124 THE COMMON FROG. [chap. 



distributed as in him. It is also so distributed in the 

 adult frog. 



In the tadpole, however, this nerve shows a very 

 different arrangement. After issuing from the skull 

 this nerve sends a branch down the outer side of each 

 branchial arch and then gives off a very long one, 

 which extends laterally, i.e. along the side of the body 

 and tail. 



Nothing like this exists in any Beast, Bird, or Rep- 

 tile, but when we come to the class of Fishes we 

 encounter a precisely similar state of things. Here 

 we find the 8th nerve sending a branch to each 

 branchial arch, and giving off a great nerve pro- 

 ceeding along the side of the body and tail, and 

 on that account named the nerviis lateralis. 



The eye of the frog is a beautiful and brilliant 

 object, and relatively large. It is furnished with two 

 eyelids, but, unlike those of man, it is the inferior one 

 which is the more moveable. In addition to these it is 

 defended by a third eyelid, called the nictitating eye- 

 lid, which is similar to that one which may be seen 

 (if watched for) so frequently and rapidly to cross the 

 eye of birds, e.g. of a hawk. 



This structure, however, Is no mark of affinity to 

 birds, as It Is one which reappears, when wanted, in 

 widely different forms. Thus we find it In the whale, 

 i.e. In the highest class of the Vertebrate sub-kingdom, 

 and In certain sharks, i.e. in the lowest class of the 

 same. 



Eyelids do not exist in all members of the frog's 

 class. Even in its order they are extremely minute, 

 in Pipa and Dactylethra, which have very small eyes. 



