152 



COMPARATIVE ANATOMY 



CHAP. 



On the other hand, such a position is the most favourable imaginable for the 

 functions of the organs belonging to the posteriorly placed pallial complex, which 

 now lie dorsally, since in this position the mantle cavity is subjected to least pressure 



<vr>U c( 



FIG. 132. 



from the viscera and from the columellar muscles. The downward pressure of 

 the visceral mass which now takes place would tend indeed to widen the cavity. 



2. The shell might be carried inclined backwards (Fig. 133). This position is 

 the most favourable imaginable for locomotion and for the functions of the organs 



Fio. 133. 



of the head, which would thus be free on all sides. It is, however, the most 

 unfavourable imaginable for the functions of the organs of the pallial complex, 

 which now lie beneath the visceral dome. The mantle cavity has to bear the whole 

 pressure of the visceral mass, and especially that of the columellar muscle ; it would 



be squeezed together, so that the 

 .* circulation of the respiratory water 

 would be prevented or at least 

 rendered more difficult, as would 

 also the ejection of the excreta, ex- 

 crement, and sexual products. 



3. Finally, the shell may be 

 carried inclined to the right or left 

 (Fig. 134). This is neither the most 

 favourable nor the most unfavour- 

 able position for locomotion, for the 

 head, and for the pallial complex. 

 It is an imaginable intermediate 

 position. 



In this position there is no dead point, as shifting of the parts would always be 

 possible, and the shell be enabled to take up the .position most suitable for locomo- 

 tion and for the functions of the cephalic organs, and the mantle cavity that best 

 suited for the exercise of the functions of the pallial complex lying within it. 



Assuming that the shell is inclined to the left (Fig. 135), the pressure brought 

 to bear on the mantle cavity would vary in amount in different areas of that cavity. 

 It would be greatest on the left side, and would continually decrease towards the 



FIG. 134. 



