VII MOLLUSGA—THE ASYMMETRY OF THE GASTROPODA 161 



points to a causal connection between the spiral coiling of the visceral dome and 

 shell on the one hand and the special asymmetry of the asymmetrical organs on the 

 other. The above-mentioned exceptions to the rule can, however, be explained as 

 follows. The spiral of a dextrally twisted shell can by degrees become flattened in 

 such a way that the shell may be simply coiled in one plane or may nearly approach 

 that condition. In this case the spiral might again assert itself, but on the side 



B 



D 



Fig. 141.— Seven forms of Ampullaria sliells (diminished in various degrees), seen in the upper 

 row from the aperture of the shell, in the lower from the dorsal side. The head, foot, and oper- 

 culum are arbitrarily drawn merely for the purpose of facilitating a comparison between dextrally 

 and sinistrally twisted shells. 



opposite to that on which the umbilicus originally lay, and in this way a false 

 spiral might form on the umbilical side and a false umbilicus on the spiral side. 



The transition from a dextrally twisted to a falsely sinistrally twisted shell, which 

 latter was, however, genetically dextrally twisted, is illustrated in Fig. 141 by 

 means of the shells of seven species of the genus Ampullaria. Ampullaria Swain- 

 soni Ph ? (G) and A. Geveana Sam (F) are dextrally twisted with distinctly project- 

 ing spiral. In A. crocastmna Ph (E) the spiral is flat, in A. (Oeratodes) rotula 

 Mss. (D) and A. (Oeratodes) chiquiteusis d'Orb (C) the spiral is already pushed 

 through or sunk, yet we iind a true umbilicus 

 on the umbilical side. In A. (Zanistes) Bol- 

 teniana Ghemn. (B), and still more in A. 

 purpurea Jon. (A), the false spiral appears on 

 the umbilical side, and on the spiral side a false 

 umbilicus is found. 



However plausible this explanation may 

 appear, it can only be proved to be correct if 

 it is found that where a spiral operculum 

 occurs, the direction of its spiral is opposite to 

 that of the spiral of the shell (Fig. 142, A, B, 

 C), and the commencement of the spiral is 

 always turned to the umbilical side of the shell, 

 operculum, but such occur in the Pteropoda. 

 VOL. II 



Pig. 142. 



Zanistes has not a spirally twisted 

 In those Pteropods which combine a 

 M 



