482 CHORDATA. 



the ordinary teeth, rudimentary teeth in mouth and pharynx. Where 

 teeth are lacking (birds, turtles, baleen whales) they have been lost. 



The respiratory organs arise from the pharynx. In the fishes and 

 some Amphibia its walls, right and left, are perforated by gill clefts, each 

 of which lies between two successive visceral arches (fig. 527). These are 

 canals which open internally into the pharynx, while the outer gill openings 

 are on the outer surface. The anterior and posterior walls of the clefts 

 bear delicate vascular folds of mucous membrane, the gill filaments.- 

 These are the internal gills, in contrast to the external gills of larvie of 

 Amphibia and several fishes which are dendritic external ectodermal 

 growths arising above and between the gill slits (figs. 4, 5). It is impor- 

 tant for the phylogeny of the vertebrates to note that reptiles, birds, and 

 mammals, which never breathe by gills, have gill clefts outlined and later 

 lost with the exception of the Eustachian cleft (fig. 3). 



Two problematical organs, the thymus and the lateral lobes of the thyreoid 

 gland, develop from the epithelium of the gill clefts. The middle unpaired part 

 of the thyreoid has been regarded as a modification of the endostyle of the 

 Tunicata (p. 443). The thyreoid, which produces iodine compounds, is 

 doubtless very important; disease or extirpation of it causes serious nervous 

 disturbances. 



The lungs also arise as two sacs (one occasionally remaining rudimen- 

 tary), which grow downwards and backwards from the pharynx. They 

 retain their opening into it either directly or by means of a trachea or 

 windpipe, which just before its entrance into the lungs usually divides 

 into two tubes (bronchi) (figs. 573, 589). At the opening into the pharynx 

 (glottis) the supporting cartilages (remnants of the visceral skeleton, p. 

 460) are strong and form the larynx, which in mammals may be closed 

 from the pharynx by a valve, the epiglottis. The lungs and trachea have 

 their counterparts in the fishes in the swim bladder (a hydrostatic appa- 

 ratus) and its duct. 



The swim bladder of fishes and the lungs of most amphibia are smooth- 

 walled sacs, but in some have greater respiratory surface since folds extend into 

 the central space. In this way the bladder may become respiratory (Lepidostens, 

 Amia, Dipnoi). There is also a small increase of respiratory surface in the 

 Amphibia (fig. 536, i). In the reptiles the peripheral folding increases at the 

 expense of the single air chamber and grows inwards. The more the central 

 chamber is divided and restricted, the more it takes on the character of a canal, 

 lengthening the bronchus (fig. 536, 2; turtles and crocodiles). In the mammals 

 (3) there is no longer a central space, since the bronchus divides again and 

 again, to fine bronchioles, which communicate with in f und^ntl a or 'air cells' (4, e) 

 by means of an alveolar duct which is lined with vesicles (alveoli) like those of the 

 infundibula. This comparative conception of the lung needs modification 

 since development shows that the higher stages are not formed by the division 

 of an air chamber by the ingrowth of walls, but by lateral outgrowths, as in a 



