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. 
