652 THE INVERTEBRATA 



The Holothuroidea are divided into six orders. Of these the 

 Aspidochirotae and Dendrochirotae contain between them the bulk 

 of the members of the class. 



Order ASPIDOCHIROTAE 

 Holothuroidea with shield-shaped tentacles; no retractor muscles, 

 but tentacle ampullae ; podia on the trunk ; the madreporite internal ; 

 and respiratory trees. 



Holothuria (Figs. 456, 457 B). 



Order PELAGOTHURIDA 

 Holothuroidea of pelagic habit ; with shield-shaped tentacles ; no re- 

 tractor muscles, but large tentacle ampullae which push out the body- 

 wall ; no podia on the trunk ; the madreporite external ; and no respira- 

 tory trees. 



Pelagothuria (Fig. 457 E). The only pelagic holothurian. The 

 animal swims by a webbed circle of projections caused by the en- 

 largement of the tentacle ampullae. 



Order ELASIPODA 

 Deep-sea, benthic Holothuroidea with shield-shaped tentacles; no 

 retractor muscles or tentacle ampullae; podia on the trunk; the 

 madreporite external or internal; and no respiratory trees. 

 Deima (Fig. 457 F, F'). 



Order DENDROCHIROTAE 

 Holothuroidea with dendritic tentacles; retractor muscles but no 

 tentacle ampullae ; podia on the trunk ; the madreporite internal ; and 

 respiratory trees. 



Cucumaria (Fig. 457 A). Body pentagonal, with two rows of tube 

 feet on each radius and usually no other podia except the tentacles. 

 British. 



Order MOLPADIDA 



Holothuroidea of burrowing habit; with slightly pinnate or un- 

 branched tentacles; tentacle ampullae, and sometimes also retractor 

 muscles; without podia on the trunk; with respiratory trees; and 

 with internal madreporite. 

 Trochostoma (Fig. 457 C). 



Order SYNAPTIDA (PARACTINOPODA) 

 Holothuroidea of burrowing habit; with pinnate tentacles whose 

 ampullae are vestigial; with retractor muscles; without radial water 

 vessels, or podia on the trunk; or respiratory trees; and with internal 

 madreporite. 



Synapta (Fig. 457 D). Ossicles anchor-shaped. British. 



