TUBIFEX. 321 



the form of a straight tube extending from the mouth, 

 situated on the ventral surface of the first segment, to 

 the anus, which is surrounded by the last segment of the 

 body, or the anal segment. 



The alimentary canal can be divided into the 

 following regions: — (1) mouth, (2) buccal cavity, 

 (3) pharynx, (4) oesophagus, (5) intestine, and (6) anus. 



1. The Mouth. The mouth is ventral in position, 

 surrounded by the first segment of the body or 

 peristomium, and overhung by the prostomium. When 

 closed it appears as a transverse slit bounded by a 

 slightly puckered wall, but when it is open the aperture 

 is rounded (PI. II, fig. 4, mo.). 



2. Buccal Cavity. The mouth leads into the buccal 

 cavity, which is short — only extending through the first 

 segment of the body. It is partly covered by the cerebral 

 ganglia, dorsally. The buccal cavity is capable of 

 extrusion, but this does not seem to take place very often 

 under ordinary conditions. I have, however, frequently 

 observed it when ether has been added to the water in 

 order to quiet the worm when it is under observation. 

 The buccal cavity can then be seen as a somewhat frilled 

 organ protruding from the mouth. When once the 

 cavity has been extruded, it is not readily drawn in again. 



A transverse section of the buccal cavity, when not 

 extruded, shows it to be of wide calibre, with a straight, 

 unfolded wall. The latter is very thin, and is composed 

 of a single layer of epithelial cells, somewhat cubical in 

 shape, with well-marked nuclei and numerous short cilia. 

 Outside this layer of epithelial cells a few muscle fibres 

 are scattered, but they are not sufficiently numerous to 

 form definite muscular layers (PI. II, figs. 4 and 7, bu.c). 



3. The Pharynx. Leading out of the buccal cavity 

 is the pharynx, which extends through segments 2 and 3. 



