4d cell—Mesentoblast; a blastomere cell that results from zygotes that have spiral cleavage divisions, and contains an unidentified cytoplasmic factor that causes the cell and its progeny to form mesoderm.

Abdomen—The posterior of the main body divisions.

Abyssal—Of, or relating to the deepest regions of the ocean.

Acanthor—First larval stage of acanthocephalans.

Aciculum—Small needlelike structure resembling a rod that supports the divisions of the parapodium.

Acoelomate—An organism, particularly an invertebrate, lacking a coelom that is characterized by bilateral symmetry.

Actinotrocha—Tentacle-like ciliated larva of phoronids.

Aestivation—A period of dormancy that is entered into when conditions are not favorable, particularly during very warm or very dry seasons.

Aflagellate—An organism that lacks a flagella.

Agamete—Nucleus within the plasmodium that divides mi-totically and gives rise to a sexual adult.

Ametabolous—Development in which little or no external metamorphic changes are noticeable in the larval to adult transition.

Anal—Relating to or being close to the anus.

Anamorphic—Development in which only part of the adult segments are present in recently hatched young.

Ancestrula—Zooid that develops from an egg.

Anecic—An earthworm known for burying leaf litter in the soil and pulling it into underground burrows for consumption.

Antibiosis—A provocative association between organisms that is detrimental, inhibitive, and preventative to one or more of them but produces a metabolic product in another.

Aphotic zone—Region of the ocean where no sun light reaches and exists in complete darkness.

Apical field—An area inside the circumapical band of rotifers that is devoid of cilia.

Arboreal—An organism that lives in, on, or among trees.

Ascidiologists—Scientists who study the Ascidiacea.

Auricularia—Primary larval stage in holothuroid development.

Benthic—An organism that lives on the bottom of the ocean floor.

Bipinnaria—Free-swimming larval stage of asteroids.

Biramous—Having two branches, such as the two appendages in crustaceans.

Bivoltine—The production of two broods or generations in a season or year.

Blastomere—Zygote cleavage divisions that result in a cell.

Blastopore—The first opening of the early digestive tract.

Blastula—Spere of blastomeres.

Brachiolaria—Second stage of asteroid larva.

Brood—When the care of eggs takes place outside or inside of the mother's body for at least the early part of development.

Buccal cavity—A cavity that is present within the mouth.

Bud—The development of new progeny cells or new outgrowth.

Caudal—Referring or pertaining to the posterior end of the body.

Cephalic—Referring or pertaining to the anterior end of the body.

Cephalothorax—The body region that consists of the head and thoracic segments.

Chelicera—Pair of appendages present in the anterior body of arachnids.

Chorion—The shell or covering of an egg.

Cilia—Outgrowth present on the cell surface that is short and produces a lashing movement capable of creating locomotion.

Cloaca—Chamber into which the intestinal and urogenital tracts discharge.

Coelom—The epithelium-lined space between the body wall and the digestive tract.

Colony—Body composed of zooids that share resources.

Commensalism—Symbiotic relationship between two or more species in which no group is injured, and at least one group benefits.

Commercial fishery—The industry of catching a certain species for sale.

Communal—Cooperation between females of one species in production and building, but not in caring for the brood.

Conspecific—Belonging to the same species.

Coracidium—Ciliated free-swimming stage of cestode.

Cosmopolitan—Occurring throughout most of the world.

Cuticle—The noncellular outer layers of the body.

Cydippid—Free-swimming ctenophore larva.

Cyphonarutes—Planktonic larva of some nonbrooding gymnolaemate bryozoans.

Definitive host—See Primary host.

Demersal—Aquatic animals that live near, are deposited on, or sink to the bottom of the sea.

Dentate—Having teeth, or structures that function, or as derived from, as teeth.

Denticles—Teeth, or structures that function as teeth.

Deposit feeders—Animals that feed upon matter that has settled on the substrate.

Detritus—Fragments of plant, animal, or waste remnants.

Deuterostome—Division of the animal kingdom that includes animals that are bilaterally symmetrical, have indeterminate cleavage and a mouth that does not arise from the blastopore.

Diapause—A period of time in which development is suspended or arrested and the body is dormant.

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