THYROID 255 



Reptiles, be split up at its distal end, being either bifurcated 

 (TrochilidiE) or having a brush-like form. In Woodpeckers (the 

 extraordinarily developed epibranchials of which have already been 

 mentioned in the chapter on the skull), the tongue may be thrown 

 far out from the mouth by means of a complicated system of 

 muscles, and it thus serves as a prehensile organ. 



The tongue is largest in predatory Birds (Rapaces) and Parrots, but its 

 size is here not due so much to the special development of muscles as to the 

 presence of fat, vessels, and glands. 



Mammals. — The tongue reaches its most complete morpho- 

 logical and physiological development in Mammals, and, as else- 

 where, undergoes the most various modifications in form. It is 

 as a rule flat, band-like, rounded anteriorly, and extensile. In- 

 trinsic as well as extrinsic muscles are well developed. A fold, 

 the so-called suhlingua (plica fimbriata), is present on the lower 

 surface of the tongue, and is especially well marked in 

 Lemurs ; in the Slender Loris (Stenops) it is supported by carti- 

 lage. This probably corresponds to the last vestige of the tongue 

 of lower Vertebrates which has been replaced by the more highly- 

 developed organ characteristic of Mammals. The latter has pro- 

 bably arisen from the posterior part of the degenerated sublingua. 



Thyroid. 



The thyroid arises primarily as a median ventral diverticulum 

 of the pharynx in the region of the first four or five visceral clefts, 

 and in the course of development may become subdivided into two 

 lobes. In addition to this unpaired diverticulum, paired portions, 

 situated more posteriorly, are developed in Mammals. 

 . In 'the Ammocoete the single diverticulum, which is lined by a 

 ciliated epithelium, opens into the pharynx between the third and 

 fourth clefts (Fig. 221), but in the adult Petromyzon the organ, 

 as in all Vertebrates, loses its connection with the pharynx, under- 

 goes a modification, and gives rise to numerous closed glandular 

 vesicles enclosing an albuminous substance. 



In Elasmobranchs the thyroid is unpaired and lies beneath the 

 mandibular symphysis; 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 

 indication of a division into right and left lobes. 



In the TJrodela and Anura the thyroid gives rise to numerous 

 vesicles situated close to the anterior end of the pericardium, 

 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 it is usually situated close to the trachea (Fig. 

 204, a), and in Chelonians and Crocodiles it often possesses right 



