72 COMPARATIVE ANATOMY 



sac is divided up and complicated by the projection into it of 

 cartilaginous offsets of the walls of the nasal chambers, and 

 three main subdivisions or sinuses have been recognised in 

 each sac a dorsal, a ventral, and a lateral sinus. These 

 sinuses are lined by an epithelial membrane which in the 

 olfactory region is modified to serve as the percipient 

 element. 



We have thus far treated of the anatomy of the frog as it 

 may be learned from simple dissection ; and we have not, in 

 dealing with the various tissues and organs, considered the 

 ultimate elements of which these are composed. But we have 

 already gathered important information respecting the plan of 

 structure of the frog's body. We have seen that it has a 

 special cavity, the coelom, in which the abdominal viscera 

 appear to be suspended, so we recognise the frog as belonging 

 to the group Coelomata. From its possessing a vertebral 

 column with a central nervous tube placed dorsally to it we 

 recognise it as being a Vertebrated animal ; from its having a 

 distinct head we recognise it as being a member of the 

 Craniate branch of the vertebrata. As it has distinct jaws it 

 must be placed among the Craniata Gnathostomata ; and, as 

 it has five-fingered arms and legs, and not fins, it must be a 

 member of the sub-grade Pentadactyla of that group. Finally, 

 the smooth glandular skin, the absence of nails or claws, the 

 fact that it breathes when a tadpole by gills, when adult by 

 lungs, show it to be a member of the class Amphibia. 



