xm PHYLUM CHORDATA 295 



Digestive Organs. The teeth are always small and ankylosed 

 to the bones : they may be singly or doubly pointed. They occur 

 most commonly on the pre- 

 maxillae, maxillae, and vomers, 

 but may also be developed on 

 the dentaries, palatines, and, in 

 one instance, on the para- 

 sphenoid. In many Anura, such 

 as the Common Toad, teeth are 

 altogether absent. In some of 

 the Stegocephala, such as Masto- 

 donsaurus, the teeth are extra- 

 ordinarily 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 



T , . .-, j f, Fia. 963. Pelvic girdle of Salamandra. a, b, 



tne term Laoynntrioaont, orten processes of epipubis; #p. epipubis ; 



fr flio Qf orrrknn>al a ia turator foramen; G. acetabulum ; //.ilium; 



to tne btegocepnala, is /,, isc hium; p. pubis; s>/. pubo-ischiatic 



derived symphysis ; f, processes of pubis present in 



, . .. . ., , some Urodeles. (From Wiedersheim.) 



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 

 nature from the so-called external gills of the embryos of Elasmo- 

 branchii and Holocephali, which are only the filaments of the 

 internal gills prolonged through the branchial apertures. 



Internal gills are developed only in the larvae of Anura. They 

 appear as papillae on the outer borders of the branchial arches 

 below the external gills. They closely resemble the internal gills 

 of Fishes and appear to be homologous with them, although it 

 seems probable that their epithelium is ectodermal. 



In most adult Amphibia lungs are formed as outgrowths of the 

 ventral wall of the oesophagus. The right and left lungs com- 

 municate with a common laryngo-tracheal chamber, supported by 

 the cartilages of the larynx and opening into the mouth by a 

 longitudinal slit, the glottis. In the more elongated forms, such 



T 2 



