HOLOTHURIANS OR SEA-CUCUMBERS. 



661 



Fig. 10. PlMTKD HOLOTRURIAN (Ps<.lll8 



diomediae). 3-2 natural size. 



rnent affect any portion of the digestive or generative systems. In most 

 other echinoderms, it will be remembered, a canal passes from the water- 

 vascular ring, and opens to the exterior by a madreporite. 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. The 

 skin is leathery, and contains a compara- 

 tively small amount of calcareous matter. 

 What there is occurs usually in small 

 spicules, which assume very definite shapes, 

 such as anchors in Synapta, or wheels in 

 Chiridota. Such spicules are represented 

 in Fig. 11 (S 1 and J> 2 ). In such forms as 



10), the spicules increase in size so as to form 

 there may also often be a ring of calcareous 

 plates round the gullet and round the anus (M and 

 As in Fig. 10). The tentacles of Cucumaria and 

 some other forms are used like a net to intercept 

 floating organisms in the surrounding water. Many 

 holothurians take a good deal of sand into the gut 

 and the intestines ; those that live near coral reefs 

 generally contain fragments of coral. They usually 

 attach themselves by their tube-feet to rocks or sea 

 weed, and wave the tentacles around. The food- 

 Inden tentacles are thrust one after the other into 

 the circular mouth. Some curious modifications of 

 form have taken place among the Holothuroidea. 

 In Psolus 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. The species shown in Fig. 

 10 is one of 



Psolus, however (Fig. 

 plated integument ; 



those that 

 were dredged 

 by the U.S. 

 steamer Al- 

 batross, and 

 occurs near 

 the Cocos 

 Islands at a 

 depth of G6 

 fathoms, ad- 

 hering to the 



rocks like a limpet. It represents a 

 highly specialised form of the genus. 



The tube-feet (P) are seen on its under side, forming a somewhat irregular 

 ring. The curious form, Psychropotes raripes, shown in Fig. 11, is one of 

 the deep-sea forms known as Elasipoda. It also was dredged by the 

 Albatross, and cornea from a depth of 1,573 fathoms, south of Cape San 

 Francisco. The left-hand figure shows the under side of it, with the mouth 



Fig. 11. DBUP-SEA HOLO- 

 THURIAN (Psychropotes 

 raripes). Two - ninths 

 natural size. 



Fig. 12. A FREK-SWIMMING HoLOTHriUAN (Pela- 

 gothuria natatrix). O^e-hal( natural size. 



