TWO AQUATIC OLIGO-CHUTES. 11 



3). The oesophagus is confined to segment IV. Its lumen 

 is flattened dorso-ventrally and its wall is similar in structure to 

 that of the pharynx, except that the endodermal cells are not so 

 long, and the muscle fibres extending between the alimentary 

 canal and the body wall are absent. The chloragogue cells are 

 rarely seen attached to the wall. 



4). The intestine begins in segment V. In cross- section its 

 lumen, for the greater part, is capacious and nearly circular, and 

 is constricted by the septa at the intersegmental lines ; but in the 

 posterior part, which is shorter, it becomes tall pentagonal in out- 

 line and finally squarish. The anus is dorso-posterior in position. 



The intestinal wall apparently consists of three layers, 

 viz. endodermal epithelium, vascular layer, and chloragogue 

 layer. The ciliated epithelial cells are large, measuring 20-25 /^ 

 by 7-1 p, and have a compact cytoplasm. The nuclei lie more 

 in the distal half of the cells, and contain each a nucleolus. 

 With Mallory's stain two kinds of epithelial cells come to view. 

 One shows a great affinity for anilin blue and the other for orange 

 G, and the interesting points are that the cilia of the latter cells 

 are stained more deeply blue than those of the former, and that 

 the blue stained cells appear to be more glandular than the 

 orange stained. There are also others of intermediate character. 

 It is therefore very probable that these cells are different stages 

 in the activities of the epithelial cells. 



The epithelial layer of the intestine is closely followed by 

 the vascular layer, and outside the latter lie the intestinal 

 muscle layers. The intestinal muscles are also disposed in two 

 layers like those of the body wall, viz. the inner circular and 

 the outer longitudinal. They are both feebly developed and the 

 individual fibres stand apart from one another. 



The chloragogues are, as is generally supposed, but modifica- 

 tions of the peritoneal epithelium. The chloragogue layer is well 

 developed in the anterior segments, especially in V-VIII. In the 

 posterior segments, however, the cells lose their characteristic 

 appearance and become ordinary connective tissue layer, as around 

 the pharynx and the oesophagus. 



