168 ZOOLOGY. 



gill-slit is supported by the presence of an open slit (the 

 spiracle) in the dogfish in an entirely equivalent position. 

 But this matter will be more intelligible when the student 

 has worked over the dogfish, and need not detain us any 

 further now. 



Moreover, the fact that the hypoglossal nerve emerges 

 behind the first vertebra in the frog, and through the ex- 

 occipital in the dog, seems to show that in the latter the 

 skull has encroached, as it were, on a region originally 

 belonging to the vertebral column. 



