516 



CHORDATA 



ratus appears. The spiracular cleft persists as a canal, opening into the 

 pharynx by the Eustachian tube, its outer end expanded into the tym- 

 panum, closed externally by the tympanic membrane, supported by the 

 tympanic annulus (dotted circle in fig. 566). The connexion of the 

 labyrinth with the tympanum is by an opening in the otic capsule, the 

 fenestra ovalis, in which is the stapes (? part of capsule), the columella 



extending from this to the tympanic membrane 

 and carrying the sound waves across to the 

 inner ear. The brain (fig. 568) has advanced 

 above that of the fishes in the stronger develop- 

 ment of the cerebrum, but remains behind in 

 the small size of the cerebellum, which is but 

 a thin lamella. 



The respiratory organs afford important 

 characters, since both gills and lungs occur. 

 The larva? of the anura, as a rule, have inter- 

 nal gills, but the dorsal part of these, as in 

 many fishes, develop early and very strongly 

 and extend out through the clefts to the skin 

 above them as three pairs of external gills. 

 These, however, are absorbed with the develop- 

 ment of the other gill leaves, which become 

 enclosed in a special chamber (atrium} by the 

 backward growth of an opercular fold from 

 each hyoid arch. The atrium usually opens 

 to the exterior by a spiracle on the left side, 

 but occasionally there are a pair of spiracles. 

 The external gills are strongly developed in 



lobes; VH, cerebrum; Z, epi- 

 physis; ZH, 'twixt brain. 



FIG. 568. Brain of frog. 

 /, line between olfactory lobes 

 and cerebrum; Frh, fossa 

 rhomboidalis; HH, cerebel- 

 lum; /, olfactory nerve; Lol, 



olfactory lobes; MH, optic the larval urodeles and caecihans, and the in- 

 ternal gills are correspondingly undeveloped. 

 The paired lungs open into the hinder part 

 of the pharynx, either directly through the glottis or more rarely by a 

 short trachea. Cartilages, the remnants of gill arches, may support 

 trachea and glottis, and on the latter support vocal cords (larynx). 

 Breathing is accomplished by a kind of swallowing, the air being 

 forced into the lungs by the muscles of the floor of the mouth and the 

 pharynx. Persistent gills and lungs are found together only in the Peren- 

 nibranchs. Usually the young breathe by gills, the adults by lungs, the 

 origin of the metamorphosis to be described below. 



Besides gills and lungs the skin is an important respiratory organ, 

 as are pharynx and mouth cavity, in which the air must remain for 



