THE LIFE-HISTORY OP NUCULA DKLPfllNODONTA. 849 



The cells of the epithelial walls of the stomach are of two 

 kinds. Those at the anterior end of the stomach carry com- 

 paratively few cilia^ and those at the posterior end carry 

 many cilia. At this stage some of the cells on the dorsal 

 side of the stomach, near its anterior end, begin to secrete a 

 mucus-like material that extends posteriorly in the lumen 

 of the stomach as a small rod that probably represents the 

 crystalline style (fig. 26). Later the posterior portion of the 

 whole dorsal division of the stomach (the part that at this 

 stage is the dorsal part of the anterior portion) is given over 

 to secreting this material, but a definite rod may not be 

 present. 



About the time that the embryo casts its test the stomach 

 gi'ows dorsally into the space above it, so that a ventral bend 

 is formed where the stomach joins the intestine (fig. 26). 

 This is the beginning of the abrupt bend that marks this 

 portion of the alimentary canal in later life. Two fibre-like 

 cells stretch across the dorsal space from the anterior end of 

 the stomach to the mantle (fig. 25). Their position suggests 

 that they may aid in moving the stomach into the more 

 dorsal position, but there is no direct evidence that this is 

 the case. 



When the test is cast awny and the adductor muscle pulls 

 the shell-valves together, the stomach is crowded further into 

 the dorsal space, and the bend in the intestine becomes 

 more pronounced (figs. 34 and 35) . The same pressure appa- 

 rently causes the liver pouches to go to pieces. Their cells 

 become more or less separated, and fill the larger part of the 

 cavity dorsal to the stomach (figs. 36—39, z.). The same 

 changes have been noticed in embryos of Yoldia limatula 

 and Nucula proxima. In all of these forms the changes 

 occur in connection with the closing of the shell. Until the 

 test is shed, tissue lies between the valves of the shell so that 

 they cannot be shut together. When the tissue is removed, 

 and the shell is closed, there is no longer room for the liver 

 pouches to lie on the sides of the stomach and retain their 

 original shape. Tliey are accordingly flattened and pressed 



