CLASSIFICATION OF AMPHIBIA. 587 



Family 4. Sirenidre. Two extant genera, Siren and Psetidobranchns, 

 both N. American, both with persistent gills, only the 

 anterior limbs. 



Order GYMNOPHIONA or APODA. 



Worm-like or snake-like forms, subterranean in habit ; without 

 limbs or girdles ; with extremely short tail ; with dermic calcified 

 scales concealed in transverse rows in the skin ; in at least some forms 

 (Hypogeophis) external gills are present in the very young stages, but 

 disappear before hatching ; there may be no larval stage ; if there is, the 

 respiration is pulmonary. There are many other striking peculiarities : 

 the eyes are small, covered up, and functionless ; there is no 

 tympanum or tympanic cavity ; there is a peculiar protrusible 

 tentacle in a pit behind the nostril ; there are only two pairs of 

 aortic arches (systemic and pulmonary). The notochord is largely 

 persistent ; the vertebras are amphiccelous ; the frontals are distinct 

 from the parietals ; the palatines are fused with the maxillae. The 

 eggs are large and meroblastic. They are altogether peculiar archaic 

 Amphibians. Examples : Ctccilia (S. America) ; Ichthyophis (Ceylon, 

 India, Malay) ; Hypogeophis (E. Africa) ; Siphonops, without scales 

 (America). 



Order STEGOCEPHALI. 



Extinct forms, occurring from Carboniferous to Triassic strata. 

 The earliest known digitate animals. 



Dermal armour is present, the teeth are frequently folded in a 

 complex manner (Labyrinthodonts). Mastodonsaurus, Dendrcrpeton, 

 ArchegosauruS) Branchiosaurus. 



LIFE OF AMPHIBIANS. 



Most Amphibians live in or near fresh-water ponds, swamps, and 

 marshes. They are fatally sensitive to salt. Even those adults which 

 have lost all trace of gills are usually fond of water. The tree-toads, 

 such as Hyla, are usually arboreal in habit, while the Gymnophiona 

 and some toads are subterranean. 



The black salamander (Salainandra atra] of the Alps lives where 

 pools of water are scarce, and instead of bringing forth gilled young, 

 as its relative the spotted salamander (S. maculosa} does, bears them 

 as lung-breathers, and only a pair at a time. The unborn young 

 have external gills ; removed from the body of the mother and placed 

 in water, they form gills like other tadpoles. Within the mother the 

 respiration (and nutrition) of the young seems to be helped by crowds 

 of red blood corpuscles which are discharged from the walls of the 

 uterus ; the debris of unsuccessful embryos seems also to be used for 

 food. 



Species of Hylodes> such as H. niartinicensis of the West Indian 



