VII MOLLUSGA—THE ASYMMETRY OF THE GASTROPODA 153 



right. On the left there -would be a pressure from the front which would, so to 

 speak, squeeze out the pallial complex backwards over to the right. It must further 

 be noted that the point subjected to least lateral pressure and to the greatest down- 

 ward pull lies on the right, 

 which has become the upper 

 side of the visceral dome. At 

 this point the mantle furrow 

 will most easily deepen, and 

 become more spacious. Into 

 such a deepening the organs of 

 the pallial complex which are 

 being pressed from the left 

 have room to move forward to 

 the right. Here we have the 

 first step in the shifting for- 

 ward of the pallial complex 

 along the right mantle furrow. 

 Further, as soon as the least 

 shifting of this sort has taken 

 place, the shell and visceral 

 dome can move slightly from 

 their present position on the 

 left, towards that backward 

 position which we have seen 

 to be the most favourable im- 

 aginable for locomotion and for 



Fig. 135.— Diagram illustrating the variations of pres- 

 sure to which the shell and visceral dome are subjected 

 when inclined to the left. The thickness of the concentric 

 lines indicates the amount of the pi-essure. a, Point of greatest 

 the functions of the cephalic pressure ; h, point of least pressure. The arrows give the 

 organs dii-ection in wliich sliifting takes place. It is evident that 



the left side of the pallial complex is subjected to greater 



If we suppose this process 



pressure than the right. 



gradually to be completed, the 

 shell and visceral dome finally gain the most favourable backward position, and the 

 pallial complex is gradually shifted forwards along the right mantle furrow. The 

 pallial comjjlex thus lies anteriorly on the upper side of the visceral dome, which 

 now points backwards. This anterior position is that of the least upward pressure, 

 or rather of the greatest downward pull, i.e. it is the point at which the mantle 

 cavity can most easily deepen and widen, and where the pallial organs can best 

 fidfil their functions. 



The position of the shell and the pallial complex characteristic of the Gastropoda 

 is now attained, and with it chiastoneury and the inverse position of the organs of 

 the pallial complex. 



4. 



The second stage in the development of the Gastropod shell is the coiling in 

 one plane of the visceral dome and shell. 



If the Gastropod visceral dome assumes the most favourable inclined position above 

 de-scribed, it will, under normal conditions, change its conical shape. The side which 

 lies uppermost will become arclied and the lower side concave. This change of form 

 is caused by the stronger growth of the integument of the visceral dome and mantle 

 on that side, whicli, in the inclined position of the visceral dome, is the most 

 stretched or pulled. The visceral dome also becomes curved in one plane, and the 

 shell naturally adapts itself to the changes of shape of the dome. Again, the shell 

 could not remain conical, because a large part of the dorsal integument (base of the 

 visceral dome) would then be uncovered, and in consequence of the increase of those 



