278 CLASS AMPHIBIA. 



Nephrostomes are very generally present and will be dealt with 

 under the urinogenital organs. ' 



Respiratory organs. The skin is an important organ of 

 respiration in all Amphibia, and in some abranchiate Urodeles 

 it is the sole * respiratory organ, lungs being entirely absent 

 {Desmogiiathinae, Pletkodontinae, Amblystoma opacum, Sala- 

 mandra perspicillata) . In such forms there is no pulmonary 

 vein and the auricular septum is perforated by a large aperture. 

 In all other members of the group lungs are present, and in some 

 of the Urodeles gills and gill-slits as well. With very few 

 exceptions the larvae are provided with gills. It is this com- 

 bination of branchial and pulmonary respiration which is found 

 in the adults of some forms and at some period in the life-history 

 of nearly all, which confers a special interest on the breathing 

 and vascular mechanisms t of the Amphibia and necessitates 

 their being treated at some length. 



With a few exceptions (for which see the systematic part and 

 p. 296), all Amphibia, are hatched as larvae, and possess while in 

 that condition gill-slits and external gills. In all cases which 

 have been fully investigated the rudiments of six visceral arches 

 and of five jiharyngeal pouches (Fig. 154) are laid down in 

 development : the arches are the mancUbular, the hyoid, and 

 four branchial ; the pouches are the mandibulo -hyoid, the 

 hyobranchial, and the jjouches between the successive branchial 

 arches, there being no pouch behind the fourth branchial arch. 

 Of these pouches the first (Fig. 154, HM) rarely t acquires an 

 external opening ; in the frog it gradually fades away and the 

 tympanic cavity is formed later in its neighbourhood. The 

 others (Fig 154) become perforate and give rise to the four 

 gill-clefts, which are found in the larvae of Urodela and Anura. 

 The external gills, which are formed before the gill-clefts become 

 perforate, are three in number on each side ; they are branched 

 structures and are attached to the dorsal ends of the three 

 anterior branchial arches. These are the only gills found in 



* It has been suggested that the hning of portions of the alimentary 

 canal may cooperate, vide Wilder, Anat. Anzeiger, 9, 1894, p. 216 and 12, 

 1896, p. 182 ; Lonnberg. Zool. Anzeiger, 19, 1896, p. 33. 



t Boas J. E. v., Morph. Jahrb., 7, 1882, p. 488 ; and 13, p. 115. Maurer 

 F., Ibid., 14, 1888, p. 175. 



X It appears to do so in the Coecilian Hypogeophis and a small dorsally 

 placed cleft (spiracle) is formed, but it is doubtful if this cleft becomes 

 perforate in any other Amphibians. 



