736 



ZOOLOGY 



around a horizontal axis which, the head and foot being regarded 

 as fixed, results in the visceral mass enclosed in the spiral' shell 

 coming to occupy a more or less dorsal position with the apex 

 directed backwards and to the right. 



i/tsc.cont 



Fig. 619. — Diagrammatic representation of the displacement of the mantle-cavity and associated 

 parts in the Gastropoda. Enteric canal blue, blood-system red. A, represents a nearly 

 .symmetrical arrangement ; in B, C, D, are represented successive stages of displacement of 

 the mantle-cavity to the right and forwards ; in E, the anus and (primarily) right ctenidium 

 have passed the middle line, an, anus: aort. aorta; cer. g. cerebral ganglion; I. cUa. left 

 ctenidium ; I. vise. com. left visceral connective ; mant. mantle ; 'tno, mouth; neph.nji. nephri- 

 dial apertures ; perf. g. pedal ganglion ; pi. g. pleural ganglion ; r. cten. right ctenidium ; r. 

 vise. com. right visceral connective ; vise. com. visceral connectives. (After Korschelt and 

 Heider.) 



The shifting of the pallial complex in many Streptoneura 

 proceeds so far that the complex completely or partially passes 

 across the middle line, and the anus comes to be situated to 

 the left of the mouth. The displacement of the pallial complex 

 involves two important series of changes in the internal organs, in 

 addition to the suppression of certain structures to be referred to 



