320 



COMPARATIVE ANATOMY 



CHAP. 



the presence of the central plate, most regular Echinoidea show the 

 following typical composition of the apical system : In the centre of 



CJO 



cvm. 



FIG. 273. Echinus sp. (1 to -2 mm. long). 

 Apical system (after Loven). For lettering see 

 p. 317. 



FIG. 274. Toxopneustes drcebachiensis, 

 O.F.M. (10 mm. long). Apical system (after 

 Loven). For lettering see p. 317. sh, Tubercles 

 carrying spines ; ap, anal plates. 



the system lies the anal area, with a few large, or many small, calcareous 

 plates. A central plate cannot be distinguished. Within the anal 



area lies the anal aperture, 

 usually excentric, less fre- 

 quently central. Round the 

 anal area are found the 

 circles of plates present in 

 all Echinoids, viz. the proxi- 

 mal circle of five basal 

 plates, and the distal circle 

 of five radial plates (Fig. 

 275). One, or several, or 

 even all of the radials may, 

 however, become wedged in 

 between the basals apically, 

 and finally may take part 

 in the limitation of the anal 

 area. 



The ontogeny of Toxo- 



FIG. 275. Toxopneustes drcebachiensis, O.F.M. pneustes shows that there is 



Forlettering at first in the anal area of 

 very young Echinoidea one 

 large central plate (Fig. 274). Near this central plate, which ceases 

 to grow and degenerates, accessory plates appear. Among these 



