14 



A PRIMER OF BIOLOGY 



a definite opening, the mouth, and not by the entire 

 surface of the body as in Vaucheria ; in its passage 

 through a tract known as the alimentary canal, it 

 undergoes certain complex changes, due to the ad- 

 mixture of secretions from glands which line or com- 

 municate with the 

 alimentary canal, 

 changes which are 

 collectively spoken 

 of as digestion. The 

 digested substances 

 are thereafter ab- 

 sorbed from the ali- 

 mentary tract by a 

 system of vessels, 

 and are carried by 

 their means to all 

 parts of the body, 

 the motive power 

 being the rhythmic 

 contractions of a 

 muscular organ on 

 the path of the cir- 

 culation, viz., the 

 heart. The mechan- 

 FIG. 7. Alimentary canal of frog : a, ism of nutrition 

 gullet ; b, liver ; c, stomach ; d, in- would thus appear to 

 testine; e, pancreas; /, rectum. be much mQre CQm . 



plicated in the frog than in Vaucheria, but a little 

 reflection will show us that the principle is the 

 same in both cases, viz., the assimilation of certain 

 organic substances by living protoplasm after these 

 have been altered within the organism into suit- 

 able forms. 



Again, at certain seasons the frog multiplies its 



