THI<; LIFE-HISTORY OF NUCULA DELPHI NODONTA. 343 



servatives, but it seems probable tbat the two spaces are 

 naturally more or less definitely connected around the sides 

 of the stomach and intestine^ and that they may be regarded 

 as a single cavity — a schizocoele. 



The side walls of the foot join the stomodseum, and are not 

 continuous with each other ventrally (figs. 20 and 28). Just 

 before the test is cast away they begin to unite dorsal to the 

 stomodseum, and the sfcomodasum becomes comparatively 

 free. This change begins at the posterior end of the foot 

 and works forward. 



The process of casting is slow, and includes a large part 

 of the stomodaeum. When it is completed, the foot consists 

 of a small mass of tissue, lying ventral to the stomach and 

 intestine (figs. 34 and 35, /.). It is not capable of executing 

 movements, and for a period of about a day, or even longer, 

 the embryo lies perfectly quiet with the shell-valves tightly 

 closed. At first I supposed that this comparatively im- 

 mature condition of the foot at the time of casting was con- 

 nected with the protected life of the embryo. The foot of 

 Yoldia executes movements before the test is shed, and bur- 

 rowing is begun almost as soon as the process is completed. 

 It seemed natural to conclude that the greater development 

 of Yoldia at this time depended upon the necessity for self- 

 preservation. It was surprising, then, to find that at a cor- 

 responding time the foot of Nucula proxima is no better 

 developed than is the foot of Nucula delphinod onta. 

 This seems very remarkable to me, for Nucula proxima 

 inliabits muddy and shelly bottoms over which flow quite 

 strong tidal currents. Under these conditions it would seem 

 that such perfectly helpless embryos would surely perish. 



The foot of Nucula delphinodonta grows rapidly, and 

 by the second day (fig. 36) performs feeble movements, but 

 it is not thrust out of the shell for some time. It becomes 

 provided with cilia (figs. 39 and 40), but they are not as 

 powerful as those on the foot of Yoldia (Text-fig. K), and 

 they are of but little service in locomotion. 



The first movements of the foot are feeble twitches. These 



