Phylum Sipuncula

Phylum Sipuncula
Phylum Sipuncula (sigh-pun´kyu-la) (L. sipunculus, little siphon) consists of benthic marine worms, predominantly littoral or sublittoral. They live sedentary lives in burrows in mud or sand, occupy borrowed snail shells, or live in coral crevices or among vegetation. Some species construct their own rock burrows by chemical and perhaps mechanical means. More than half the species are restricted to tropical zones. Some are tiny, slender worms, but the majority range from 15 to 30 cm in length. Some are commonly known as “peanut worms” because, when disturbed, they can contract to a peanut shape (Figure 21-1).

Sipunculans have no segmentation or setae. They are most easily recognized by a slender retractile introvert, or proboscis, which is continually and rapidly being run in and out of the anterior end. Walls of the trunk are muscular. When the introvert is everted, the mouth can be seen at its tip surrounded by a crown of ciliated tentacles. Undisturbed sipunculans usually extend the anterior end from the burrow or hiding place and stretch out their tentacles to explore and feed. They are largely deposit feeders living on organic matter collected in mucus on the tentacles and moved to the mouth by ciliary action. The introvert is extended by hydrostatic pressure produced by contraction of the bodywall muscles against the coelomic fluid. The lumen of the hollow tentacles is not connected to the coelom but rather to one or two blind, tubular compensation sacs that lie along the esophagus (Figure 21-2). The sacs receive fluid from the tentacles when the introvert is retracted. Retraction is effected by special retractor muscles. The surface of the introvert is often rough because of surface spines, hooks, or papillae.

There is a large, fluid-filled coelom traversed by muscle and connective tissue fibers. The digestive tract is a long tube that doubles back on itself to end in the anus near the base of the introvert (Figure 21-2). A pair of large nephridia opens to the outside to expel waste-filled coelomic amebocytes; the nephridia also serve as gonoducts. Circulatory and respiratory systems are lacking, but the coelomic fluid contains red corpuscles that contain a respiratory pigment, hemerythrin, used in transportation of oxygen. The nervous system has a bilobed cerebral ganglion just behind the tentacles and a ventral nerve cord extending the length of the body. Sexes are separate. Permanent gonads are lacking, and ovaries or testes develop seasonally in the connective tissue covering the origins of one or more of the retractor muscles. Sex cells are released through the nephridia. The larval form is usually a trochophore. Asexual reproduction also occurs by transverse fission, the posterior onefifth of the parent constricting off to become a new individual.

There are approximately 330 species and 16 genera, which are placed by some authorities into four families. The best-known genera are probably Sipunculus, Phascolosoma (Gr. phaskolos, leather bag, pouch, + soma, body), Aspidosiphon (Gr. aspidos, shield, + siphon, siphon), and Golfingia (named by E. R. Lankester in honor of an afternoon of golfing at St. Andrews, Scotland).