ir. YERTEDRATA. 547 



A striking vertebrate characteristic occurs in the dentition. 

 In the cyclostonies tliere are horny teetli — strongly cornifled epi- 

 tlielial products seated on conneotive-tissne papilhie; in the higher 

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

 vascular i>ulp. They occur in places where the underlying skele- 

 ton affords them a firm supp)ort, 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 ariseii from a diffuse dentition, recalling the scales of 

 the skin, since 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 

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

 The anterior and posterior walls of the clefts bear delicate vascular 

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

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

 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 

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

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

 causes serious nervous disturbances. 



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

 casionally remaining rudimentary), which grow downwards and 

 backwards. They retain their oi^ening into it either directly or 

 Ijy means of a trachea or windjiipe, which just before its entrance 

 into the lungs usually divides into two bronchi (figs. 579, C2(J). 

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

 (remnants of the visceral skeleton, p. 5.34) are strong and form 

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

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

 counterparts in the fishes in the swim bladder, a hydrostatic 

 apparatus, and its duct. 



