9. Petalostoma
V. No tentacles, no vascular system. One retractor, and one segmental organ.
A. Introvert long. Body small, pear-shaped.
10. Onchnesoma
B. No introvert (?). Body cylindrical, thickly covered with papillae, which are larger and more crowded at both ends of the trunk.
11. Tylosoma
Species of Sipunculoidea.—The genus Phymosoma (Fig. 214) contains more species than any other genus of Sipunculoidea, and they are all of fair size. Twenty-seven species are known, of which seventeen occur in the Malay Archipelago, thirteen being found there alone. Phymosoma affects shallow water, the deepest specimens being taken at a depth of about 50 fathoms; this may be due to the fact that they flourish only in comparatively warm water. With very few exceptions, they are found only in tropical seas, very often living in tubular excavations made in soft coral rock.
The genus Sipunculus contains sixteen species. They are the largest and the most conspicuous members of the group. They have a very wide distribution, some species, as S. nudus (Fig. 212) and S. australis, being almost cosmopolitan. They are most common in temperate and tropical seas, but S. norvegicus and S. priapuloides are found far north, but always at considerable depths, 100 to 200 fathoms.
The following account of the habits of Sipunculus gouldii is taken from Mr. Andrews'[[482]] paper on that species:—
"This Sipunculus is very abundant in certain small areas of compact, fine sand darkened by organic matter and not laid bare at ordinary low tide. In such places, only a few square metres in extent, they pierce the sand in all directions to a depth of more than half a metre, making burrows with persistent lumen running from the surface downward and then laterally, but with no regularity in direction.