270 



Mr. G. A. Drew on the 



mantle. The mid-gut, mg^ and the ventral tube, vt^ are both 

 shown in cross-section. 



This stage is of interest in showing the beginnings of the 

 ectodermal thickenings that form the pedal ganglia and the 

 ectodermal invaginations that form the otocysts. 



Fig-. 3. 



Transverse section of an embryo of fifty-eioht hours, taken just in front 

 of the second band of cilia, mg, mid-gut ; sg, shell-gland ; 

 t, test ; vt, ventral tube. 



As the shell-gland spreads out into the mantle, the embryo 

 becomes slightly compressed laterally, the foot appears, and 

 the visceral ganglia may be distinguished. 



Fig. 4 represents a reconstructed embryo of one hundred 

 and five hours. The test-cells, t, are now very flat and much 

 thinner than at an earlier stage. The shell, 5, indicated in 

 outline, is well formed. The anterior adductor muscle, aa^ 

 occupies a ratlier isolated position. The posterior adductor 

 muscle, pa, has just made its aj)pearance. The foot, /, is 

 pretty well formed, and has running over and united to its 



