148 THE STORY OF ANIMAL LIFE 



the jaw-teeth are used for biting. Among the 

 Carps, the jaw-teeth are reduced, and the fish de- 

 pends upon its throat-teeth only. In the Wrasses, 

 one pair of the bones that bear throat-teeth (the 

 inferior pharyngeal bones) are fused, so as to form 

 a stronger apparatus : and from this circum- 

 stance, the group of Fishes to which they belong 

 has been given the name of Pharyngognathi, 

 fishes possessing throat-jaws. They have, how- 

 ever, biting teeth as well, in the true jaws. The 

 grinding teeth are apparently used for consuming 

 the food in a leisurely manner when once it has 

 been taken into the mouth. 



A curious circumstance in connection with 

 these " throat-jaws " is, that they produce musical 

 sounds. Fishes have other means, however, of 

 producing a voice usually by means of the swim- 

 ming-bladder and muscles in connection with it. 

 Probably they are able, to some extent, to effect 

 communication with each other in this way. 



It has already been stated that teeth, in their 

 primitive form, are to be regarded as skin-struc- 

 tures. Certain fish, which are looked upon as an- 

 cestral types, have, dispersed throughout the skin, 

 a number of bony plates, or granules (placoid 

 scales), more or less formidable, and tipped with 

 a hard enamel-like substance. Teeth are regarded 

 as but a special form of these. But if they are 

 skin-structures, how come they in the mouth and 

 throat ? Because the mouth and throat are lined 

 by an ingrowth from the external skin ; the origin 

 and growth of this is seen in the embryo. 



In the Mammalia the teeth, though restricted 

 in number, attain the greatest possible variety of 

 form, so that the jaws of different but allied 

 species may be distinguished by their teeth. 



