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Hind-gut. 



The Rectum is from two to three centimetres long in 

 a full-grown animal. It is thick-walled and muscular, 

 and packed with the rosette-shaped glands mentioned 

 above. The walls are considerably folded internally and 

 have a thick chitinous lining. They are richly supplied 

 with blood from the plexus of vessels which covers them. 



The AniiS opens on the under surface of the telson 

 and is surrounded by a sphincter muscle. In the living 

 animal the rectum exhibits vigorous peristaltic movement. 



Histology of Alimentary Canal. 



The walls of the oesophagus are formed of very long 

 narrow cells, with a thick base of fibrous connective 

 tissue and an outer layer of muscle fibres. The lumen is 

 lined with cuticle. Mention has already been made of 

 the glands on the oesophageal walls. The cuticular 

 lining is continued through the stomach, and the gastric 

 mill is formed by calcifications in its substance. No 

 other histological feature of the stomach calls for notice. 



The lining of the mid-gut is very characteristic. 

 The cells are columnar, with large nuclei and consider- 

 able contents of fatty matter. They have a striated 

 border which appears in sections as a dark lining to the 

 cells. The muscle layers at the base are frequently 

 thrown into small plications. 



The epithelium of the hind-gut (fig. 22) is more 

 regular than that of the achitinous gut. The nuclei are 

 smaller and there are very scanty cell contents. A thick 

 chitinous layer lines the gut. Behind the basement 

 membrane is a wide layer of muscle bundles and glands 

 which are arranged in very definite clumps. A layer of 

 connective tissue with blood vessels and nerves surrounds 

 the whole. The structure of the paired and abdominal 



