450 GNATHOSTOMATA [CH. 



are jointed rods developed from the inner or splanchnic wall of the 

 coelom ; they cannot therefore be considered as corresponding to 

 the branchial basket of Cyclostomata. They are placed in the 

 forms which retain gill-slits between these openings, and hence are 

 often called gill-bars. The first pair of visceral arches lie in the 

 sides of the mouth, and consist on each side of two pieces, hinged 

 on one another and called the upper and under jaws respectively. 

 By the motion of these on one another the mouth can be opened 

 and closed. The nose is always represented by two sacs" and the 

 ear has three semicircular canals. 



The Gnathostomata are divided into five classes which can be 

 grouped into two sub-divisions. In the first sub-division, termed 

 Anamnia, which includes Pisces and Amphibia, the whole of the 

 egg is converted into the body of the embryo, and though a ventral 

 protrusion of the hind gut termed the allantois may be formed, 

 this does not function as a special embryonic organ. In the second 

 sub-division, termed Amniota, a portion of the egg is converted 

 into a hood termed an amnion, which envelops the body of the 

 embryo and is cast off at birth, and the allantois is enlarged during 

 embryonic life and functions as respiratory or nutritive organ. The 

 greater part of the allantois is cast off with the amnion at birth, 

 and only the stump persists in the adult and functions as urinary 

 bladder. In the first three of these the temperature of the body 

 varies with that of the surrounding medium. 



SUB-DIVISION I. ANAMNIA. 

 Class I. PISCES. 



Gnathostomata with fins supported by fin-rays and breathing 

 chiefly by gills. 



Class II. AMPHIBIA. 



Gnathostomata with pentadactyle or five-fingered limbs and 

 without fin-rays. Gills and gill-slits functional in the young but 

 generally entirely lost in the adult. An allantois is formed which 

 functions as a urinary bladder in the adult, but has no function in 

 the embryo. The skin is soft and moist. 



SUB-DIVISION II. AMNIOTA. 



Class III. REPTILIA. 



Gnathostomata with pentadactyle limbs. The young are born 

 similar to the adult. The allantois is greatly enlarged in the 



