ECHINODERMATA. 45 



Two rows of ambulacral alternating with two 



rows of interambulacral plates. 

 Anus central ........................................ ENDOCYCLICA. 



Anus not central .................................... EXOCYCLICA. 



Three to six rows of plates in each ambulacral 



space ................................................ PAL^CHINOIDEA. 



Order I. ENDOCYCLICA. 

 DESMOSTICHA. REGULARIA. 



Mouth and anus central. Two rows of ambulacral and two 

 rows of interambulacral plates alternating with each other. 



The plates overlap one another in some of the extinct forms ; 

 in Asth&nosoma and Phormosoma they are movably connected by 

 membrane. 



In our common sea-urchin (Echinus sphtsra) there are about 

 300 plates and 4000 spines (Forbes). The Mediterranean E. 

 escutentus is extensively used for food ; the ovaries are the parts 

 selected. Toxopneustes lividus is found in holes of limestone, 

 sandstone, granite, &c., which it is supposed to form for itself. 



Cidarid(B. EchinotJiuriidce, Echinidce. 



Cidaris. *Echinothuria. Toxopneustes. 



*Goiiiocidaris. Phormosoma. Heliocidaris. 



Salenia. Asthenosoma=Cal- Hipponoe. 



Arbaciidce. veria. Echinus (Sea-urchin). 



Arbacia. Echinometridce. 



JXadematid*. Echinometra. Echnohrix. 



Diadema. Podophora. 



Order II. EXOCYCLICA. 



PETALOSTICHA. IRREGULARIA. 



Anus not central. Two rows of ambulacral and two rows of 

 interambulacral plates alternating with each other. 



The masticatory apparatus is frequently absent. The mouth 

 is central in Clypeastridae and Mellitidas, but excentric in the re- 

 maining families. The two former are placed by A. Agassiz 

 among the Endocyclica, by Claus they are ranked as a separate 

 order (Clypeastridea). 



The English species of this order are known as " heart- 

 urchins." As they mostly bury themselves in the sand or mud, 

 the greater part of their spines are directed backwards. 



