TYPE MOLLUSCA. 



295 



tended to produce a retardation in the. growth or even the 

 complete abortion of the organs lying in that region. Accord- 

 ingly the original left nephridium is in many Gasteropoda com- 

 pletely suppressed as well as the original left brauchia, and 

 in accordance with the disappearance of this latter structure 



V 



I) 



FIG. 131. DIAGRAMS TO ILLUSTRATE THE ROTATION OF THE MANTLE-CAVITY 

 AND ITS ORGANS IN A GASTEROPOD (after figures by BUTSCHLI and LANG from 

 KOESCHELT aud HEIDER). 



a anus. in mouth. 



ao = aorta. n == uephridial pore. 



ct = cteuidium. peg = pedul ganglion. 



eg = cerebral ganglion. pig = pleural ganglion. 



vc = visceral connective. 



there is a disappearance also of the left auricle of the heart 

 which receives blood from it. 



The visceral hump does not, however, retain its original 

 conical form, but, owing perhaps to unequal pressure, grows 

 more rapidly upon one surface, the anterior, and so becomes 

 coiled into a right-handed spiral, the shell covering the 

 hump naturally assuming a similar form. In the majority of 



