RESPIRATORY ORGANS. 247 



ally becomes follicular. In most vertebrates, the anlage, after separa- 

 tion, forms a network of epithelial tubes before becoming follicular. 

 Usually it exhibits a marked bilaterality, and in amphibia and birds 

 it becomes divided into two glands. 



In the elasmobranchs the thyreoid lies between the end of the ventral 

 aorta and the symphysis of the lower jaw; in teleosts the groups of 

 follicles lie around the ventral aorta, extending out on the anterior aortic 

 arches. In the urodeles the gland lies just behind the second arch and 

 in the anura on the hinder margin of the thyreoid process of the hyoid 

 plate. In reptiles it is ventral to the trachea (at about its middle in 

 lizards, nearer its division in other groups), while in the birds the two 

 glands occur at the base of the bronchi. In the mammals it is usually 

 near the larynx, and while generally two-lobed, it is here and there 

 (monotremes, some marsupials, lemurs, etc.) paired. 



Like the other ductless glands, the thyreoid supplies the blood with 

 substances necessary to the well-being of the organism, in the case of 

 mammals at least, an iodine-containing albumen. Degeneration or 

 extirpation of the thyreoid result in cerebral trouble. In the ancestral 

 vertebrate the thyreoid apparently had to do with some part of the 

 digestiv.e work, as is shown by its late connexion with the pharynx 

 in the ammoccete. 



In the pharynx and at the entrance of the mouth into the pharyn- 

 geal cavity (isthmus of the fauces) occur certain lymphatic structures 

 called tonsils, concerning which our knowledge is yet very deficient. 

 One account says they arise from inwandering epithelial cells, the 

 other maintains that they are formed from the sub-epithelial meso- 

 derm. Two different groups of organs are included under this name, 

 'the true tonsils at the isthmus of the fauces, and the pharyngeal 

 tonsils. The latter may be represented by lymphoid structures in the 

 floor or roof of the pharynx of urodeles and anura. They are well 

 developed in reptiles and birds, occurring in the latter behind the 

 choanae. In mammals they are inconstant structures. The true 

 tonsils of mammals lie one on either side of the isthmus. Both 

 types of tonsils consist of an adenoid ground substance containing 

 numerous lymph cells, and become follicular after birth. 



THE SWIM BLADDER. 



While the air or swim bladder (pneumatocyst) is not respiratory, 

 it is included here from its possible connexion with the lungs. It 



