492 



PEOF. ALLMAN 01^ THE STETICTURE AND 



uniformly cover this layer. On the tentacular sheath and on 

 the under part of the body-wall they are disposed in separate 

 groups, each of which is seated on a small elevation determined 

 by the presence of a nucleus. 



Structure of Alimentary Canal. — The histological structure of 

 the alimentary canal is very similar to that of the endocyst. Its 

 walls, for the greater part of their extent, are here also composed 

 o£ three layers. The outer is an epithelial layer continuous with 

 the epithelial or inner layer of the body-wall. It is, however, 

 destitute of cilia, and is composed of flattened fusiform cells, con- 

 taining a nucleus, but without any cell-membrane. At the blind 

 end of the stomach it increases in thickness, and is here continued 

 over the funiculus. 



Next to the epithelium is a muscular layer composed of fibres 

 supported by a transparent homogeneous foundation membrane. 

 The fibres are flat bands pointed at each end, and having for the 

 most part a longish nucleus in the middle. They run trans- 

 versely round the alimentary canal, with the pointed ends of each 

 wedged into the intervals of others. Nitsche has noticed in them 

 a kind of striation, but has not satisfied himself that this depends 

 on an essential structure of the fibre. The muscular fibres are 

 absent on the extreme point of the stomach where this passes 

 into the funiculus. 



The most internal stratum of the alimentary canal consists in 

 the stomach of a single layer of cells. Here this layer is thrown 

 into longitudinal ridges, whose cells contain brown granules, which 

 possibly indicate a hepatic function. The ridges have been shown 

 by Nitsche to consist of certain cells of this layer which have be- 

 come elongated, and are sometimes enlarged at their free extre- 

 mities so as to present a club-shaped form*. 



The inner layer of the rectum appears also to consist of a simple 

 layer of cells. These are prismatic, and sit vertically on the mus- 

 cular layer. Each is provided with a nucleus at its base ; and as 

 they are all of equal length, the longitudinal ridges of the stomach 

 are here wanting. 



In the oesophagus the cells which correspond to this layer pre- 

 sent, according to Nitsche, a very remarkable condition. He 

 describes them as being of long prismatic shape with the long 

 This view is apparently the right one, and is a rectification of a somewhat 

 different description of the longitudinal ridges given formerly by myself (' Mo- 

 nograph of the Freshwater Polyzoa '). 



