fertilized ovum.
Cnidocyte
A prey-capturing and defensive cell in the epidermis of cnidarians.
compound eye
The eye of most insects and some crustaceans, composed of many faceted, lightsensitive units called ommatidia fitted closely together, each having its own refractive system and each forming a portion of an image.
determinate cleavage
A type of cleavage in protosomes in which each cell's developmental path is determined as the cell is produced.
Deuterostome
A group of animals in which cleavage usually follows a radial pattern, mesoderm arises from cells in the roof of the archenteron, the coelom forms in a space pinched off by the developing mesoderm, and the blastopore forms the anus.
digestive tract
Part of an extracellular digestive system; it is tubelike with two openings that form a separate mouth and anus. Digestive contents move in one direction through specialized regions of the tube, from the mouth to the anus.
Dorsal
Indicating the back side of an animal.
ecdysis
Shedding of the cuticle, exoskeleton, or skin; molting.
Ecdysozoa
A monophyletic group of protostomes that periodically shed their exoskeleton or cuticle.
Ectoderm
The outermost of the three primary germ layers of an embryo, which develops into epidermis and nervous tissue.
Endoderm
The innermost of the three primary germ layers of an embryo, which develops into the gastrointestinal tract and, in some animals, the respiratory organs.
Enterocoelom
In deuterostomes, the body cavity pinched off by outpocketings of the archenteron.
Epidermis
A complex tissue that covers an organism's body in a single, continuous layer or sometimes in multiple layers of tightly packed cells.
Eumetazoa
A subgroup of Animalia, comprising species that have tissues.
Exoskeleton
A hard external covering of an animal's body that blocks the passage of water and provides support and protection.
Foot
The muscular structure in the bodies of many mollusks that is responsible for crawling or burrowing locomotion.
gastrovascular cavity
A saclike body cavity with a single opening, a mouth, which serves both digestive and circulatory functions.
Head
The anteriormost part of an organism's body, containing the brain, sensory structures, and feeding apparatus. For a bacteriophage, the usually polyhedral part of the virus containing the genetic material.
Hemolymph
The circulatory fluid of invertebrates with open circulatory systems, including mollusks and arthropods.
hydrostatic skeleton
A structure consisting of muscles and fluid that, by themselves, provide support for the animal or part of the animal; no rigid support, such as a bone, is involved.
indeterminate cleavage
A type of cleavage, observed in many deuterostomes, in which the developmental fates of the first few cells produced by mitosis are not determined as soon as cells are produced.
Larva
A sexually immature stage in the life cycle of many animals that is morphologically distinct from the adult.
Lophophore
The circular or U-shaped fold with one or two rows of hollow, ciliated tentacles that surrounds the mouth of brachiopods, bryozoans, and phoronids and is used to gather food.
Lophotrochozoa
A lineage of protostomes that includes phyla that do not shed their exoskeleton or cuticle.
Mesoderm
The middle layer of the three primary germ layers of an animal embryo, from which the muscular, skeletal, vascular, and connective tissues develop.
Mesophyl
The gelatinous middle layer of cells lining the body cavity of a sponge.
Metamorphosis
A reorganization of the form of certain animals during postembryonic development.
Oscula
One or more openings in a sponge through which water is expelled.
Parapodia
Fleshy lateral extensions of the body wall of aquatic annelids, used for locomotion and gas exchange.
Parazoa
A subgroup of Animalia, comprising species that lack tissues.
Pinacoderm
In sponges, an unstratified outer layer of cells.
Polyp
The tentacled, usually sessile stage in the life cycle of a cnidarian.
Protostomes
A lineage of animals, in many of which cleavage follows a spiral pattern; mesoderm arises from cells near the blastopore, the coelom arises from a split in mesoderm tissue, and the blastopore forms the mouth.
Pseudocoelomate
A fluid- or organ-filled body cavity between the gut (a derivative of endoderm) and the muscles of the body wall (a derivative of mesoderm).
Pupa
The nonfeeding stage between the larva and adult in the complete metamorphosis of some insects, during which the larval tissues are completely reorganized within a protective cocoon or hardened case.
radial symmetry
A body plan of organisms in which structures are arranged regularly around a central axis, like spokes radiating out from the center of a wheel.
Radiata
A branch of the Eumetazoa comprising animals with radial body symmetry.
Radula
The tooth-lined "tongue" of mollusks that scrapes food into small particles or drills through the shells of prey.
Segmentation
The production of body parts and some organ systems in repeating units.
Sessile
Unable to move from one place to another.
Setae
A chitin-reinforced bristle that protrudes outward from the body wall in some annelid worms.
spiral cleavage
The cleavage pattern in many protostomes in which newly produced cells lie in the space between the two cells immediately below them.
Spongocoel
The central cavity in a sponge.
Thorax
The central part of an animal's body, between the head and the abdomen.
Tissue
A group of cells and intercellular substances with the same structure that function as a unit to carry out one or more specialized tasks.
triploblastic
An animal body plan in which adult structures arise from three primary germ layers, endoderm, mesoderm, and ectoderm.
Trochophore larva
The small, free-swimming, ciliated aquatic larva of various invertebrates, including certain mollusks and annelids.
Ventral
Indicating the lower or "belly" side of an animal.
visceral mass
In mollusks, the region of the body containing the internal organs.