PRIAPULID

(phylum or class Priapulida), any of some 15 species of predatory, marine, mud-inhabiting, unsegmented worms of uncertain classification. Sometimes considered a class of the phylum Aschelminthes, priapulids are perhaps best considered to be a very small phylum with no clear relationship to any other. The largest of the priapulids are 10 to 15 cm (4 to 6 inches) long and inhabit the colder seas, while the smallest, several millimetres long, inhabit warmer seas. The presoma, or anterior end of the body, with the mouth at the tip, can be retracted into the trunk and is used in locomotion as well as in feeding. The body is covered with a cuticle that is secreted by the hypodermis. Beneath this cuticle lie body-wall muscles that enclose a spacious body cavity. The cuticle hardens into a ridged case (the lorica) during the larval stage. It forms spines on the presoma, especially around the mouth, within the pharynx, and to a lesser degree elsewhere on the body, and it molts as the worm grows to an adult. The mouth of the priapulid leads into a large muscular pharynx, a short esophagus, a larger intestine (with musculature), and a rectum with the anus at the hind end. There is no circulatory system. The excretory system consists of flame cells (solenocytes) opening by ducts to the exterior. The nervous system is very simple, consisting of a nerve ring surrounding the mouth, a ventral nerve cord, and peripheral nerves. The reproductive organs are tubular, with posterior openings, and internal fertilization is known to occur in one species. A number of fossil species that look much like modern forms are known from the Cambrian Period (540 to 505 million years ago). The evolutionary relationships and therefore the zoological classification rest on the interpretation of the epithelial lining of the body cavity. If it is an epithelium, as some zoologists hold, the priapulids are coelomates; if the nuclei belong instead to the musculature, as others claim, the priapulids are pseudocoelomates, possibly Aschelminthes.