ON THE ANATOMY OV HISTEIOBDELLA HOMAEI. 315 



than in the adult. A more or less slender oesophagus leads 

 dorsalwards and backwards from a quadrilateral-shaped 

 mouth (text-fig. 4). This^ although small, is capable of 

 considerable expansion. It is completely everted in allowing 

 the jaws and teeth to be protruded in the act of biting. It 

 commences in a slight ciliated depression, which rapidly 

 deepens into a groove in the anterior part of the head. The 

 oesophagus terminates, on a line with the posterior boundary 

 of the jaw musculature, in a narrow constriction leading into 

 the stomach. It is difficult to say where the mouth ends and 

 the oesophagus commences. The mouth and oesophagus are 

 lined throughout with fine cilia, those of the oesophagus 

 being much stouter than those of the mouth. 



Text-fig. 4. 



4. 



Showing the outline of the mouth when partially closed. 



The stomach may be defined as that portion of the intestinal 

 tract lying between the first and the third segment. Its wall 

 is composed of a single layer of cubical cells. It is for the 

 most part uniform in thickness. The rounded internal ends 

 of cells project irregularly into the lumen and are ciliated. 

 At the anterior end, near the oesophagus, the cells are very 

 columnar and contain many granules. They have probably 

 to do with the elaboration of the digestive secretions, as they 

 are seen to be very opaque after tlie animals have taken food. 

 Those of the ventral wall in this part are somewhat larger 

 than the dorsal. The nuclei are always placed at the bottom 

 of the cell, that is, farthest from the internal ciliated surface. 

 In the anterior region they are long and oval in shape, while 

 in the middle and posterior regions they are spherical, and 

 the cells themselves cubical in outline. In the posterior 



