VI.] THE COMMON FROG, 67 



them away without inducing any apparent injurious 

 effects. In the whole class of Batrachians skin re- X- 

 spiration seems, then, to be of very great importance. 

 The internal skeleton (or the skeleton commonly 

 so called) of the frog presents some points of con- 

 siderable interest, especially as exhibiting its inter- 

 mediate position between fishes on the one hand, and 

 higher vertebrates on the other. First, as regards the 

 backbone^ it may be remembered that it is made up of 

 distinct bony joints (or vertebrce), in which it agrees 

 with all animals above fishes and with bony fishes ; 

 its hinder termination, however, is essentially fish-like. 



Fig. 32. — Coccyx of Frog, lateral view, a black line indicates the course of the 



sciatic nerve. 



It is fish- like because the terminal piece, as it is 

 called, or ^^ coccyx'' (unlike the coccyx in man or in 

 birds) is not composed of rudimentary vertebrae which 

 subsequently blend and anchylose together, but is 

 formed by the ossification continuously of the mem- 

 brane investing (or sheath of) the hindermost part of 

 that primitive continuous rod, or notochord,^ which, as 

 has been said, precedes, in all vertebrate animals, the 

 development of the backbone, making its appearance 

 beneath the primitive groove. 



The vertebrae are shaped like rings, and enclose 

 within their circuit the spinal marrow upon which, as 

 it were, these rings are strung. From the side of each 



' From NwTos, back, and XopSTy, chord. 



F 2 



