462 CHARACTERS OF INVERTEBRATE ANIMALS 



Class V.— SEA-CUCUMBERS (Holothuroidea) 



These are for the most part leathery elongated forms, some 

 of which look uncommonly like cucumbers, a similarity which 

 has suggested the name of the group. They are regarded by 

 some authorities as representing most nearly of living animals the 

 ancestral stock from which all Echinoderms must be supposed 

 to have taken origin. This, however, is extremely doubtful; but 

 as unfortunately the class, unlike the other ones included in the 

 phylum, is but poorly provided with hard parts, fossil remains are 

 infrequent, and throw but little light upon the question. 



In a typical Holothurian (such as the genus Cucumaria) (fig. 

 285) the elongated body is somewhat angular, and the two open- 

 ings of the digestive organs are at opposite ends of the body, 

 the mouth being surrounded by a circlet of ten branching tentacles, 

 which can be retracted. The ambulacral areas are marked by five 



double rows of tube- 

 feet protruding along 

 the side of the body. 

 No trace of a madre- 

 porite can be discerned. 

 The body-wall consists 

 of skin and underlying 

 muscles, the latter 



Fig. .85.-A Sea-Cucumber [Cucun,aria), reduced ^eing aS WCll deVelopcd 



here as they are scanty 

 in those forms which have a well-developed exoskeleton. The 

 hard parts of a Holothurian are comparatively insignificant, the 

 most important of them making up a calcareous ring round the 

 gullet; and there are, besides, minute calcareous spicules scattered 

 through the skin. 



The mouth, as already stated, is placed in the middle of the 

 crown of tentacles, and these are arranged in five pairs, one for 

 each ambulacral area. One pair is smaller than the rest, and 

 is used in shovelling food into the mouth. They correspond to 

 the under surface of the body, which in some other members of 

 the group is very sharply defined, though here the radial symmetry 

 is not much interfered with. The mouth leads into a mouth- 

 cavity, which opens into a gullet, and this again into a looped 



