The trunk has from thirty to thirty-two longitudinal furrows, the elevations between which correspond with a similar number of muscles lying in the skin. This longitudinal marking is crossed at right angles by a circular marking of similar origin, the elevations of which correspond with the circular muscles in the skin. These two sets of markings thus divide the skin of the trunk into a number of small square areas, very regularly arranged (Fig. 212).
The outline of the trunk is more or less uniform, but it is capable of considerable change according to the state of contraction of its muscles. The circular muscles, for instance, may be contracted at one level, thus causing a constriction at this spot. The colour of S. nudus is a somewhat glistening greyish-white.
Fig. 211.—Right half of the anterior end of Sipunculus nudus L., seen from the inner side and magnified. a, Funnel-shaped grooved tentacular crown leading to the mouth; b, oesophagus; c, strands breaking up the cavity of the tentacular crown into vascular spaces; c', heart; d, brain; e, ventral, and e', dorsal retractor muscles; f, ventral nerve-cord; G, vascular spaces in tentacular crown.
The anterior end of the fully-expanded Sipunculus may be termed the head; here the skin is produced into a frayed fringe which stands up in the shape of a funnel round the mouth. This fringe is grooved on its internal surface with numerous little gutters, all of them lined with cilia, which by their constant motion keep up a current which sweeps food into the mouth. The fringe may be in the form of a simple ring round the mouth, or the ring may be folded in at the dorsal side so as to take the form of a double horse-shoe (Figs. 211 and 212).
Body-wall.—The glistening appearance of Sipunculus is due to the cuticle, a chitinoid layer which is secreted by the external layer of cells, the epidermis. Beneath this lies a layer of connective tissue, which is not always present in other Gephyrea; within this lies a layer of circular muscles arranged in bundles, then comes a very thin sheath of oblique muscular fibres, then a thicker layer of longitudinal muscles, and finally a layer of peritoneal epithelial cells, which in Sipunculus are for the most part ciliated.
Scattered over the surface of the body, and opening by narrow tubes which pierce the cuticle, are a number of glandular bodies which may be either bi- or multi-cellular. The glandular cells are apparently enlarged and modified epidermal cells; they are arranged in a cup-shaped manner, with their apices directed towards the orifice. They are crowded with granules, which are presumably poured out over the cuticle, but the exact function of the secretion is entirely unknown. They have a well-developed nerve supply.
Digestive System.—The mouth lies in the centre of the fringe, and is not provided with any kind of jaw or biting armature; it leads directly into the thin-walled alimentary canal, the first part of which is ciliated. The alimentary canal is not marked out into definite regions, but passes as a thin-walled semi-transparent tube to the posterior end of the body, and then turns forward again and opens to the exterior by an anus situated about an inch below the junction of the introvert with the trunk, on the median dorsal line. The descending and ascending limbs of the alimentary canal are coiled together in a spiral, which may be more or less close in different individuals. The whole is supported by numerous fine muscular strands, which pass from the walls of the intestine to the skin, and by a spindle-muscle, which runs from the extreme posterior end of the trunk up the axis of the spiral and terminates in the skin close to the anus.
No glands open into the alimentary canal at any point of its course, but near the anus a simple diverticulum, or pocket, of unknown function arises. The size of this outgrowth differs enormously in different individuals. The alimentary canal near the anus also bears two tuft-like organs, which, however, do not open into the intestine, but probably have some function in connexion with the fluid in the body-cavity.
Along the whole course of the alimentary canal there runs a ciliated groove, into which the food does not pass, but the cilia of which probably keep in motion a current of water whose function may be respiratory.