320 



CHAP. 



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

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



m 



ICC 



cvm 



FIG. 273. Echinus sp. (1 t<> _' nun. 1..H-). 

 Apical system (uttiT Lovjm). For lettering see 

 p. 317. 



Fio. 274. Tozopneustes drcehachiensis. 

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

 Lovfcn). For lettering see p. 317. sh, 

 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 



-- 111 ana ^ area are f un( l tne 

 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 drcehachiensis, O.F.M. pneustes shows that there IS 

 Apical system of the adult (after Lov6n). For lettering a ^ 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 



cvvt 



