768 COMPARATIVE ANATOMY 



lions of the epithelium that are highly vascular and play a pulmonary 

 role. 



DOGFISH 



Here branchial respiration is carried on in the six pairs of branchial 

 clefts. These branchi are primitive respiratory organs, consisting of 

 mere diverticula of mucus membrane, richly vascular, and supported by 

 cartilaginous processes called gill-rays. The water enters the mouth 

 and is forced out through the gill slits. In doing so it aerates the gill 

 filaments, and provides oxygen for the blood that circulates rapidly 

 through them. 



AMPHIBIA 



External gills are found in the perennibranchiate urodeles (Fig. 

 374) throughout life and in practically all amphibians while in the larval 

 stage. 



The epithelium covering these external gills is ectodermal so that 

 they are really cutaneous and not pharyngeal gills. They are, therefore, 

 of a totally different nature from the so-called external gills of the em- 

 bryos of Elasmobranchs and Holocephali, in which case the external 

 gills are only filaments of the internal gills prolonged through the 

 branchial openings. 



Internal gills develop only in the larvae of Anura and are probably 

 homologous with the internal gills of fishes, although even here the 

 epithelium may be ectodermal. In many species of Salamanders lungs 

 are absent, but in most amphibians they develop as ventral outgrowths* 

 from the oesophagus. The left is usually the longer. The lungs are 

 united at their base although true bronchi are absent. In the lungless 

 Salamanders respiration is exclusively cutaneous and pharyngeal. The 

 lings are supposed to have secondarily disappeared in these animals. 

 The air ducts enter the anterior end of the lungs in amphibia, while in 

 amniotes the lungs extend cephalad to the entrance of the bronchi which 

 is on the medial side. This change is due to the transfer of the heart 

 into the thorax so that the pulmonary arteries then force the bronchi 

 toward the center of the lungs. In the amniotes the ducts have cartilage 

 in their walls, thus being true bronchi. These bronchi often extend 

 into the lungs where they divide into secondary and tertiary bronchi. 



REPTILIA 



Gills are absent, and gill-slits disappear, in all animals higher than 

 arodeles. The lungs are large and complicated and often non-symmet- 

 rical, sometimes one is even lacking. In the snakes the lungs consist 

 of a single sac lined with infundibula either in part or throughout. In 

 the lizards there are one or nftore verticle septa dividing the lung into 

 chambers lined with alveoli, while a part of the bronchus may extend 



