HOLOTHUROIDEA 575 



five interradial, which has been thought to represent the auriculae and 

 lantern of an echinoid. The oesophagus is followed by a short mus- 

 cular region known as the stomach, this is succeeded by a thin-walled 

 intestine which forms the greater part of the canal, and finally there is 

 a short, wide cloaca. Into the latter usually open two long, branched 

 respiratory trees, whose ramifications end in thin-walled ampullae 

 through which water, when pumped in by contractions of the cloaca, 

 passes into the body cavity, carrying oxygen to the coelomic fluid, and 

 so to the organs. In Holothuria and a number of other genera the lower 

 branches of the respiratory trees are converted into Cuvierian organs, 

 tubes covered with a sticky substance which in sea water elongate and 

 form a mass of sticky threads. When the animal is attacked or other- 

 wise irritated a violent contraction of the muscles of the body wall sets 

 up in the perivisceral cavity a pressure which ruptures the cloaca and 

 drives out the Cuvierian organs (and often subsequently the rest of the 

 alimentary canal). The enemy is entangled by the sticky threads. 

 Except in the Dendrochirotae, the food is extracted from sand or mud 

 which is shovelled into the mouth by the tentacles. Dendrochirotae 

 entangle small organisms on their sticky tentacles and, putting the 

 latter one by one into the mouth, contract upon them and, by drawing 

 them out, strip off the catch. 



The axial organ is represented only by a cord-like genital stolon 

 near the gonoduct. The aboral sinus and vascular ring, genital rachis, 

 and apical nervous system are absent. The lacunar system consists of 

 an oral ring, radial " vessels", "dorsal and ventral vessels" of the 

 alimentary canal, and a plexus on the latter. In the middle part of 

 the intestine of Holothuria and many other genera, the "dorsal 

 vessel" hangs from it on a perforated fold of the peritoneum, but 

 remains connected with it by a plexus known as the rete mirahile. 

 In perforations between the strands of this plexus the branches of 

 the left respiratory tree are entangled. The condition of the water 

 vascular system is that described on p. 551 ; for that of the genital 

 system see p. 553. 



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. 431, 432 B). 



