CHARACTERISTIC DEEP-SEA TYPES. HOLOTHURIANS. 85 



Owing to the absence of fossil holothurians we are unable, as 

 in the case of other echinoderms, to trace the groups from which 

 this peculiar deep-sea order of Elasipoda has been derived. 

 While during earlier geological periods the holothurians un- 

 doubtedly made their way by gradual migration from the shore 

 into deep water, their shallow-water progenitors have left us no 



Fig. 338. Psolus tuberculosus. f . (Th<$el.) 



trace of their existence. The whole tribe of Elasipoda, which 

 stands out apparently isolated from the other orders of holo- 



Fig. 339. Echinocucumis typica. f. (Thel.) 



thurians, is found associated with such 

 genera as Psolus (Fig. 338), Echinocu- 

 cumis (Fig. 339), StichopUS (Fig. 340), Fig. 340. Stichopus natans. J. 



Trochostoma l (Fig. 341), and Caudina, < Koren & Danielssen -> 



all of which have representatives in deep water, and some even 



in very deep water. 



1 Trochostoma arcticum is of a greenish violet color; the tentacles are much lighter; 

 and the skin is comparatively tough. 



