The enormous amount of sand and mud which passes through the bodies of the Sipunculids shows that they must take a considerable part in modifying the mineral substances which form the bottom of the sea. Just as earthworms, as shown by Darwin, play a considerable rôle in the formation of soil, so must these animals, in conjunction with Echinids and Holothurians, effect considerable modifications in the sand and mud which pass through their bodies. Mr. J. Y. Buchanan[[480]] is "led to believe that the principal agent in the comminution of the mineral matter found at the bottom of both deep and shallow seas and oceans, is the ground fauna of the sea, which depends for its subsistence on the organic matter which it can extract from the mud." The minerals at the bottom of the sea are exposed to a reducing process in passing through the bodies of the animals which eat them, and subsequently to an oxidising process due to the oxygen dissolved in the sea-water acting on the minerals extruded from the animals' bodies.
The rate at which the sand passes through the body of Sipunculus is unfortunately unknown, but that at any one moment a considerable quantity is contained in the intestine is shown by the fact that the average weight of five specimens of S. nudus from Naples, taken at random, was 19.08 grms., whilst the average weight of sand washed out of their alimentary canal was 10.03 grms. The sand contained in five other specimens of the same species measured respectively 6 c.c., 7 c.c., 6.5 c.c., 7.5 c.c., and 7.5 c.c., giving an average of 6.9 c.c. for each individual.
Onchnesoma and Tylosoma have only one retractor muscle; Aspidosiphon and Phascolion have, as a rule, two; Phymosoma and Sipunculus have four, and perhaps this is the more usual number.
Phascolion, Tylosoma, and Onchnesoma have but one "brown tube"; in Phascolion this is the right, in Onchnesoma it is sometimes the right and sometimes the left that persists. Most other genera retain two, but there are many exceptions; for instance, Phascolosoma squamatum has but one, and so has Aspidosiphon tortus, and in both cases it is that of the left side. No Sipunculid has more than two. It has been pointed out by Selenka that those species which have but one brown tube are, as a rule, inhabitants of tubes or shells, and do not move actively about in the sand.
The eggs of all members of the family, with the exception of the genus Phymosoma, are spherical, but those of the last-named genus are elliptical. They are always surrounded by a thick membrane, the "zona radiata," pierced by numerous pores.
Aspidosiphon (Fig. 215) is easily recognised by the presence of two symmetrically-arranged cuticular shields, one at each end of the trunk. These are formed by the fusion of minute cuticular plates, such as exist in the skin of most Sipunculids. The posterior shield is radially symmetrical, but the anterior is somewhat like the shell of a Pecten, and symmetrical only about one plane. The introvert is protruded from the acute angle of the anterior shield, and when extended lies almost at right angles to the trunk, instead of being, as is usually the case, in the same straight line with it. In many specimens, and these seem as a rule to be the older ones, a deposit of calcium carbonate takes place over these shields, covering over and concealing their external markings.
Cloeosiphon (Echinosiphon) has a calcareous ring, consisting of four or five rows of lozenge-shaped calcareous bodies forming a close mosaic, arranged round the base of the introvert, which when extended is in the same straight line as the trunk. Each piece bears a brown spot, which is said to be the pore of a gland (Fig. 217). Golfingia Lankester, has a cylindrical horny thickening at the anterior end of the trunk and another at the posterior.
Fig. 215.—Aspidosiphon truncatus Kef. × 2. a, Introvert partially extended, but not sufficiently to show the head.
Key to the Genera of Sipunculoidea.[[481]]