226 ARTHUR E. SHIPLEY. 



alimentary canal (fig. 12). When the introvert is extended, 

 the first part of the digestive tube or the oesophagus forms a 

 straight tube with smooth walls ; when, however, the introvert 

 is retracted, the walls of the oesophagus are thrown into a 

 number of circular folds with intervening depressions. The 

 cells lining this part of the alimentary canal are cubical, and 

 thickly beset with cilia. 



Throughout the intestine the lining epithelium is surrounded 

 by a layer of connective tissue, which is in its turn covered by 

 the peritoneal epithelium ; the connective tissue varies in thick- 

 ness in different parts of the tube, but it is especially thick on 

 the dorsal surface of the anterior end of the oesophagus : it is 

 just here that the single retractor muscle is inserted. 



The oesophagus passes into the descending intestine, whose 

 walls are lined by large glandular cells : these have, when the 

 intestine is comparatively empty, a columnar shape ; but if the 

 intestine is full of food its walls are stretched, and the lining 

 cells become cubical, or even depressed. Owing to the small 

 size of the animal it is not possible to wash the food out of 

 the alimentary canal, and the nature of the food rendered it 

 very difficult to cut satisfactory sections of the walls of the 

 alimentary canal. These were in most cases torn ; hence I have 

 not been able to settle quite definitely whether the cells lining 

 the descending intestine are ciliated or not, but I am inclined 

 to think they are. 



The ascending intestine is certainly lined with ciliated cells. 

 It is distinguished by the possession of a longitudinal groove, 

 which is lined by cells bearing especially long and large cilia. 

 A similar groove is described by Mr. E. A. Andrews in Sipun- 

 culus Gouldii.^ He states that "in it a current of liquid 

 passes from the action of cilia, and possibly also of the radiat- 

 ing fibres, towards the anus during life." The absence of this 

 groove is the only thing which distinguishes the short rectum 

 from the descending intestine. 



^ "Notes on the Anatomy of Sipunculus Gouldii, Pourtales," E. A. 

 Andrews, 'Studies from the Biological Laboratory,' Johns Hopkins Uni- 

 versity, Oct., 1890. 



