368 ECHINODERMATA [CH. 



Class III. ECHINOIDEA. 



Eleutherozoa in which the arms have coalesced with the hody, 

 the radii being arranged like meridians on a sphere. The ambulacral 

 grooves are closed. The body has a complete armour of closely 

 adjusted plates and the spines are movably articulated with these 

 and assist in locomotion. 



Order 1. Endocyclica. 



Echinoidea in which the anus is in the centre of the aboral 

 pole and teeth are present. 



Ex. Echinus. 

 Order 2. Clypeastroidea. 



Echinoidea in which the anus is excentric, the dorsal tube- 

 feet are flattened and teeth are present. 



Ex. Clypeaster, Echinocyamus, Echinarachnius. 



Order 3. Spatangoidea. 



Echinoidea with an excentric anus and flattened dorsal 

 tube-feet but without teeth. 

 Ex. Echinocardium. 



ClaSS IV. HOLOTHUROIDEA. 



Eleutherozoa resembling Echinoidea in the coalescence of the 

 arms with the body and in the closed condition of the ambulacral 

 grooves ; but with rudimentary skeleton, highly developed muscular 

 body-wall and greatly enlarged buccal tube-feet by means of which 

 all the food is obtained. 



Order 1. Aspidochirotae. 



Buccal tentacles with shield-shaped ends; front part of 

 body not retractile, gill-trees present. 



Ex. Holothwia. 

 Order 2. Dendrochirotae. 



Buccal tentacles branched in tree-like fashion ; front part of 

 body retractile, gill-trees present. 



Ex. Cucumaria. 

 Order 3, Synaptidae. 



Buccal tentacles feather- shaped, no other tube-feet ; radial 

 canals and gill-trees absent. 

 Ex. Synapta. 





