xiii PHYLUM CHORDATA 305 



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 larvse 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 pharynx. 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 

 as Siren, Amphiuma, and the Gymnophiona, the laryngo-tracheal 

 chamber is prolonged into a distinct trachea or wind-pipe, sup- 

 ported by cartilages. In many species of Salamanders the lungs are 

 absent, and respiration is exclusively cutaneous and pharyngeal. 



Circulatory Organs. — The heart always consists of a sinus 

 venosus, right and left auricles, ventricle, and conus arteriosus. 

 The sinus venosus opens into the right auricle, the pulmonary 

 veins enter the left, and the two are separated by a septum 

 auricularum which forms a complete partition in Anura, but in 

 Urodela and Gymnophiona is more or less fenestrated, i.e. formed 

 of a network of muscular strands with intervening spaces. The 

 conus arteriosus has no longitudinal valve in the lower Urodela 

 and the Gymnophiona, but is separated both from the ventricle 

 and from the bulbus aortse by transverse rows of valves. 



In the perennibranchiate Urodela and in the larvse of the air- 

 breathing forms the circulation is essentially like that of a Fish. 

 The bulbus aortse (Fig. 953, A, h. ao.), which represents an abbre- 

 viated ventral aorta, gives off four afferent hra7ichial arteries (af. 

 hr. a. 1 — 4), three to the external gills, and a fourth which curves 

 round the gullet and joins the dorsal aorta directly. From 

 each gill an efferent branchial artery brings back the purified 

 blood, and the efferent arteries unite, in a somewhat irregular 

 way, to form the dorsal aorta (d. ao.). Each afferent with 

 the corresponding efferent artery constitutes an aortic arch. Short 

 connecting branches unite the afferent and efferent arteries of 

 each gill, carotids {ext. car., int. car.) arise from the first efferent 

 artery, and, when the , lungs appear, a pulmonary artery {pul. a.) 

 is given off from the dorsal portion of tbe fourth aortic arch 

 of each side. In those Urodela which in the adult condition 

 are devoid of gills, when the latter atrophy (B) the first aortic 



