FUNCTION AND STRUCTURE 13 
is much longer than the body of the insect, and hence 
much bent and coiled, consists of a pharynx, esophagus, 
fore-stomach or proventriculus, 
true digesting stomach or ven- 
triculus, intestine, and rectum 2% 
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- 16. 36.—Cockroach dissected to show 
Pre alimentary canal, ad. c.—After Hat- 
acteristics of an organ.—In © scuex and Cort. 
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 
speciai 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 
