332 



COMPARATIVE ANATOMY 



just in front of which is the tuber- 

 culum impar of the tongue-rudiment. 



In the Ammocoete, the simple 

 diverticulum, which is lined by cili- 

 ated epithelium, opens into the 

 pharynx between the third and 

 fourth clefts (Fig. 260), but in the 

 adult Petromyzon the organ, as in 

 all Craniata, loses its connection with 

 the pharynx and undergoes consider- 

 able reduction and a change of 

 function, giving rise to numerous 

 closed follicular masses. 1 



In Elasmobranchs the thyroid is 

 unpaired and lies behind the man- 

 dibular symphysis, just in front of 

 the bifurcation of the ventral aorta ; 

 in adult Teleosts it is paired, and is 

 situated in the region of the first 

 branchial arch. In Dipnoans it lies 

 anteriorly to the muscles of the 

 visceral skeleton and shows an in- 

 dication of a division into right and 

 left lobes. 



In the Urodela and Anura the 

 thyroid is situated close to the 

 anterior end of the pericardium : it 

 undergoes subdivision and forms 

 numerous vesicles lying posteriorly 

 to the second ceratobranchials in the 

 former, and on the ventral side of the 

 posterior cornua of the hyoid in the 

 latter. 



In Lizards the thyroid is situated 

 close to the trachea, slightly behind 

 the middle of its course (Fig. 242), 

 and in Chelonians and Crocodiles it 



usually possesses right and left lobes 

 FIG. 243 THYM us AND THYROID J " & , . c , 



OF A YOUNG STORK. lying on the great vessels just alter 



they leave the heart. In Birds (Fig. 



B. bronchi ; H. heart ; Oe, ceso- _.;, j 11 



phagus ; T\ trachea ; Tm, 243) the organ is paired, and has a 



thymus ; Tr, thyroid. similar position. 



The thyroid of Mammals (Fig. 

 227) consists of two lobes often connected by a median isthmus 



1 In Myxine the thyroid consists of rounded or oval capsules surrounded by 

 fat ; they are nearly all median and are arranged separately or in groups 

 between the gill-sacs on either side, the gullet above, and the ventral aorta 

 below. 



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