SECT, vii THE ALIMENTARY CANAL 115 



as showing a perfect transition stage between the 

 simple digesting diverticula of the Annelids, and the 

 hepato-pancreatic glands of the higher Crustacea. 

 In Apus, particles of food are found in all the wider 

 parts of the diverticula which, like the rest of the 

 mid-gut, are lined with ciliated epithelium. Smaller 

 branching invaginations from these diverticula (see 

 Fig. 29) contain large glandular cells, which occur in 

 great numbers towards their tips. The secretion of 

 the glands is no doubt forwarded by the ciliated 

 epithelium, which is everywhere present when not 

 entirely displaced by the glandular cells. In the 

 preserved specimens this secretion formed brown 

 crystals. 



In order to turn these digesting diverticula, pro- 

 vided with glandular cells at their branched distal 

 ends, into the lobate hepato-pancreas of the higher 

 Crustacea, we have only to imagine the glandular 

 cells increasing so as entirely to displace the ciliated 

 digesting epithelium, and the lumen of the diverticula 

 themselves narrowed to form glandular ducts. 



In Astacus, where the mid-gut has almost en- 

 tirely disappeared, these diverticula are highly deve- 

 loped tassel-like hepato-pancreatic glands. Apus 

 supplies us with a perfect transition stage, showing 

 the origin of these livers out of the digesting diver- 

 ticula of the Annelidan mid-gut. 



The epithelium of the mid-gut, like that of the 

 diverticula, is composed of minute thread-like cells 

 with nuclei near their basal ends. They stand on a 

 basal membrane, round which run at short intervals 



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