NUTRITION PROPER. 3 , r 



One more example: In fishes the whole digestive 

 process is very simple, and the intestines are correspond- 

 ingly short. But in the shark, although externally the 

 intestine is almost a straight tube running through the 

 body, yet internally ■ its surface of absorption is made very 

 large by means of a curious spiral valve. This gives 

 the peculiar spiral marking on the dung of these and 

 of some other lower vertebrates, both living and extinct, 

 for fossil dung of sharks and reptiles is not uncommon. 



SECTION V. 

 Digestive System in Invertebrates. 



General Remarks. — i. As already explained, we 

 shall pursue a different plan here. Instead of following 

 each stage of digestion through the different classes, we 

 shall run through the whole digestive process in each 

 class as taken up. 



2. The complexity, and especially the diversity, 

 among invertebrates is so great that if we took them up 

 with anything like the fullness that we have taken up 

 vertebrates, our object — viz., to make a small volume, 

 giving only an outline of the most interesting points — 

 would be defeated. We will, therefore, only give a few 

 very striking examples from different departments. 



ARTHROPODS. 



Insects : Mouth Parts. — Insects are wonderfully 

 specialized animals. This is especially true of their mouth 

 parts. From this point of view insects may be divided 

 into two groups — viz., the biting and the sucking in- 

 sects, or the mandibulate and the haustellate. The former 

 include the orthopters (grasshoppers), the neuropters 

 (dragon flies), and the coleopters (beetles). The latter 

 include the lepidopters (butterflies and moths), the dip- 



