513 



than the embryos of Yoldia, and externally they are evenly ciliated 

 with short cilia. These cilia are very much alike on the tests and 

 apical plates. They serve to keep current of water moving, but they 

 are not sufficiently powerful to lift the embryos from the surfaces on 

 which they rest. 



-~— MM hUu 



int 



ap 



std' 



Fig. 18. Median sagittal section of an embryo of Nucula delphinodonta, at a stage 

 just before the test is cast off. aa anterior adductor muscle, ap apical plate, int intestine. 

 sto stomach, std stomodseum. t test. 



Inside of the test, the shell has been formed, and the two valves 

 gape ventrally. The anterior adductor muscle, Figs. 17 and 18 aa, 

 occupies an isolated position. The apical plate, ap. lies close to the 

 stomach, sto, and is relatively quite large. The stomodseum, std, is 

 quite widely separated from the rest of the alimentary canal and, as 

 the foot has hardly begun to develop, there is quite an unoccupied 

 space between them. The stomodamm lies on the ventral side of the 

 embryo, close to the test, and opens postero-ventrally between the 

 test-cells. The intestine opens posteriorly by a separate opening be- 

 tween the test-cells. Which opening represents the position of the 

 blastopore, has not been determined. The pouches of the digestive 

 glands, Fig. 17 rl, communicate with the stomach. A large unoccupied 

 space has been left by the developing mantle, dorsal to the intestine. 

 The posterior adductor muscle, the ganglia, and the otocysts have not 

 been formed. The test is attached to the body of the embryo, around 



