4S2 ClIORDATA. 



the onliiiarv tcclh, nuliinentary teeth in mouth nitd pharynx. ^^ here 

 teeth are laekini,' (l)irds, turUes, l.uUeen \vhales) thev ha\e heon lost. 



The respiratory origans arise from tlie plrar\-]ix. lu the lishes ami 

 some Ampliiliia ils walls, riijht and left, are perforated li\" .L;ill clefts, each 

 of wliiclr lies between two suecessixe \asceral arehes (li;-;- 5-7)- Tliese are 

 canals which ojK-n internalh' into the pharynx, wliile the outer gill openings 

 are on the outer surface. The anterior aird [tosterior walls of the cielts 

 bear delicate wtscular folds of mucous niembrarie, tiie gill hiamenls. 

 These are the inlerrial gills, in contrast to the exterrtal gills of lar\'a~ of 

 Amphibia and se\-eral iishes which are tlemlritic e.xtenral ectodermal 

 growths arising al)o\-e and Ijetween the gill slits (ligs. 4, 5). It is impor- 

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

 mammals, which ne\'er lireathe by gills, have gill clefts outlined and later 

 lost with the exceptiort of the Eustachian cleft (tig. 3). 



Two prnlilematical organs, the tliyiiuis and the lateral lobes of the Ihvrcoid 

 ghiiid, develop from the epithelium of the gill clefts. The middle unpaired ]iart 

 of the thyreoid has been regarded as a modiheation of the endostvle of the 

 Tunica ta (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 (oite occasioirallv remaiiting rudimen- 

 tary), whicli grow ilo\\aiwards and l)ackwards front the phar3nix. They 

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

 ■windpipe, wliich just liefore its entrance into the luirgs usually divides 

 into two tubes (broiiflil) (figs. 573, 580). At the opening into the pharynx 

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

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

 from the pharynx by a vah-e, the cpiglollis. The litirgs and trachea Intve 

 their counterparts in the hshes in the siciin bladder (a Inalrostatic appa- 

 ratus) and its duct. 



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

 walled sacs, but in some have greater res[iirator\' surface since folds extend into 

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

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

 Amphibia (hg. 536, i). In the reptiles the peri])lieral folding' increases at the 

 expense of the single air chamber and grows inwards. 'J"hc more the central 

 chamber is divided and restricteil, the m.ire it takes on the character of a canal, 

 lengthening the bronchus (lig. 530, 2; turtles and croculilcs). In the mammals 

 (3) there is no lon,L;er a central space, since the brnnehus divides ai,'ain and 

 again, to fine britiilnolrs. which communicate with Iniiiidihiila or 'air cells' ( 4, c) 

 by means of an ntvrolar duel which is lined with vesi.Ies (alveoli) like those of the 

 infundibula. This comparative conception of the hmn- needs modification 

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

 of an air chamber by the in.i^q-owth of walls, but by lateral out-niwths, as in a 



