Psolus is a most extraordinary genus. There is a well-marked sole, to which the tube-feet are confined, whilst the dorsal radial canals, and consequently all the dorsal tube-feet, are absent. The dorsal ossicles are enlarged to form a complete mail of plates, recalling the corona of a Sea-urchin. The two British species are small, and found in comparatively deep water, but a fine large species is found in the Gulf of St. Lawrence, and extends into brackish water up the estuary. The species figured (P. ephippifer) is an Antarctic one, which carries the eggs until development is complete in a dorsal brood-pouch.

Order V. Molpadiida.

Holothuroidea with simple, finger-shaped feelers, provided with ampullae; retractor muscles occasionally present; respiratory trees present. Besides the feelers, the only podia are five minute papillae terminating the radial canals in the neighbourhood of the anus.

Fig. 262.—Trochostoma violaceum. × 1. m, Mouth.

This order includes six genera and about thirty species. Its peculiarities seem to be due to the fact that its members are burrowers, leading a life like an earthworm. Hence the absence of the tube-feet, and the small, almost vestigial character of the feelers. Trochostoma (Fig. 262) and Caudina are remarkable for the presence of a tail. This appendage is in reality only the narrow posterior end of the body, and is especially long in Caudina; and observations on a species found off the coast of Maine, U.S.A.,[[511]] have shown that the tail, like the siphon of a Mollusc, projects up from the burrow to the surface in order to maintain the respiratory current of water.

Order VI. Synaptida.

Holothuroidea with short bipinnate (i.e. feather-shaped) feelers, provided with only vestigial ampullae, and with well-developed retractor muscles. No other podia; radial canals absent in the adult. Respiratory trees absent, and transverse muscles of adjacent interradii continuous, so as to form circular muscles. Otocysts attached to the nerve-ring as in Elasipoda.

Fig. 263.—Synapta digitata. A, animal viewed from the side, × 7⁄13; B, anterior end, with tentacles extended, × 2.