208 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



THE ORAL CAVITY. 



The cavity of the mouth is limited anteriorly by the line of the stom- 

 odeal involution and extends back to the pharynx. It is lined in part 

 by ectoderm, in part by entoderm, the line between the two, as stated 

 above, not being recognizable in the adult. In the amphibia the lining 

 is ciliated, the cilia extending back to the stomach. In the cyclostomes 

 the oral cavity is funnel-shaped, with a circular or quadrangular open- 

 ing supported by a cartilaginous ring and has the name of oral hood. 

 It is permanently open, there being no jaws capable of closure (see 

 skeleton, p. 73) thus furnishing a marked contrast to all other verte- 

 brates in which there are jaws and which are consequently known 

 as gnathostomes. (Development gives little support to the view that 

 the cyclostome tongue is the homologue of the lower jaw of the 

 gnathostomes.) 



In development the mouth arises on the ventral side of the head, 

 some distance from the anterior end of the body. This position 

 is retained throughout life in most elasmobranchs and in the sturgeons; 

 but elsewhere, by the development of the bony upper jaw in front of 

 the pterygoquadrate (p. 69) and the concomitant extension of Meckel's 

 cartilage, the mouth opening is gradually transferred to the anterior 

 end and becomes terminal. 



In most lower gnathostomes (the holocephali and other isolated 

 forms are exceptions) the mouth opening is bounded by folds of epithe- 

 lium which meet when the mouth is closed. Usually these folds are 

 soft and are supported below by connective tissue, but in birds, turtles 

 and monotremes they are cornified. It is only in the mammals that 

 true lips occur. These are fleshy folds around the mouth and their 

 development in this group is correlated with the presence of the dermal 

 facial muscles (p. 134), by which they are moved. With the develop- 

 ment of lips there is formed a space between lips and teeth, the vesti- 

 bule of the mouth, which sometimes (e. g., some rodents) forms cheek 

 pouches, lined with hair, of considerable size. 



Teeth. 



The primitive function of the teeth was apparently to hold the prey 

 taken into the mouth and this is their sole use in many forms. In 

 other species they have become efficient organs for the comminution of 

 food, either by cutting or by crushing it. 



