TYPE M0LLU8CA. 



295 



tended to produce a retardation in the growth or even the 

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

 inglj' the original left nephridium is in many Gasteropods com- 

 pletely suppressed as well as the original left branchia, and 

 in accordance with the disappearance of this latter structure 



Fig. 131. — Diaqbams to illustbate the Rotation of the Mantlb-cavitt 



AND ITS ObgANS IN A GasTEROPOD {after figures by BiJTscHLi and Lang from 

 KoRscHELT and Heider). 



a = anus. in = mouth. 



ao = aorta. n = nephridial pore. 



ct = ctenidium. peg = pedal ganglion. 



eg — cerebral ganglion. pig = pleural ganglion. 



ve = 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 



