SUB-CLASSES OF ECHINOIDS 305 



Classification. -- 



The class may be divided into three sub -classes or groups of orders. 

 Sub-Class I. Regularia Endobranchiata. Mouth and anus at opposite 

 poles ; the anus surrounded by the apical system of plates if these 

 are developed ; no external gills. 



e.g. the somewhat primitive Cidaris. 

 Sub-Class II. Regularia Ectobranchiata. Mouth and anus at opposite 

 poles ; a double circle of apical plates surrounds the anus ; there 

 are external gills. 



e.g. the common genera Echinus, Sirongylocentrotus, Arbacia. 

 The Echinothurinffl have flexible tests and powerful muscles. 

 e.g. Asthenosoma and Phormosoma. 

 Sub-Class III. Irregularia. The anus lies outside the apical system 

 of plates in the posterior inter-radius. 



e.g. the heart -urchins, Spatangus and Echinocardium, without 

 lanterns. In the related Echinoneus there is a lantern in the 

 young forms. It is interesting to contrast the large massive 

 Clypeaster with the minute Echinocyamus pusiUns, common 

 in the stomach of cod-fishes. 



Class HoLOTHUROiDEA. Sea-Cucumbcrs 



Cylindrical or worm-like Echinoderms, elongated in the 

 direction of the main axis^ with more or less tendency to 

 bilateral symmetry, with a usually soft or leathery skin, with 

 irregularly scattered microscopic calcareous bodies, with a 

 terminal month surrounded by tentacles, with a posterior anus, 

 with or without tube-feet, with no external madreporite, with 

 a muscular body wall. 



The Holothurians do not at first sight suggest the other 

 Echinoderms, for they are Hke plump worms, and the 

 carcareous skeleton is not prominent. But closer examina- 

 tion shows the characteristic pentamerous symmetry, and 

 the occurrence of calcareous plates in the skin. These 

 seem to be absent in the unique pelagic Pelagothuria. 



Holothurians occur in most seas, from slight to very 

 great depths. Their food consists of small animals, and of 

 organic particles from the sand. Some of them catch these 

 in their waving tentacles, which are then plunged into the 

 pharynx. The muscles of a captured Holothurian often 

 over-contract and eject the viscera at the ends or through 

 a side rupture ; in this way the animal may sometimes 

 escape, and the viscera can be regrown. 



In Synapta the rupture of the body takes place very rapidly, and is 

 probably defensive, the anterior portion re-forming a complete in- 



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