254 PHYLUM ECHINODERMA. 



adjacent sea-urchins, and the free-swimming larva is called 

 a Pluteus. 



From the nature of the case there can be no autotomy 

 in sea-urchins, but the spines can be regenerated. 



Classification. - 



The class is 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, Strongylocentrotits, Arbacia. 

 The Echinothurinaj have flexible tests and powerful muscles. 



e.g. Asthenosojna, and Phormosoma. 



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

 of plates in the posterior inter-radius. 



e.g. the heart urchin, Spatangns, the massive Clypeaster, the 

 minute Echinocyaiinis pnsilhis, common in the stomach of cod- 

 fishes. 



Class HOLOTHUROIDEA. Sea-Cucumbers. 



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, ivith 

 irregularly scattered microscopic calcareous bodies, with a 

 terminal mouth surrounded by tentacles, with a posterior anus, 

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



The Holothurians do not at first sight suggest the other 

 Echinoderms, for they are like plump worms, and the 

 calcareous 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. 



