XIII 



PHYLUM CHORDATA 247 



downwards, outwards, and forwards from the thigh -joint, the 

 shank inwards, backwards, and upwards from the knee. The foot, 

 consists of two parts, a tarsal region directed downwards from the 

 heel-joint, and five long, slender digits united by thin folds of skin 

 or webs. Thus the limbs are placed in such a way that the elbow 

 and knee face one another, and the first digit, that of the hand 

 probably representing the index-finger, that of the foot, the liallux 

 or great toe, is turned inwards or towards the median plane of 

 the body. 



The skin is greyish-brown in R. temporaria, greenish in R. 

 esculenta, and is mottled, in both species, with dark brown or black ; 

 in R. temporaria there is a large black patch over the tympanic 

 region. Sexual differences occur in both species ; in R. temporaria 

 there is a large, black, glandular swelling on the inner side of the 

 hand of the male, and in R. esculenta the male has, at each angle 

 of the mouth, a loose fold of skin, the vocal sac, which can be 

 inflated from the mouth into a globular form. The skin is soft 

 and slimy owing to the secretion of mucous glands ; there is no 

 trace of exoskeleton. 



Endo-skeleton. The vertebral column (Fig. 870) is remark- 

 able for its extreme shortness ; it consists of only nine vertebrae 

 (V.1--V.9), the last followed by a slender, bony rod, the urostyle 

 (u. ST). The second to the seventh vertebrae have similar cha- 

 racters. The centrum (B, en) is somewhat depressed and has a 

 concave anterior and a convex posterior face, a form known as 

 proccelous. Each half of the neural arch consists of two parts, a 

 pillar-like pedicle (pd) springing from the centrum and extending 

 vertically upwards, and a flat, nearly horizontal lamina (lm\ 

 forming, with its fellow, the roof of the neural canal. When 

 the vertebrae are in position wide gaps are left between succes- 

 sive pedicles ; these are the inter-vertebral foramina and serve for 

 the transmission of the spinal nerves. The zygarjophyses (a. zyg) or 

 yoking processes are far better developed than in any Fish ; they 

 spring from the junction of pedicle and lamina, the anterior 

 zygapophysis having a distinct articular facet on its dorsal, the 

 posterior on its ventral surface. Thus when the vertebrae are in 

 position the posterior zygapophyses of each overlap the anterior 

 zygapophyses of its immediate successor. Laterally the neural 

 arch gives off on each side a large outstanding transverse process 

 (tr. pr) ; its crown is produced into a very small and inconspicuous 

 neural spine (n. sp). 



The first or cervical vertebra (v. 1) has a very small centrum and 

 no transverse processes. There are no anterior zygapophyses, but 

 at the junction of centrum and arch there occurs on each side a 

 large oval concave facet for articulation with one of the condyles 

 of the skull (vide infra). The eighth vertebra has a biconcave 

 centrum; that of the ninth or sacral vertebra (v.9) is convex in 



