510 COLLEGE ZOOLOGY 



later, and, when the external gills disappear, these function in 

 their stead, the water entering the mouth, passing through the 

 gill-slits, and out of an opening on the left side of the body, called 

 the spiracle. 



The hind limbs appear first (Fig. 426, 5). Later the fore 

 limbs break out (6). The tail decreases in size as the end of the 

 larval period approaches and is gradually resorbed (7). The 

 gills are likewise resorbed, and the lungs develop to take their 

 place as respiratory organs. Finally the form resembling that 

 of the adult frog (S) is acquired. 



2. A Brief Classification of Living Amphibia l 



There are about one thousand different species of Amphibia — 

 a number very much smaller than that of the other principal 

 classes of vertebrates. Approximately forty of these belong to 

 the order Apoda, one hundred to the Caudata, and nine hun- 

 dred to the Salientia. 



Order 1. Apoda (Gymnophiona, Fig. 427). — Ccecilians. — 



Worm-like Amphibia without limbs or limb-girdles; 



usually with small scales embedded in the skin; tail 



short or absent. 



Family Cceciliid^:. — With the characters of the order. 



Examples: Dcrmophis, Ccecilia, Gymnopis, Siphonops, 



Ichthyophis (Fig. 427). 



Order 2. Caudata (Urodela, Figs. 428-433). — Tailed Am- 

 phibia. Amphibia with a tail ; without scales; usually 

 two pairs of limbs; the adults with or without external 

 gills and gill slits. 

 Suborder 1. Proteida (Fam. Protehle, Fig. 428). — Mud- 

 puppies.— Tailed Amphibia with two pairs of limbs; 

 three pairs of external gills and two pairs of gill-open- 

 ings persistent; no eyelids. 



*I am indebted to. Dr. Alexander G. Ruthven for the main divisions of this 

 classification. 



