154 HISTORY OF THE HUMAN BODY 



scales, or as jaws equipped with teeth, since their origin as 

 the first, and their present and future function as the second, 

 are both so clearly indicated. 



We have thus revealed in these organs a valuable bit of 

 history, since we can here observe the jaws and teeth of ver- 

 tebrates almost at their birth, and can learn the source from 

 which the material was derived and how it was first trans- 

 formed. It cannot be said, however, that the jaws as used 

 here have developed directly into those of the higher verte- 

 brates, since in the intermediate history there is much addition 

 of new material and replacement of old, with the one object 

 in view of increased physiological -effectiveness. 



The assumption of an upper and lower jaw marks an epoch 

 in early vertebrate history, since these organs, once acquired, 

 replace forever the circular jawless mouth, and hood-like lip, 

 characteristic of both Amphioxus and cyclostomes. So radi- 

 cal must have been the change that some think that even the 

 mouth opening is a different one, that the one associated 

 with the new jaws was once merely a gill-slit like the 

 rest, through which some ancestor acquired the habit of ad- 

 mitting food, and that its manifest advantage over the other 

 mouth in its convenient skeletal equipment, caused the dis- 

 appearance of the old one and the perfection of the new; 

 there are, however, certain indications that point to the former 

 possession of a still older mouth, the palaostoma, homologous 

 with that of the tunicate, and with this both the hood-like 

 mouth of the cyclostomes and the slit form of other verte- 

 brates may be contrasted as the neostoma, or secondary mouth. 

 This last expression applies simply to the opening, which is 

 probably homologous in all vertebrates, but becomes com- 

 pletely metamorphosed by the addition of jaws. [Cf. Chapter 

 X., sub Hypophysis.] 



The use of placoid scales as teeth is also a new idea, and 

 they are certainly a great advance over the horny epidermic 

 excrescences that arm the circular lip of the cyclostomes. Pla- 

 coid scales are composite structures, composed of dentine cov- 

 ered over by enamel, and they are so perfectly fitted for the 



