3280 



ECHINODERMS 



feet passing from mouth to vent. The five-rayed symmetry is not obscured, and is 

 traceable in the arrangement of nerves and muscles, although it does not affect any 

 portion of the digestive or generative systems. Around the mouth, in Cucumaria, 

 is a fringe of branched tentacles, connected with the water-vascular ring. In 

 most other echinoderms, it will be remembered, a canal passes from this ring and 

 opens to the exterior by a madreporite; and in a few holothurians of primitive 

 structure this is similarly the case. But in Cucumaria, as in most, the connection 

 with the exterior is lost, and the canal, with its madreporite, hangs down into the 

 body cavity. In Cucumaria the tentacles are used like a net to intercept floating 

 organisms in the surrounding water. Many holothurians swallow a great deal of 

 sand, and the intestines of those that live near coral reefs generally contain frag- 



A DEEP-SEA HOI.OTHURIAN, 

 (Natural size.) 



ments of coral. They usually attach themselves by their tube feet to rocks or sea- 

 weed, and wave the tentacles around. Holothuria atra, which lives on the great 

 Australian barrier reef, inserts its hinder extremity within a crevice of the rock, 

 into which on being disturbed it speedily retreats. 



Some curious modifications of form have taken place among the holothurians. 

 In the plated sea cucumbers (Psolus}, of which a specimen is illustrated on p. 3281, 

 the animal has become flattened, and the tube feet restricted to three 

 out of the five ambulacra, and by these three the animal creeps about, 

 or holds itself fixed to the rock. A similar modification is carried to 

 excess in the deep-sea holothurians known as Klasipoda. Here, as in the 

 illustrated Scotoplana, there are a couple of rows of thick tube feet, forming little 

 stumps, with which the animal moves, as a centipede moves by its legs. In front 



