572 ECIIINODERMATA — HOLOTHUROIDEA chap. 



the soft ooze forming the ocean floor. The ooze forms their food, 

 and as their weight must to a certain extent immerse them 

 in it, we can understand why the stiff, long dorsal podia have 

 been specialised as respiratory organs, since there are no respiratory 

 trees. These respiratory podia sometimes undergo extraordinary 

 development ; thus in Peniagone several very long ones coliere 

 to form a huge vertical sail, whilst in Psychropotes one or two 

 cohere to form a hackwardly i)rojecting tail. On the other 

 hand, in Ilyodacmon (Fig. 258) the dorsal podia are numerous 

 and slender. 



Order III. Pelagothuriida. 



Holothuroidea with shield-shaped feelers provided with long 

 ampullae which project outwards, pushing the skin before them 

 so as to form external appendages, connected at the base by a 

 web. Calcifications absent. No retractor muscles. No respira- 

 tory trees. The external madreporite is retained, but all podia 

 other than the feelers have disappeared, although the radial 

 canals have been retained. 



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. 

 Eespiratory trees well developed. This order includes twelve genera 

 and over 180 species, and, like the Aspidochirota, is of world- 

 wide distribution. So ftir 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 branclied 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. 



