IV. VERTEBRATA. 547 



A striking vertebrate characteristic occurs in the dentition. 

 In the cyclostomes there are horny teeth strongly cornified epi- 

 thelial products seated on connective-tissue papilla?; in the higher 

 groups occur true teeth of dentine and enamel, enclosing a richly 

 vascular pulp. They occur in places where the underlying skele- 

 ton affords them a firm support, especially on the upper or lower 

 jaws, but they may occur on other bones of the mouth and 

 pharyngeal cavities (roof of the mouth, gill arches). They have 

 apparently arisen from a diffuse dentition, recalling the scales of 

 the skin, sinco many elasmobranchs possess, besides the ordinary 

 teeth, rudimentary teeth in mouth and pharynx. Where teeth are 

 lacking (birds, turtles, baleen whales) they have been lost. 



The respiratory organs arise from the pharynx. In the fishes 

 and some Amphibia its walls, right and left, are perforated by 

 gill clefts, each of which lies between two successive visceral arches- 

 (fig. 570). These are canals which open internally into the^ 

 pharynx, while the outer gill openings are on the outer surface. 

 The anterior and posterior walls of the clefts bear delicate vascular 

 folds of mucous membrane, the gill filaments. These are the in- 

 ternal gills, in contrast to the external gills of Amphibian larvae,, 

 which are dendritic external ectodermal growths arising above and! 

 between the gill slits (figs. 4, 5). It is important for the phylogeny- 

 of the vertebrates to note that reptiles, birds, and mammals, which 

 never breathe by gills, have gill clefts outlined and later lost with 

 the exception of the Eustachian cleft. 



Two problematical organs, the thymus and the lateral lobes of the thy- 

 roid gland, develop from the epithelium of the gill clefts. The middle- 

 unpaired part of the thyroid has been regarded as a modification of the- 

 endostyle of the Tunicata (p. 506). The thyroid, which produces iodine- 

 compounds, is doubtless very important ; disease or extirpation of itr 

 causes serious nervous disturbances. 



The lungs also arise from the pharynx as two sacs (one oc- 

 casionally remaining rudimentary), which grow downwards and 

 backwards. They retain their opening into it either directly or 

 by means of a trachea or windpipe, which just before its entrance^ 

 into the lungs usually divides into two bronchi (figs. 579, 620). 

 At the opening into the pharynx (glottis) the supporting cartilages- 

 (remnants of the visceral skeleton, p. 524) are strong and form 

 the larynx, which in mammals may be closed from the pharynx 

 by a valve, the epiglottis. The lungs and trachea have their 

 counterparts in the fishes in the swim bladder, a hydrostatic 

 apparatus, and its duct. 



