322 



COMPA If A TIVE ANA TOMY 



CHAP. 



be sought in the shifting" of the anus and anal area out of the 

 apical system into the posterior interradius ; by this shifting the anus 

 may come to lie at any point between the (aboral or dorsal) apical 

 system and the oral (or ventral) area. In its posterior and downward 

 shifting the anus thus does not carry the apical system with it, but 

 the latter remains on the dorsal side, although it is often shifted 

 somewhat excentrically anteriorly, rarely posteriorly. The whole body 

 is then bilaterally symmetrical, and when seen from above it is oval or 

 heart-shaped, etc., in outline. The line connecting the mouth with the 

 anus, which in the regular endoeyelie Echinoidea altogether or nearly 



FIG. 276. Holectypus clepressus, Cot- 

 teau. Apical system and neighbouring 

 parts of the perisome (after Lov&n). For 

 lettering see p. 317. 



FIG. 277. Clypeaster rosaceus, L. Apical 

 system and neighbouring parts of the perisome 

 (after Lov&n). For lettering see p. 317. 



coincides with the vertical (principal) axis, now becomes the more in- 

 clined, i.e. approaches the more nearly to the horizontal, the further the 

 anal aperture is removed from the apical system into the posterior 

 interradius, and is shifted on to the under side (into the oral or actinal 

 region). Those Echinoidea in which the anal aperture has been 

 shifted outside the apical system are called exoeyelie or irregular. 



Among the Palceechinoidea the genus Echinocystls (Cystocidaris) 

 alone is exocyclic. It appears that in this form the whole apical 

 system consisted merely of one madreporic plate. 



Among the Euechlnoidea the three orders Holectypoida, Clypeastroida, 

 and Spatangoida are exocyclic. 



a. Holectypoida (Fig. 276). In consequence of the wandering of the anus out 

 of the apical system, the posterior basal plate has lost its genital aperture, probably 

 in connection with the disappearance of the related genital gland (the place of which 

 has been taken by the rectum) ; in Cunocli/pcus and G(ifi'rnji//:/iis this plate has even 



