580 



CHORD ATA. 



Fe 



Class III. Amphibia. 



There are two views as to the origin of the Amphibia. Accord- 

 ing to the one they have descended from 

 Crossopterygian ganoids (and this seems the 

 better supported); the other is that they 

 have come from the Dipnoi. The group is 

 distinguished at once from the fishes by the 

 absence of fins. There is, it is true, a median 

 fin in larval life, and this may persist (Peren- 

 nibranchs, Triton], but it is never divided 

 into dorsal, caudal, and anal, and it lacks 

 any skeletal support (figs. 4, 5). The paired 

 fins are replaced by pentadactyle feet (p. 529). 

 These are often webbed and are used for 

 swimming; they are also used for creeping 

 and leaping, and are consequently jointed 

 between the separate skeletal elements (fig. 

 610). Besides the shoulder and hip joints, 

 which alone occur in fishes, there occur also 

 elbow (knee), wrist (ankle), and finger joints. 

 The number of digits is not always five, for 

 a reduction to four, three, or eve a two 

 occurs. 



The connexion of the girdles with parts 

 of the axial skeleton (lacking in most fishes) 

 is of importance. The pelvic girdle is con- 

 c, oentrale; F, nected with the vertebral column by means 

 ; i intermedium; f~ of the ilium, which articulates either directly 



metacarpals and digits. 



or by a sacral rib with the single sacral ver- 



tebpa> y entrall y the t WQ halveg of the gj rd](> 



fuse, and usually the limits of ischium and pubis cannot be traced. 



The attachment of the pectoral girdle is less firm (fig. 564, A}. 

 The dorsal portion, the scapula, ends free in the muscles; the 

 ventral, differentiated into coracoid and clavicle, is often connected 

 with the sternum, but this is 'not connected with the vertebral 

 column, since the ribs are too short to reach it. The sternum is 

 frequently connected with an episternum. 



The vertebral column often (Perennibranchs, Derotremes, 

 Caecilians, and many Stegocephali) resembles that of fishes in 

 amphicoelous centra and persistence of notochord. The notochord 

 may disappear, there then occurring opisthocoelous (Salamandrina) 



