532 



AMPHIBIA. 



dermis differs markedly from that of a fish, for there is no 

 exoskeleton, but this was present in the extinct Labyrintho- 

 donts ; there are imdtkellular glands, whose secretion makes 

 the skin moist ; and there is a stratum of unstriped muscle 

 fibres. In the dermis there are also branched pigment 

 cells, usually in two strata. Through a reflex nervous action 

 they are slightly affected by the colour of the surroundings, 

 the pigment-bearing internal cell 

 substance contracting or expanding, 

 and thus producing colour change. 

 There are cutaneous blood vessels, 

 by means of which the frog can, to 

 a certain extent, breathe by its skin. 

 The tadpole has sensory cells ar- 

 ranged in distinct lateral lines, but 

 of these the adult retains no definite 

 trace, though there are many nerve- 

 endings and " touch spots " in the 

 skin. 



Skeleton. 



The vertebral column consists of 

 nine vertebrae, and an unsegmented 

 portion called the urostyle. 



The first vertebra bears two facets 

 for the two condyles of the skull 

 and an odontoid process which lies 

 between the condyles. Its arch is 

 incompletely ossified. Each of the 

 next six has an anteriorly concave 

 or proccelous centrum, a neural arch 

 surrounding the spinal cord, a trans- 

 verse process from each side of the base of the arch, an 

 anterior and a posterior pair of articular processes, and 

 a short neural spine. The eighth vertebra has a biconcave 

 or amphiccelous centrum. The ninth is convex in front, 

 with two convex tubercles behind, and bears large trans- 

 verse processes with which the hip girdle articulates. The 

 urostyle has anteriorly a dorsal arch enclosing a prolont^a- 

 tion of the spinal cord, but both arch and nerve cord 

 disappear posteriorly. The notochord, around which the 



Fig. 175. — Vertebral 

 column and pelvic girdle 

 of Bull-frog. 



^./, Transverse processes of 

 sacral verLebra ; //, ilium ; U, 

 urostyle ; Fc^ femur ; Iscli, 

 ischiac region. 



