39 



the exterior, but not because the external surface is 

 thrown into folds. It is only the internal layers that are 

 folded, so that as a result the walls are alternately thick 

 and thin. There is no external epithelial layer bounding 

 the alimentary canal, for it runs through a haemocoele 

 and not a true body cavity bounded by an endothelial 

 layer. One finds, therefore, most externally, an attenuated 

 connective tissue layer (fig. 25, Oe. con. ). This gives 

 place to compact dense connective tissue, through which 

 run many muscle fibres (fig. 25, Oe. con.'). Externally, 

 the circular or transverse muscle fibres predominate, 

 internally one finds longitudinal fibres, and this layer is 

 particularly thick and forms the main substance of the 

 folds. The connective tissue sheath is divided, therefore, 

 into two distinct layers by reason of the muscles which 

 run through it. The inner layer with the longitudinal 

 muscles varies in thickness according as to whether 

 it is under a groove or a fold, and forms the support for 

 the epithelium which lines the gut. 



The Ejrithelium is composed of regular columnar 

 epithelial cells, the height of which is about twelve or 

 more times the thickness. There are three kinds evident, 

 viz. : — (1) Ciliated cells, (2) Eosinophilous cells, and (3) 

 Gland cells, and their frequency is in the order given 

 above, the gland cells being least numerous. 



The ciliated cells are very narrow basally, but 

 increase in thickness towards the lumen, and their 

 surfaces form a distinct unbroken plane. They are 

 typical ciliated cells and show very distinctly the double 

 row of desmochondria at the base of the cilia, and the 

 connecting fibres in the cytoplasm. 



Between these cells occur the eosinophilous cells in 

 great numbers. They are more common here than any- 

 where else, and are crowded with small granules which 



