2cj8 CLASSIFICATION OF ECH I N ODE RM ATA 



present, and there are spines. The skeleton usually makes a rigid 

 box of 'calcareous plates. Most of the volume of a sea urchin is 

 taken up by the body cavity, in which the spiral gut is slung. A 

 common example is Echinus escnlentus of British coasts. Some, 

 such as Spatangiis, are heart-shaped, with slight bilateral 

 symmetry. 



CLASS IV—HOLOTHUROIDEA 



The sea cucumbers bear a superficial resemblance to the 

 vegetables of the same name. The body is elongated in the direc- 

 tion from anus to mouth, which opens towards the direction in 

 which the animal moves, so that there is a tertiary bilateral 

 symmetry. The adambulacral area is reduced and the madreporite 

 usually internal. The ambulacral grooves are covered, some tube- 

 feet have suckers and others are modified into oral tentacles. 

 There are neither spines nor pedicellarise. The skeleton is reduced 

 and the skin leathery in texture. Holothuria is a common genus. 



CLASS V—CRINOIDEA 



The sea-lilies, or feather stars, resemble most of the fossil 

 forms in being, for at least part of their life, sessile by a stalk 

 which grows from the aboral side. The arms, which are sharply 

 marked off from the disk, are branched. The ambulacral grooves 

 are open, the tube-feet have no suckers, and there are neither 

 pedicellariae nor spines. There is no madreporite. Rhizocrinus is a 

 deep-water form with a long permanent stalk. Antedon rosacea 

 lives at about ten fathoms, and when adult breaks loose and 

 swims by waving its arms. Round the stump of the stalk is a ring 

 of cirri, which are used for temporary attachment. 



