FUNCTION AND STRUCTURE 



73 



is much longer than the body of the insect, and hence 



much bent and coiled, consists of a pharynx, oesophagus, 



fore-stomach or proventriculus, 



true digesting stomach or ven- 



triculus, intestine, and rectum 



which opens at the posterior 



tip of the body. The inner 



lining of the canal shows much 



differentiation in the different 



parts of the canal, and there 



are numerous accessory glands 



connected with various parts of 



the canal. 



Finally, among the highest 

 animals, the vertebrates, we 

 find still more elaborate special- 

 ization of the alimentary canal. 

 As an example the alimentary 

 canal of a cow has already been 

 described in detail. 



43. Stable and variable char- 

 acteristics of an organ. In 

 spite of all this variation in 



the structure and general character of the alimentary 

 canal, there are certain characteristics which are features 

 of all alimentary canals. In the examination of an organ 

 we must ever distinguish between its so-called constant or 

 stable characteristics and its inconstant or variable charac- 

 teristics. The constant characteristics are the fundamen- 

 tally essential ones of the organ ; the variable ones are the 

 special characteristics which adapt the organ for the pecul- 

 iar habits of the animal possessing it habits which may 

 differ very much from those of some other animal of similar 

 size, similar distribution, similar abundance. 



44. Stable and variable characteristics of the alimentary 

 canal. A tiger or a lion has an alimentary canal not more 



PIG. 36. Cockroach dissected to show 

 alimentary canal, al. c. After HAT- 

 SCHEK and Com. 



