S30 ECHINODERMATA ECHINOIDEA chap. 



the part of each order have undoubtedly taken place ; but still the 

 statement would remain roughly true, and would give a fair idea 

 of the differences in habitat which have led to the differentia- 

 tion of the group. 



Order I. Endocyclica {Regular Urchins). 



The principal variations concern (1) the peristome, (2) the 

 periproct, (3) the corona, (4) Aristotle's lantern and its 

 appendages, (5) the spines, (6) the pedicellariae, and lastly, (7) 

 the tube-feet. We shall consider these points in order. 



Peristome. — In the vast majority of species this region is 

 covered only with flexible skin in which ten small plates are 

 embedded, pierced by pores for the buccal tube-feet ; besides 

 these there 'are irregularly arranged thin plates. In the 

 Cidaridae both the ambulacral and the interambulacral series 

 of plates are continued on it ; these plates differ from those 

 of the corona in being movable on one another. In Echino- 

 thuriidae only the ambulacral series of plates is continued on to 

 the peristome. In the ease of both these families there are a 

 considerable number of tube-feet within the region of the peri- 

 stome which may be classed as buccal. 



Periproct. — This area, which represents the whole dorsal 

 surface of Asteroidea, is very large in the Cidaridae, where, as 

 in JEchinus, it is covered with leathery skin in which small 

 plates are embedded. In the Saleniidae it is covered with a 

 single large sur-anal plate, in the edge of which the anus is 

 excavated ; in the Arbaciidae it is covered with four valve-like 

 plates; whilst in the remaining species its condition is similar 

 to that described in the case of Echinus esculentus. 



Corona. — In Echinothuriidae all the plates are separated by 

 slips of membranous skin, so that the test is flexible. In all 

 other families it is an unyielding cuirass. In the Cidaridae the 

 pore -plates remain separate throughout life, and are therefore 

 identical with the ambulacral plates. These are small and 

 placed in two vertical rows, and so the ambulacra are 

 exceedingly narrow. In Echinothuriidae there is some tendency 

 to adhesion amongst the pore-plates ; these are of different sizes, 

 and usually one larger and one smaller adhere to one another. 

 In all other species regular ambulacral plates are formed at least 



