1914] Sharp: Diplodinium ecaudatum 81 



When empty the caecum cannot be distinguished in the living animals, 

 but when well filled it may be discerned as a round or balloon-shaped 

 structure whose walls become more and more definite as they approach 

 the rectum. We noted (p. 68) under the description of the boundary 

 layer that in the stained specimens this layer, together with its asso- 

 ciated alveolar layers, is lost upon the sides of the caecum. The caecum 

 empties directly into the rectum. 



Rectum. — The rectum {red., pi. 4, fig. 3 ; pi. 6, figs. 16-19 ; pi. 

 7, figs. 31-33), short, but well defined, elliptical in cross-section, leads 

 from the caecum to the cytopyge or anal opening {an., pi. 4, fig. 3). 

 In the case of the rectum the three layers, i.e., (1) cuticular, (2) 

 alveolar, and (3) boundary layer, may be seen. 



It is intensely interesting to note, just at this point, that during 

 the process of organ formation, in the predivision stage, the new oral 

 cilia form in a little cavity which is situated in the ectoplasm between 

 the ventral edge of the oesophageal wall and the ventral surface of 

 body at about the level of the posterior contractile vacuole. As 

 development progresses this ventral edge of the oesophageal wall is 

 forced more and more toward the central axis of the body. When 

 division is completed the little pocket in which the oral region of the 

 posterior animal was developed now becomes the caecum of the an- 

 terior animal, and the right and dorsal wall of this caecum is formed 

 by that part of the ventral edge of the oesophageal wall which was 

 pushed in by the developing oral cilia as described above. The ventral 

 wall is formed by the boundary layer. This becomes all the more 

 interesting when, as will be described under observations on the living 

 animals, it will be noted that the internal posterior current (current 

 No. 3) of the entoplasm is directed obliquely from the left ventral 

 side above towards the right ventral side below, i.e., towards the most 

 open side of the caecum. Further it is to be noted that lower down 

 the whole dorsal wall of the rectum is formed by what was once the 

 ventral edge of the oesophagus, hence this wall is richly supplied with 

 fibrillae which, moreover, before division were oesophageal retractor 

 strands. That these strands or fibrillae {red. f., pi. 4, fig. 3) have 

 undergone a certain amount of atrophy or degeneration owing to 

 disuse, or possibly a change in function is indicated by the fact that 

 with Mallory's modified connective tissue stain they no longer stain 

 as distinctly and as intensely as the oesophageal fibers from which 

 they were derived. After iron haematoxylin, however, they are quite 

 distinct. Whether these strands still retain some of their retractile 



