aschelminths

aschelminths , large assemblage of loosely related, wormlike organisms of extremely varied structure and habits. Formerly considered an animal phylum, these organisms are now more commonly referred to as pseudocoelomates and are divided in to a number of different phyla, including Nematoda , Gastrotricha, and Rotifera . All are covered by a noncellular coat, or cuticle, and have a pseudocoelom, i.e., a fluid-filled cavity separating the body wall from the gut but lacking a peritoneal lining. In many species the digestive, excretory, and reproductive systems join in a cloaca, or discharge chamber, near the posterior end. Many aschelminths also show cell constancy, a condition in which each organ of the adult contains the precise number of cells characteristic of the species.

Aschelminthes The name sometimes given to a phylum of worm-like animals that are typified by possessing a pseudocoelom. True segmentation is not present. The intestine is straight with a highly developed pharynx and anus. Cell constancy is characteristic of the group: after the young has hatched (or been born) the number of cells in the body never increases although the size of the body usually does. There are six classes, some free-living and some parasites. The group contains several miscellaneous groups whose classification has always been controversial; the classes are now generally thought to be unrelated and are usually regarded as distinct phyla. See GASTROTRICHA; KINORHYNCHIA; NEMATODA; NEMATOMORPHA; PRIAPULIDA; ROTIFERA.

Cyclorhagida (phylum Aschelminthes, class Kinorhyncha) An order of worms in which the head is covered on retraction by platelets on the second segment of the body.