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 
digestive work, as is shown by its late connexion with the pharynx 
in the ammoceete. 
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 
choane. 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 
