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. 388. — Psolus tuberculosus. 2. (Théel.) 
trace of their existence. The whole tribe of Elasipoda, which 
stands out apparently isolated from the other orders of holo- 
Fig. 339. — Echinocucumis typica. 2. (Theel.) 
thurians, is found associated with such 
genera as Psolus (Fig. 338), Echinocu- va 
cumis (Fig. 339), Stichopus (Fig. 340), Fig. 340. — Stichopus natans. $ 
Trochostoma' (Fig. 341), and Caudina, E eco pm. 
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. 
