554 



COMPAEATIVE ANATOMY. 



the whole of the branchial cavity, it appears to have been reduced, 

 in comparison with the same organ in the Tunicata. When the 

 organ which functions as a tongue is differentiated, the groove under- 

 goes still further reduction, and is converted into a canal, which is 

 gradually cut off from the superior cavity (Fig. 311), and is at last 

 completely separated from it. In the adult animal it is converted into 

 a complex of follicles, covered with epithelium, which extend from 

 the second to the fourth pair of branchial sacs. They form an 

 organ with unknown physiological relations — the thyroid gland. 



In the Gnathostomata a groove, remaining for some time, is 

 no longer developed, but at the homologous region a process of the 



lumen of the cephalic enteron is 

 nipped off, and forms an azygos 

 follicle invested by epithelium. By 

 a process of gradual gemmation this 

 is broken up into a number of sepa- 

 rate follicles, which are united to- 

 gether by connective tissue. In 

 Fishes the organ is placed not far 

 from the point at which it was 

 formed ; that is, at the anterior end 

 of the trunk of the branchial arteries, 

 and between it and the copula of the 

 hyoid arch. In the Amphibia the 

 thyroid is placed near the larynx, 

 where it forms a paired coil (unpaired 

 in Proteus), and is set on the inner 

 surface of the posterior cornua of the 

 hyoid. It is sometimes broken up 

 into several groups. In the Eeptilia 

 it is unpaired, and lies in front of the 

 aortic arches ; in Birds, however, it 

 is paired (Fig. 312, t), and lies 

 near the commencement of the 

 carotids. In both these divisions, therefore, it is removed some 

 way from the point at which it was developed. This appears to be 

 due to the shifting backwards of the great arterial trunks. Among 

 Mammals it is separated into two parts in the Monotremata, many 

 Marsupials, and various other forms ; while in the rest its two 

 lateral masses are united by a median bridge (isthmus). It always 

 lies just below the larynx, and on the trachea. 



The preservation of this organ, which lost its primitive signifi- 

 cance even in the lower Vertebrata, throughout the long series of 

 higher forms, is explicable from the fact that it has been inherited 

 from what is phylogenetically a very early period ; it is an arrange- 

 ment, indeed, which was physiologically of great importance to the 

 ingestion of food in the Tunicata. 



MuLLEB, W., Die Hypobranchialrinne der Tunicaten, etc. Jen. Ztscir. Bd. VII. 

 The same, Entw. d. Schilddriise. Jen. Ztscir. Bd, VI. 



Fig. 312. Thymus (th) and thyroid 



(f) of a mature embryo of Buteo 



vulgaris, tr Trachea. 



