SIPUNCULOIDEA, marine animals of uncertain affinities formerly regarded as a subdivision of the class Gephyrea (q.v.). Some authorities have linked them with the Phoronidea (q.v.) as the Podaxonia, but they are here treated as autonomous. A number of fossil forms occur in the Middle Cambrian of British Columbia.
The body is fusiform with a glisten ing cuticle, and may attain a length of about 12 inches. The body-wall comprises a thick cuticle, an epidermis containing glandular bodies, a thin sheet of connective tissue, several layers of muscle, and finally a coelomic epithelium. The anterior part, the proboscis or introvert, is capable of being retracted into the remainder as the tip of a glove-finger may be pushed into the rest : at the top of the introvert is the mouth, surrounded in Sipunculus by a laciniated funnel, but more usually by tentacles each with its ciliated groove creating a current in the direction of the mouth. The mouth leads into the oesophagus surrounded by the retractor muscles of the introvert, which arise in the body-wall about one third of the body-length from the anterior end. The alimentary canal is a thin-walled tube, not marked off into definite regions, which runs to the posterior end of the body and then turns for ward to end in the dorsal anus near the insertion of the retractor muscles. The descending and ascending limbs of the gut are twisted into a spiral coil, the axis of which is often traversed by a muscle arising from the posterior end of the body. Along most of the gut runs a ciliated groove, into which food does not enter. A diverticulum of variable size opens into the rectum. On the dorsal surface of the oesophagus lies a contractile body, the Polian vesicle, filled with corpusculated fluid and closed pos teriorly; it opens in front into a circumoesophageal ring, giving off branches to the tentacles, which by its contraction are caused to expand. The excretory organs consist usually of two sac cular brown tubes attached by one side to the ventral wall of the body and opening to the exterior by a pore. The ventral lip of the internal ciliated funnel is a portion of the coelomic epithel ium; the funnel leads by a canal into the nephridial sac. These organs serve both for excretion and for the passage of the genital products : the former function is also served by certain cellular bodies, the urns, which may be free in the coelom or attached to the outer wall of the gut. The essential part of these organs is a
ciliary mechanism by which the waste products in the coelomic fluid are agglutinated and brought into contact with the pha gocytes, and so eliminated. J. Cantacuzene has shown that the urns are important in the production of immunity. The Sipuncu loids are dioecious, and the germ cells arise from the coelomic epithelium near the insertion of the retractor muscles; they drop into the coelom at an early stage and there develop. The coelom is rich in corpuscles and contains haematids carrying a respiratory pigment, haemerythrin, rich in iron. There is a well developed dorsal brain, usually with two pigmented eye-spots at the bottom of an epidermal invagination passing deeply into it ; in some genera this cerebral tube is divided into two : the brain is also connected with the heart-shaped nuchal organ, to the lat eral ciliated lobes of which it gives off two large nerves. A nerve ring joins the brain to the ventral nerve-cord, in which there are no distinct ganglia but a series of ganglion cells. The nerve-cord gives off numerous nerves to the periphery.
The Sipunculoids are divided into a number of genera separated on the arrangement and number of the longi tudinal muscles and tentacles, the presence of calcareous bodies on the posterior end, etc.
Segmentation is spiral. The larva is a trocho phore but with no protonephridia and flame-cells. The develop ment is characterized by the enormous elongation of the post anal region which brings about the dorsal and anterior situation of the anus.
Some Sipunculoids live in mud and sand and feed on the organic matter contained in them (Sipunculus) ; others (Phascolosoma, Physcosoma) occupy crevices and rock-crannies; others again (Phascolion, Aspidosiphon) inhabit the shells and tubes formed by other small marine animals. The two latter groups feed on detritus, etc., which are conveyed to their mouths by means of the cilia on their tentacles. Only the mud and sand dwellers are capable of free movement.
BIBLIOGRAPHY.-The best account of the group as a whole is that by J. W. Spengel, 1913, in the Handworterbuch der Naturwissen schaften, Jena, vol. ix., pp. 97-106, 14 figs. with a bibliography.
See also L. Cuenot, 1922, "Sipunculiens," Faune de France, iv., pp. 1-17, figs. and bibl. (C. C. A. M.)