266 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



it is 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 — 

 fig. 284, A — nearer its division in other groups), while in birds (fig. 

 284, B) the two glands occur at the base of the bronchi. In mammals 

 it is usually near the larynx, and, while it is usually two-lobed (fig. 

 284, c, C), in some cases (monotremes, some marsupials, lemurs, etc.) 

 it is paired. The remains of the place of its involution can often be 

 distinguished through life as a pit, the foramen caecum (fig. 286, ic) 

 at the base of the tongue. 



Like the other ductless glands the thyreoid supplies the blood with 

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

 of the mammals at least, an albumen containing iodine. In the 

 young it stimulates growth, and the extirpation or degeneration of 

 the gland results in cerebral trouble. In the ancestral vertebrate the 

 thyreoid apparently had a function connected with the collection or 

 metabolism of food, as is shown by its late connexion with the 

 pharynx in the ammocoete stage of the lamprey, to say nothing of the 

 probable homologues in the non-vertebrate chordates. 



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

 geal cavity (isthmus of the fauces) are certain lymphoidal structures 

 called tonsils, concerning which our knowledge is very deficient. 

 According to the older accounts they arise from subepithelial meso- 

 derm; more lately they are stated to arise in part from the oral 

 epithelium. The true tonsils (tonsillae palatinae), at the isthmus of 

 the fauces arise as thickenings of the epithelium of the first gill cleft 

 (fig. 286, i). They consist of an adenoid ground substance and be- 

 come follicular after birth and contain numerous lymph cells. Less 

 is known of the lingual tonsils at the base of the tongue. 



Both lingual and palatine tonsils occur only in the mammals. Besides these 

 there are pharyngeal tonsils, inconstant in mammals, but well developed in 

 reptiles and birds, where they lie behind the choanae. They may be the same 

 as certain lymphoid structures occurring in the pharyngeal roof of amphibians. 



/ THE SWIM BLADDER 



While the air or swim bladder of fishes (pneumatocyst) is not a 

 respiratory organ, it is included here from its possible relations to the 

 lungs. It occurs only in teleostomes, and while found in most species 

 (frequently absent from bottom-feeding forms — pleuronectids, etc.), 

 is it lacking here and there, even among species classed as physosto- 

 mous {Loricaria, etc.). In the young of a few sharks {e.g., Scyllium) 



