516 CHORDATA 



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

 pharynx by the Eustacliian tuhe, its outer end expanded into the tym- 

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

 tympanic annulus (tlotted circle in tig. 566). The connexion of the 

 labvrintli with tlie tympanum is l.iy 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 

 ■\li\l/f And carrying the sound waves across to the 



fif/i^ inner ear. The brain (fig. 56S) has advanced 



--^ above that of the fishes in the stronger develop- 

 Xol nrent of the cerelirum, but remains l)ehind in 

 the small size of the cerebellum, which is but 

 •^ a thin lamella. 



The respiratory organs afford important 



^^" F' Ife * characters, since both gills and lungs occur. 



The larvK of the anura, as a rule, have inter- 

 Z — J^^O^^^ ^'^^^ g'lls, but the dorsal part of these, as in 



ZlS' / ■^'^ %k many fishes, develop early and very strongly 



ilffi -XxS: A, ;7 aiid extend out through the clefts to the skin 

 „ , ^,^ ^_ , above them as three pairs of external eills. 



jsM [--^ Ste i M j^' i ^ 



These, however, are absorbed with the develop- 



-^Tff -*^'^H / ment of the other gill lea^■es, which become 



ji<j.j^ l"-^» I enclosed in a special chamber {atrium) by the 



backward growth of an opercular fold from 

 each hyoid arch. The atrium usually opens 



Fig. 56S.-Brain of frog, to the exterior by a spiracle on the ijft side, 



7, line between olfactory lobes ' 



and cerebrum; Frh, fossa but occasionally there are a pair of spiracles. 



rhomboidaUs, HH^ cerebel- -phe external gills are strongly developed in 



mm; /, ollactory nerve; Lol, ^ '- - ^ 



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



SSs!1^:'lwixt b;^n:^'''- f^'^^'l gi"« -'^ 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 kintl of swallowing, the air being 

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

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

 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 



