AMPHIBIA. 277 



cular fold, traces of which are found in some urodeles, grows 

 back over the gill slits in such a way as to enclose them in an 

 extrabranchial or atrial chamber on either side, the two com- 

 municating by a passage beneath the throat, and opening to the 

 exterior usually by a single opening upon the left side. 1 



The lungs, which are absent from several salamandars which 

 respire by means of the skin, are thin-walled sacs, which may be 

 either smooth internally or folded into alveoli and infundibula 

 (Fig. 33). The shape is correlated with that of the body, 

 elongate in the longer species, shorter in the more compact 

 forms. Occasionally (gymnophiona, Amphiitma) the left lung is 

 small or rudimentary. The trachea may be long or the bronchi 

 may unite just behind the glottis. The glottis is supported by 

 a pair of arytenoid cartilages, and in the anura a ring-like cricoid 

 is added. 



In many stegocephals the vertebral column is poorly de- 

 veloped, the centra being sometimes represented by pleuracen- 

 tra and hypocentra arcale and pleuralia (rhachitomous type, 

 p. 1 36) ; or again by an embolomerous condition where centralia 

 and intercentralia alternate. These two conditions, sometimes 

 used as a basis of sub-division, may occur in the same species. 

 In the living species the centra are well developed, and are 

 either amphicoelous (perennibranchs, gymnophiona, and some 

 salamanders), opisthocoelous (most salamanders and aglossate 

 anura), or procoelous (most anura). The number varies from 

 9, plus the urostyle, in living anura to 2 50 or more in the gym- 

 nophiona. At most but four regions can be recognized, cer- 

 vical, trunk, sacral, and caudal; the single cervical is without 

 ribs, but bears in front an odontoid process derived from an an- 

 terior vertebra which early fuses with the skull. There is also 

 a single sacral vertebra in all except one group of fossil anura 

 (Palaeobatrachidae), where there are two. In the urodeles the 

 vertebrae bear dia- and parapophyses (p. 141), but in the anura 

 only the diapophysis persists. 



The ribs are small, bicipital in urodeles and gymnophiona, 

 anchylosed to the vertebrae in the anura. In the stegocephals 

 they are larger, but in no case do they reach the ventral surface. 



1 Paired openings occur in the aglossa, a median ventral opening in a few forms. 



