XIII 



PHYLUM CHORDATA 



283 



The limbs of Urodela differ from the typical structure already 

 described only in details : there are usually four digits in the 

 fore-limb and five in the hind-limb. In Anura the limbs are 

 modified by the fusion of the radius and ulna and of the tibia 

 and fibula, and by the great 

 elongation of the two proxi- 

 mal tarsals. A pre-hallux 

 is frequently present. 



Myology. In the lower 

 Urodela the muscles of the 

 trunk and tail occur in the 

 form of typical myomeres 

 like those of Fishes. In 

 the higher forms the my- 

 omeres become converted 

 into longitudinal dorsal 

 bands, the extensors of the 

 back, paired ventral bands, 

 the recti abdominis, and a 

 double layer of oblique FlG> 8 97.-PeMc girdle of saiamandra. A,I, 



processes of epipubis ; Ep. epipubis ; Fo. ob- 

 urator foramen ; G. acetabulum ; II. ilium ; 

 Is. ischium ; P. pubis ; Sy. pubo-ischiatic sym- 



physis ; t. processes of pubis present in some 

 Ur ' 



Jrodeles- (From-Wiedersheim.) 



muscles, covering the flanks. 

 Digestive Organs. The 



teeth are always small and 

 ankylosed to the bones : 

 they may be singly or doubly pointed. They occur most com- 

 monly on the premaxilla3, maxillae, and vomers, but may also 

 be developed on the dentaries, palatines, and, in one instance, 

 on the parasphenoid. In many Anura, such as the Common 

 Toad, teeth are altogether absent. In some of the Stegocephali. 

 such as Mastodonsaurus, the teeth are extraordinarily complex in 

 structure, the tissues being folded in such a way as to produce in 

 section a complex tree-like pattern. It is from this circumstance 

 that the term Labyrinthodont, often applied to the Stegocephali, 

 is derived. 



The enteric canal is divisible into buccal cavity, pharynx, gullet, 

 stomach, small intestine, rectum, and cloaca. The stomach and 

 duodenum together form a U-shaped loop in which the pancreas 

 lies. The tongue in many Urodeles is fixed and immovable, like 

 that of a Fish : in most Anura it is free behind, as in the Frog, 

 but in Xenopus and Pipa (hence called Aglossa) it is absent. 



Respiratory Organs. With very few exceptions Amphibia 

 possess external gills in the larval state, and, in the perenni- 

 branchiate Urodela, these organs are retained throughout life. 

 They are branched structures, abundantly supplied with blood, 

 and springing from the dorsal ends of the first three branchial 

 arches. The epithelium covering them is ectodermal, so that they 

 are cutaneous and not pharyngeal gills, and are of a totally different 



