VERTEBRAL COLUMN. 



265 



burrowing (Gymnophiona), arboreal (tree frogs) and purely 

 terrestrial forms are found. 



The vertebral column consists of separate, ossified vertebrae, 

 which vary in number from over 250 in large specimens of some 

 Gymnophiona, a variable but considerable number in the Urodela 

 (Fig. 142), to nine in the Anura.* The first vertebra, sometimes 

 called the cervical, has two concave surfaces for articulation 

 with the paired occipital condyles. 

 In the Urodela it has a forwardly 

 directed process of its centrum which 

 has been identified as the odontoid 

 process ; in this case it must be re- 

 garded as the axis vertebra, the atlas 

 having fused with the skull. It is 

 without ribs and usually without 

 transverse processes. The remaining 

 vertebrae may be divided into trunk 

 vertebrae, one sacral vertebra (two in 

 Pelobates, Pipa, Hymenochirus, absent 

 in Siren and Proteus) and, except in 

 Anura, caudal vertebrae. Most or all 

 the trunk vertebrae carry short ribs 

 (Fig. 143, It) which are two-headed in 

 Gymnophiona and most Urodeles. The 

 sacral vertebra is without ribs in the 

 Anura, but has them in the Urodela. 

 In the Anura, in which there are 

 almost always eight presacral verte- 

 brae, the sacral vertebra is followed FIG. 143. vertebral column of 

 by an unsegmented bony rod, the 

 os coccygis or urostyle, which is with- 

 out spinal cord or any trace of 

 notochord in the adult. Indications 

 of segmentation may sometimes be 

 discerned in the front end of this 



(Fig. 143, Pte). The caudal vertebrae of the Urodela ^are 

 provided with complete haemal arches enclosing the tail 

 blood vessels. The vertebrae are amphicoelous in the 

 Gymnophiona and lower Urodela, the notochord being persistent 



* Hymenochirus has only seven vertebrae. 



Off 



Discoglossus pictus (after Wie- 

 dersheim) Ob neural arch ; 

 Oc urostyle ; Pdzygapophysis; 

 Po supposed odontoid process ; 

 Ps neural spine ; Pt transverse 

 process ; Pte transverse pro- 

 cesses of caudal vertebrae 

 (urostylar) ; R ribs ; Sy arti- 

 cular concavities for occipital 

 condyles ; SW sacral vertebra. 



