19 



Alimentaky Canal. 



The oesophagus is a short, narrow, curved tube which 

 leads ventrally to the stomach — a large, thick-walled 

 organ, lying on the left-hand side of the branchial sac, 

 imbedded in the body-wall and projecting into the atrium, 

 (as shown in fig. 4, PI. II.). From the other (ventral) 

 end of the fusiform smooth-walled stomach arises the 

 intestine, a long serpentine tube which ends by opening 

 into the dorsal or cloacal part of the atrium, from which 

 the undigested portions of the food are carried to the 

 exterior through the atrial aperture, by the water current. 



The intestine is curved so as to form two loops (PL II., 

 fig. 4), a first between the stomach and intestine, open 

 posteriorly, and in which the ovary lies ; and a second 

 between the intestine and rectum, open anteriorly, and in 

 which the renal vesicles lie. The external convex edge of 

 the intestine is thickened internally to form the "typhlosole," 

 a large pad which runs along its entire length (PI. IL, figs. 

 2 and 4, tij.), reducing the lumen to a crescentic slit. 



The walls of the stomach are glandular ; and, in addi- 

 tion, a system of delicate hyaline microscopic branched 

 tubules with dilated ends (the " refringent organ"), 

 which ramifies over the outer wall of the intestine and 

 communicates with the cavity of the stomach at the 

 pyloric end by nieans of a duct, is possibly a digestive 

 gland. There is in AscicUa no separate large gland to 

 which the name " liver " can be applied as in some other 

 Tunicata {e.g., Molgitla). Over these viscera, on the left 

 side of the body, the body- wall is thin and gelatinous, and 

 has usually no muscle fibres visible. 



The wall of the alimentary canal, throughout its length, 

 consists of an epithelial lining (endoderm), a thick layer 

 of highly vascular connective tissue, and, over that, the 

 flattened epithelium lining the peribranchial cavity. The 



