366 



OILMAN A. DREW. 



shrinkage caused by the treatment with preservatives^ it 

 seems most likely that the spaces are normally connected 

 with each other. It is just possible that these spaces may be 

 traced back in their formation to the blastocoele, but it is 

 more probable that the blastocoele entirely disappears, and 

 that they represent a schizocoele. At first the space ventral 

 to the gut is larger than that dorsal to it, but the latter 

 grows as the mantle arches dorsally, and the ventral space 

 remains practically unchanged. 



As the foot begins to take form the ventral space becomes 

 quite small, and about the same time that the embryo sheds its 

 test it disappears altogether. A short time before the test 

 is shed the dorsal space reaches its greatest size (figs. 25 and 

 26). About the time that the test cells begin to break apart, 

 the stomach is carried dorsally some distance into this space 

 (fig. 84). Two fibres, that in shape suggest muscle-fibres, 

 extend from the anterior end of the stomach to the mantle. 

 Their position suggests that they may aid in moving the 

 stomach dorsally, but of this I have no proof. As the 

 stomach moves dorsally they become shorter and thicker, but 

 there is no evidence that they are moving factors. Until 

 casting is completed, the apical plate and the stomodaeum lie 

 between the edges of the shell-valves, and keep them from 

 being closed. When they are removed, the contraction of 

 the adductor muscle closes the shell, and the body, which has 

 until now been lying between gaping valves, is made to 

 change its shape and position. The stomach and liver 

 pouches are forced into the dorsal space until the dorsal end 

 of the stomach comes in contact with the mantle. 



This divides the space into anterior and posterior parts 

 (fig. 35). There is no longer room for the liver pouches to 

 retain their form and position, and as the body continues to 

 move dorsally they are flattened out and soon go to pieces 

 (fig. 36) . Most of the cells that formerly composed their walls 

 become scattered and rounded, and the anterior space 

 becomes entirely filled (fig. 39). The posterior space, some- 

 what diminished in size, persists, and finally becomes the 



