XVIII 



ENDOCYCLICA 



533 



widely different results. Agassiz, 1 for instance, places the genera 

 Echinus (the common British form) and Strongylocentrotus (the 

 commonest American form) in different families. Bell, 2 on the 

 other hand, considers them to be closely allied. Bell's system, 

 based as it is on the development of the peristome, seems to the 

 present author the most justifiable, for the peristome is un- 

 doubtedly a differentiation of the corona, which has been 

 brought about by the manner in which the animal breathes 

 and masticates, two functions of prime importance. The 

 periproct is also of importance, representing as it does the 



Fig. 235. Oral view of 

 dried and cleaned test 

 of Cidaris. p, Pores 

 for tube-feet arranged 

 in single series ; per, 

 peristome with both 

 ambulacra! and inter- 

 ambulacral plates ; t, 

 tubercle of a large 

 interambulacral spine. 



whole aboral surface of the Starfish, and so are to a less 

 extent the arrangements of the spines and of the tube -feet. 

 Proceeding in this way, living Endocyclica can be divided into 

 six families, which are briefly described below. 



Fam. 1. Cidaridae. Endocyclica with a large peristome 

 and a large periproct. The peristome is covered with a regular 

 series of both ambulacral and interambulacral plates, the former 

 pierced by tube-feet. No special buccal tube-feet and no external 

 gills. The periproct is large, and is covered with irregular 

 plates (Fig. 236, A). The lantern eoelom is provided with large 

 Stewart's organs. 



1 "Revision of the Echini," Illustrated Catalogue of Museum of Comp. Zool. 

 Harvard, No. 7, 1874, p. 423. 



2 British Museum Catalogue, "British Echinoderms," 1892, p. 30. 



