Elements of Comparative Anatomy (1878) Elements of Comparative Anatomy elementsofcompar78gege Year: 1878 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 exte
Elements of Comparative Anatomy (1878) Elements of Comparative Anatomy elementsofcompar78gege Year: 1878 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 la
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