320 



COM FA HA TI VE A NA TO MY 



CHAP. 



the presence of the central })late, most regular Echinoidea show the 

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



m 



Fig. 273. — Echinus sp. (1 to 2 mm. long). 



Apical system (at'tf'v Loven). For lettering see 



p. 317. Fig. 274. — Toxopneustes droebachlensis, 



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

 Lov^n). For lettering see p. 317. sh, Tubercles 

 carrying spines ; o/>, 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- 

 2Mcmtes shows that there is 

 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 



c\-\\. 



Fig. 275. — Toxopneustes droebachlensis, O.F.M, 

 Apical system of the ailult (after Loven). Fur letterinf 

 see p. 317. 



