145 



a double thickness of mid- gut wall as well as the 

 fore-gut. There is an increase in the muscular 

 fibres of the mid-gut at this point, especially the 

 circular fibres, so that a very distinct mass is 

 formed homologous to the proventriculus of many 

 insects. There is no chitinous development, how- 

 ever, and the structure would appear to act only 

 as a muscular sphincter (Fig. 30). 



The Hind-gut. — The nature of the epithelium 

 and arrangement of the muscular fibres differs 

 somewhat in different portions of the hind- gut. 

 Structurally the small and large intestine are 

 similar, whilst the dilatation beyond the pylorus, 

 and especially the rectum, differs from these. 



The dilatation which occurs at the origin of 

 the malpighian tubules is thin-walled and poorly 

 supplied with muscle fibres. The cells lining it 

 are small and flattened. 



The intestine is lined with a single layer of 

 large cubical cells ; external to these is a muscular 

 coat. The cells of the intestine have large nuclei 

 which have a similar, though more open, arrange- 

 ment of the chromatin than the nuclei of the mid- 

 gut. The protoplasm is finely reticular, and stains 

 less deeply than the cells of the mid-gut. Stained 

 with Heidenhain's haematoxylin, no granules are 

 present as in the cells of the mid- gut. They have 

 no striated border. 



In the rectum the cells become small and 

 flattened. There are here, however, bodies usually 

 termed rectal glands. These are papillae covered 

 with a single layer of much hypertrophied cells 

 resembling those lining the small intestine and 

 colon. 



The muscular system of the hind- gut is very 



