1914] Sharp: Diplodinium ecaudatum 79 



what larger form and one, moreover, in which these oesophageal struc- 

 tures are especially clear. 



As the ventral or external wall of the oesophagus comes to lie 

 against the skeletal structure (pi. 7, figs. 28, 29) it is separated from 

 it only by the boundary layer, and, as the inner wall disappears, or, 

 more accurately speaking, separates along its mid-line and its sides 

 also become flattened against the boundary layer, it becomes more 

 and more difficult to distinguish the latter from the oesophageal wall. 

 As this oesophageal wall, if such it may still be called, approaches the 

 posterior end it comes to extend over more and more of the circum- 

 ference of the boundary layer until (compare figures 29 and 32, plate 

 7) at the posterior extremity it seems to completely encircle the ento- 

 plasm and is distinct from the boundary layer only in the region 

 of the rectum which seems to pass down between the two (pi. 7, figs. 

 31, 32). The manner in which the rectal sheath is formed will be 

 considered below. It will suffice here to note that it contains fibrillae 

 (red. f., pi. 4, fig. 3) which appear to be of the same origin and take 

 the iron-alum haematoxylin stain in the same intensity as do the 

 oesophageal rectractor strands. At the extreme posterior end of the 

 entoplasm which is just dorsal to the anal opening all these fibrillae 

 or retractor strands meet in a point which is probably the region of 

 final constriction at the time of division. As noted above, the oeso- 

 phageal wall comes to lie against the boundary layer and the skeletal 

 structure at about the middle of the body, i.e., below the fusion of the 

 right, ventral, and left skeletal structures. At just what point or 

 points the oesophagus is attached to the skeletal structures has so far 

 defied an exact determination, but that such attachments are made 

 seems altogether certain. 



A study of the retracted forms (fig. D) justifies this belief and 

 also furnishes evidence which tends to prove that the function of these 

 oesophageal strands is one of retraction. This conception would at 

 least explain how the whole oral and adoral region is pulled into the 

 body when the organism is irritated. Either such a complicated 

 oesophageal structure does not exist in the heretofore described ciliates 

 from the horse and the ruminant, or it has been overlooked by previous 

 investigators. 



We have seen that in the normal, active condition a large portion of 

 the anterior end of the body is taken up with the organs of locomotion 

 and nutrition. Three other structures, however, which are also situated 

 at the anterior end of the animal, deserve description. To these struc- 



