This order contains one species, Pelagothuria natans, which is the only free-swimming Holothuroid known, the muscular web connecting the freely projecting ampullae being the organ of locomotion.
Order IV. Dendrochirota.
Holothuroidea with long repeatedly branched feelers terminating in fine pointed twigs. No feeler-ampullae; but retractor muscles are present, which can introvert the anterior end of the body. Respiratory trees well developed. This order includes twelve genera and over 180 species, and, like the Aspidochirota, is of worldwide distribution. So far as can be safely generalised from the few species whose habits have been closely observed, it seems that this order is adapted to catch swimming prey—it is an order of fishers. The long branched tentacles are extended like the lines of an angler. Their surface is coated with adhesive slime, and before long becomes covered with small organisms which have come in contact with it. When a feeler has captured in this way a large enough haul, it is turned round and pushed into the mouth, which is closed on it. It is then forcibly pulled out, during which process the prey is, so to speak, stripped off it. Four genera (Cucumaria, Thyone, Phyllophorus, and Psolus) and sixteen species have been recorded from British waters.
Fig. 259.—Cucumaria crocea, carrying its young, × 1. (From Wyville Thomson.)
Cucumaria is remarkable for being the only genus of Holothuroidea in which the body is pentagonal in cross-section. In the majority of its species the tube-feet are confined to two rows along each radius, but in a few there are some scattered tube-feet in addition. There are only ten buccal tentacles. The species figured (C. crocea) is an Antarctic one which carries the young on the back. Thyone differs in being circular in cross-section and in having the tube-feet scattered evenly over the whole surface. In Phyllophorus (Fig. 260) the tentacles are more than fifteen, and are disposed in two circles, an inner of smaller and an outer of larger tentacles. The other podia are, as in Thyone, scattered.
Fig. 260.—Phyllophorus urna. × 1.
Fig. 261.—Psolus ephippifer. × 3. A, with feelers retracted and brood-pouch closed; B, with feelers extended and some of the plates of the brood-pouch removed. (From Wyville Thomson.)