3 1 2 ECHINODERMS. 



between the membrane round the mouth and the plates of the test. In the 

 irregular urchins some tube-feet are modified for respiration, becoming broad, 

 flat, and somewhat lobed ; the hinder end of the intestine seems to be respiratory 

 in function. Some sea-urchins possess eyes. In a Diadema there are five 

 ovate pigment-masses of a brilliant ultramarine blue, placed at ecpaal distances 

 around the vent. There are certain other peculiar bodies supposed to be sense- 

 organs of some kind, called sphseridia, which are of microscopic size, and in 

 structure not unlike tiny spines. They lie near the mouth and on the lower 

 ambulacral plates, are often set in small holes, and are provided with special nerves. 

 Perhaps they test the water in which the sea-urchin lives, and thus may be said 

 to serve the sense of smell. Sea-urchins are both animal and vegetable feeders, 

 and are even cannibals when opportunity offers. 



The Sea-Cucumbers, — Class Holothuroidea. 



Sea-cucumbers are, as we have seen, elongated and worm-like creatures, with 

 a mouth at one end and a vent at the other. The skin is leathery, and 

 contains a comparatively small amount of calcareous matter. Usually this 

 occurs in small spicules, which assume very definite shapes, such as the anchors of 

 Synapta, or the wheels of CJtiridota; but in such forms as Psoitis the spicules 



increase in size, so as to form a plated integument. 

 There may also often be a ring of calcareous plates 

 round the gullet, five of which plates have the same 

 relation to the radial water- vessels as the auricles around 

 the jaws of a sea-urchin, and they likewise serve for the 

 attachment of muscles. In such a common form as 

 Cucumaria planci there are five rows of tube-feet pass- 

 ing 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 

 ^"natTsizeiT" 1 ^ ne 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 

 fragments of coral. They usually attach themselves by their tube-feet to rocks 

 or seaweed, 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 (Psoitis), of which a specimen is illustrated on p. 314. 





U-SHAPED SEA-CUCUMBER 



