THE FROG 19 



balls ; (s) the tongue, thin and fleshy, attached to the front 

 part of the floor of the mouth with its free end, which is 

 " forked," projecting backwards towards the throat. 



Whilst there is a general correspondence between the mouth 

 and buccal cavity of the frog and our own, there are consider- 

 able differences, of which the most important are, the absence 

 in the frog of mobile lips ; the marginal teeth of the upper 

 jaw correspond in position to ours, but they are more 

 numerous, are not implanted in sockets, are all alike simple 

 and pointed, instead of being of various shapes as ours are. 

 The vomerine teeth of the frog have no counterpart in man. 

 The posterior nares, situated right forward in the upper jaw of 

 the frog, are carried backwards in man by the formation of the 

 "roof of the mouth" or hard palate, followed by a fleshy 

 curtain or soft palate, so that they open far back at the 

 entrance to the throat. The hard palate in man also excludes 

 the orbits altogether from the region of the mouth. The 

 tongue of man, fixed at the back of the mouth, with its free end 

 projecting forward, is obviously different from that of the frog. 



Already, after an examination of the external features of the 

 frog, we are able to make a comparison of its structure with 

 our own, and to recognise a general correspondence of plan, 

 with numerous differences in detail. We cannot escape from 

 the conclusion that the frog is built on the same principle as 

 ourselves ; and yet not a single organ or feature that we have 

 examined is exactly or even nearly the same as it is in man. 

 The science of comparative anatomy takes account, both of 

 the resemblances and the differences, and embodies the results 

 of its comparison in a system of classification. The frog, because 

 of the resemblances which we have already noted, and many 

 others of greater importance relating to the internal organs, is 

 placed in the phylum vertebrata along with fishes, reptiles, 

 birds, and beasts. But the differences compel us to place it 

 far apart from man in a class comprising many other animals 

 such as newts and salamanders : in this class. Amphibia, 

 the component members resemble one another in all their 

 more important characters, and differ only in those which are 

 relatively unimportant. But comparative anatomy does more 

 than this. It seeks to give an explanation of the resemblances 

 and of the differences, and it finds the explanation in the 

 doctrine of the common descent of all the animals composing a 



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