716 



ZOOLOGY 



SECT. 



retarded, and, in consequence, it is on the right side that the 

 pallial complex comes to travel forwards. The effect is as if, the 

 bead retaining its symmetry, the parts between it and the anus 

 had undergone a process of rotation or torsion through about 180 

 around a vertical axis passing in a dorso- ventral direction the 



C 



r. vise. com 



FIG. 629. Diagrammatic representation of the displacement of the mantle-cavity and asso- 

 ciated 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 dis- 

 placement 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 ; 1. cten. left ctenidium ; 1. vise. com. left visceral connective ; rnant. mantle ; 

 mo. mouth ; neph. ap. renal apertures ; ped. g. pedal ganglion ; pi. g. pleural ganglion ; 

 r. cten. right ctenidium ; r. vise. com. right visceral connective ; vise. com. visceral con- 

 nectives. (After Korschelt and Heider.) 



direction of torsion being opposite that of the movement of the 

 hands of a watch (Fig. 629). 



With regard to the spiral form assumed by the shell in all highly 

 developed Gastropods, it can only be pointed out here that, given 

 the necessity for complete protection, compactness, and a provision 

 for continuous growth, the spirally-coiled cone is the form of shell 



