348 



OILMAN A. DREW. 



by the separation of some of the larger cells, accompanied 

 by their division into smaller cells. This ponch is the first 

 indication of the alimentary canal (fig. 8, mg.). Partly by 

 the division of cells forming it, and partly by the addition of 

 ectodermal cells around the blastopore, the gut is carried 

 further into the interior (figs. 9 and 11), and comes to lie at 

 the end of a narrow tube, the stomodseum (fig. 15, std.). The 

 blastopore never closes, so from its first appearance the 

 stomoda3um is connected with the gut. 



The blind end of the gut turns dorsally beneath the shell- 

 gland (fig. 11, mg.), and soon begins to grow posteriorly 

 (fig. 15, int.). It finpJly comes to the surface at the posterior 

 end of the embryo at a point ventral to the shell-gland and 

 dorsal to the blastopore (fig. 24), where the anus is formed. 

 The anus does not open directly to the exterior, but opens 

 into the mantle chamber near an external opening in the test. 



The alimentary canal at this stage consists of three distinct 

 parts (fig. 24) : a slender tube, the stomodseum {std.), opening 

 through the blastopore and extending forward nearly to the 

 apical plate that is formed from the ectoderm ; a rather thick- 

 walled stomach {sto.) that lies dorsal to the anterior end of 

 the stomoda3um, and ventral to the shell-gland j and the 

 intestine [int.), which joins the posterior end of the stomach, 

 and at first has rather thick walls. 



Dorsal to the stomach and intestine, between them and the 

 shell-gland, a cavity makes its appearance that communicates 

 by lateral passages with another cavity that lies ventral to 

 the stomach and intestine, between them and the stomodseum. 

 The ultimate fate of these cavities has been referred to in the 

 sketch of the life-history, and in connection with the foot, 

 and will again be referred to in connection with the peri- 

 cardium. For some time they are rather large, and a portion 

 of the alimentary canal is left quite free from surrounding 

 tissue, except where it seems to rest on the walls of the 

 developing foot (fig. 28). A short time before the test is shed 

 the liver pooches make their appearance (fig. 25, I.). These 

 are formed from the sides of the anterior end of the stomach. 



