No. 522] THE CANADIAN OYSTER 



position, for after the loss of velum and foot there is no 

 longer any need of such free movement, consequently 

 these muscles may lag behind other parts in growth, ex- 

 erting a tension as they do so sufficient to cause the rota- 

 tion. In harmony with this view is the similar upward, 

 movement of the anterior adductor muscle and the rapid 

 downward movement of the posterior adductor muscle. 



In 1 mm. spat the two sides of the mantle, meeting- 

 above at a broad angle, appear to be suddenly thickened 

 (Fig. 4) into a sort of upper lip for the widely open, 

 transversely crescentic mouth, that opens a little farther 

 back on the right side than on the left. The anterior 

 end of the body, between the first gill filaments, serves 

 as lower lip, while very short upper and lower palps, 

 possessing only a couple of ridges and furrows, are con- 

 tinuous with the lateral margins of the upper and lower 

 lips, and project on each side of the anterior end of the 

 median left gill. In 2 mm. spat (Fig. 13) the palps have 

 increased perceptibly, doubled the number of furrows, 

 turned down, and are seen to be anterior to, but not con- 

 tinuous with, the gills. Judging from the great progress 

 made between these two sizes, I believe that the palps 

 originate sometime before the spat reaches 1 mm. in 

 height, but are not present as such in the larva, where 

 upper and lower lips and palps are represented by the 

 internally-ciliated, funnel-like, projecting rim of the 

 mouth. When this rotates upwards it comes to lie be- 

 tween the mantle above and the forward growth of the 

 abdomen below, while its side-angles fold outwards and 

 backwards becoming the palps, their like surfaces turned 

 towards each other and their cilia continuous with that 

 of the mouth and oesophagus. Later they become ad- 

 herent to a greater extent along the lateral walls of the 

 anterior part of the enlarging abdomen, and direct food 

 from the gills into the mouth. 



Rice (33, p. 28) curiously made the observation: "During the first 

 period of attachment when the shell itself is not firmly attached, but 



