360 Proceedings of Boyal Society of Edinhurgli. [sess. 
in different forms. This has an important bearing on the topo- 
graphy of the body. 
Fig. 1 represents the relative positions of the organs in the pallial 
cavity of a Prosobranch. The figure is diagrammatic. It must be 
remembered that the gill and osphradium are attached to the roof 
of the mantle cavity, and are thus drawn up into a position in which 
the osphradium is to the actual left of the gill. It will be noted 
that the gill and osphradium are situated to the left in the mantle 
cavity, and that the latter is in a direct line, with a funnel-like pro- 
longation of the mantle (the siphon). This left side may be regarded 
as the inhalent side, and the right side as the excretory j and it 
would appear that the necessity (endeavour*?) of getting a pure 
supply of water, and the formation of an external organ (siphon) to 
appreciate the direction of stimuli, have something to do with the 
position of this organ. As a result we have the pallial cavity 
brought forward and to the left. 
These considerations will explain the condition which we find in 
the first Opisthobranch we take up, viz.. Bulla. There is no turreting 
of the shell, a condition associated with the position of the mantle. 
Fig. 2 will show that the position of the pallial organs is quite 
different from that in the case just mentioned. The gill is now 
