ECHINODERMATA. 79 



Class IV. Holothuroidea. Sea-cucumbers. 



Elongated forms with tough muscular body- wall containing calcareous 

 bodies, and with a circlet of retractile oral tentacles. Gullet sur- 

 rounded by a ring of calcareous ossicles. The madreporic plate external 

 in the larvae and also in many adult Elasipoda, but directed into 

 the body-cavity in other adults. A blood-vascular system. Larva 

 (Auricularia) most like that of Stellerida. 



Order 1. Pedata, with respiratory trees, and external ambulacral ap- 

 pendages (tube-feet, papillae), which are arranged in rows or scattered. 

 Sexes separate. 



Dendrochirotae, arborescent tentacles ; five retractors to the 

 pharynx ; generative caeca in two bundles. Cucumaria Planci, ten 

 tentacles. 



Aspidochirotse, 20-30 shield-shaped tentacles, by means of which 

 the food is shovelled into the mouth ; only one bundle of generative cfceca. 

 Holothuria tubulosa, 20 tentacles; Miilleria, 25 tentacles, five cir- 

 cumanal plates. 



Order 2. Elasipoda, all primitive deep-sea forms, with reduced or 

 rudimentary calcareous ring ; without respiratory trees or ciliated funnels ; 

 stone-canal often opening to the exterior. Sexes separate. Move by 

 creeping, and the dorsal and ventral surfaces sharply marked off in 

 consequence ; tube-feet limited to the latter. Deima, the calcareous 

 ring a brittle network, long stiff papillae on the back. 



Order 3. Apoda, without tube-feet or papillae. 



Pneumonophora, with respiratory trees. Molpodia. 



Apneumona, ciliated funnels in the mesentery instead of respiratory 

 trees ; mostly hermaphrodite. Chirodota, with calcareous wheels in the 

 skin. Synapta digitata, calcareous ossicles in the form of plates and 

 anchors. 



