278 CLASS AMl'HIBIA. 



Nephrostomes are very generally present and will be dealt with 

 under the urmogenital organs. 



Respiratory organs. The skin is an important organ of 

 respiration in all Amjyhibia, and in some abranchiate Urodeles 

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

 {Desmognathinae, Plethodontinae, 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 pharyngeal pouches (Fig. 154) are laid down in 

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

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

 hyobranchial, and the pouches between the successive branchial 

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

 Of these pouches the first (Fig. 154, HM) rarely + 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 tliree 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 lining 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. 



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

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

 perforate in any other Amphibians. 



