OUR FACE FROM FISH TO MAN 



shown to be the highly speciaHzed, eel-Hke, and 

 in some respects degenerate derivatives of the 

 ostracoderms of Silurian times. Even today (Fig. 

 59) in the early larval lamprey (Ammocoetes stage) 

 the pharynx is provided with a prominent "ciliated 

 groove," which (like that of Amphioxus) appears 

 to be reminiscent of the earlier days of feeding on 

 microscopic organisms; the adult lamprey, on the 

 other hand, is a cruel pirate, rasping off chunks 

 of flesh from the sides of helpless fishes and occa- 

 sionally eating its way, it is said, into their 

 interiors, finally reducing them to floating shells. 



The lampreys and their allies are enabled to 

 carry on their nefarious business by means of 

 thorny teeth, set in concentric rows about the 

 mouth and flanking a protrusile rasp, which is 

 likewise covered with horny teeth and can be 

 drawn back and forth like the rasp of a whelk. 



The teeth of the lampreys (Fig. 60A) are of 



extraordinary interest, since they have always 



been regarded as representing a very early stage 



in the evolution of the teeth of vertebrates. Each 



tooth consists of a thick, horny, epithelial thorn 



with a pulp cavity within, which is ready to grow 



another thorn as soon as the outer one is broken 



98 



