The skin is folded in a series of rings, and the body is usually somewhat swollen posteriorly. P. caudatus bears a curious caudal appendage, beset with a number of hollow lobes somewhat grape-like in appearance. This is situated ventral to the anus; its lumen is continuous with that of the body-cavity, but it can be separated from it by the action of a sphincter muscle. Two such appendages exist in P. bicaudatus.

There cannot be said to be any head in the Priapuloidea; they have no tentacles or tentacular fringe, no proboscis, and no distinct brain; simply a round aperture, the mouth, which is surrounded by a groove in the skin, at the bottom of which the circumoesophageal nerve-cord lies. The mouth leads into a very muscular pharynx lined with stout chitinous teeth; this passes into an intestine, which is as a rule straight, but in P. glandifer it has a single loop.

The Priapuloidea possess no vascular system and no brown tubes. Their skin has in the main the same structure as that of the Sipunculids, with spines, glandular bodies, and papillae with sensory hairs which resemble similar structures on Phymosoma varians. Retractor muscles arise from the longitudinal muscles of the skin, and are inserted into the pharynx; they are short and not constant in number.

The nervous system has retained throughout its primitive connexion with the epidermis. In almost all animals the nervous system is formed from the epiblast or outermost cellular layer of the embryo; it usually, however, breaks away from this and sinks into the body. Thus in Sipunculus it lies within the body-cavity, and has retained its primitive connexion with the outer layers of the skin only in the region of the brain; but in the Priapulids the nervous system, which consists of a ring round the mouth and of a ventral cord, lies embedded in the skin, and the nerve cells are directly continuous with the cells of the epidermis. The nerve-ring lies at the base of a groove in the skin, which forms a kind of gutter round the mouth; the ventral nerve-cord is visible exteriorly as a light line which marks the ventral surface of the animal. In no place is the ring or cord differentiated in any way, and there cannot be said to be any brain or special sense-organs. Numerous nerves are given off from the ring to the pharynx and intestine, and from the cord to the body-wall.

Fig. 218.—Priapulus caudatus Lam. Nat. size. a, Mouth surrounded by spines.

The sexes are distinct, but they differ from the other Gephyrea in the nature of their reproductive organs. In mature specimens the ovaries or testes are easily recognisable, lying to the right and left of the alimentary canal. The reproductive glands are continuous with ducts, which act as oviducts and vasa deferentia respectively. Both glands and ducts are attached to the body-wall by a mesentery.

The excretory function is performed in the Priapuloidea by the ducts of the generative organs. These are primarily connected with a number of branching canals of small size which project into the body-cavity. According to Schauinsland,[[485]] one or more pear-shaped cells are found at the end of each branch, and each is continued into a long cilium which hangs down into the lumen of the canal, and by its movement produces a flickering motion. Beyond the free end of the large cilium the canal is lined with ciliated cells. The remarkable resemblance this form of excretory organ presents to that of the Platyhelminthes (vide p. [25]) and of certain Chaetopods is worthy of attention. In the young Priapuloidea the duct with its branching canals is not masked by the generative organs, but as the animals become mature, diverticula from the duct arise, and the cells covering these become modified into ova in the female, and into spermatozoa in the male. The presence of these follicles masks the excretory part of the gland. The ova and spermatozoa escape through the ciliated ducts which open to the exterior one on each side of the anus, and, contrary to what is the case with other Gephyrea, leave the body without having ever been in the body-cavity.

Nothing is known of the embryology of either member of this family, but both genera appear to be sexually mature from the end of May until October.

Classification.—The two genera which make up the Order Priapuloidea are characterised as follows:—