The eggs break away from the ovary in a very undeveloped condition, but whilst floating about in the body-cavity they increase in size and secrete a thick membrane around them. They have a well-marked nucleus, and are oval in outline.

The mother-cells of the spermatozoa also break away in an immature condition, and complete their development in the nutritive fluid of the body-cavity. They divide into a number of spermatozoa, usually eight or sixteen, which remain in contact. They each develop a tail, which projects outwards, and aids the cluster in swimming along. These clusters of spermatozoa are about the same size as the ova of the female, and, like them, make their way into the "brown tubes." The exact way in which this is accomplished is not very clear, but the cilia on the funnel-shaped internal opening of the tube seem to have some power of selecting the generative cells when they come within their reach, and of passing them on, whilst they reject the much smaller corpuscles of the perivisceral fluid, which are never found in the nephridia.[[477]] Once inside the internal opening, the clusters break up and the spermatozoa escape singly into the sea. Here they meet with and fertilise the eggs which have escaped from the body of the female.

Fig. 213.—Larva of Sipunculus nudus L. × 150. (After Hatschek.) a, Mouth; b, anus; c, excretory organ; d, glandular appendage of oesophagus; e, wall of stomach over which the retractor muscle runs; f, invaginated sense-organ at aboral pole.

Development.—Hatschek,[[478]] who investigated the development of Sipunculus nudus at Pantano, an inlet of the sea near Messina, states that the spawning takes place during the night, and ceases about July 10. The rate of development depends upon the temperature, but the larvae usually free themselves from the egg-membrane during the third day. When hatched the embryos lengthen out a good deal, and take the form represented in Fig. 213. The larva swims actively by means of a ring of stout cilia, which encircle the body just behind the mouth. Other shorter cilia are found on the head, continuing into the lining of the mouth, and a little bunch of them is situated at the extreme posterior end. The alimentary canal is already formed, and is twisted, so that the anus lies dorsally, but not so far forward as it does in the adult. A glandular structure opens into the mouth, and another body of unknown function is connected with the oesophagus; both these disappear during larval life. A pair of excretory tubules, the forerunners of the brown tubes, are found, and the chief muscle tracts are already established. The nervous system is still in close connexion with the skin, from the outer part of which it is derived; the cerebral thickening bears two eye-spots.

The fluid of the body-cavity contains corpuscles, which are kept in active circulation by the constant contractions of the body-wall, and by numerous tufts of cilia which are borne on the inner surface of the skin. The dorsal blood-vessel is one of the latest organs to arise.

The larva swims actively about for a month, during which time it increases greatly in size; it then undergoes a somewhat sudden metamorphosis. The ciliated ring and the structures related to the oesophagus begin to disappear, the distinction between the head and the rest of the body is obliterated, and the head becomes relatively small. The mouth changes its position, and becomes terminal instead of being somewhat ventral, and the tentacular membrane begins to appear. At the same time the larva relinquishes its free-swimming life, and sinks to the bottom; it begins creeping amongst the sand by protruding and retracting the anterior part of its body, and takes on all the characters and habits of the adult.

I. Order Sipunculoidea.

Besides the genus Sipunculus, the Order Sipunculoidea includes ten other genera. A key to these, taken for the most part from Selenka's admirable monograph, is given on page 424.

Phascolosoma contains, in comparison with Sipunculus, only small species, and it is easily distinguished by the fact that the longitudinal muscles are fused into a continuous sheath. As a rule the skin is smooth. A few species bear hooks, which are generally scattered irregularly and not arranged in transverse rows, as in Phymosoma (Fig. 214) and most of the other genera.