OUR FACE FROM FISH TO MAN 



ancestors and taken to living on the bottom in 

 shallow water. The simple pulsations of a bell- 

 shaped body, which were sufficient for jellyfishes, 

 were modified into writhings or contractions in 

 definite directions. Anyhow, radial symmetry 

 gave way to bilateral symmetry, the animals began 

 to progress in a fore-and-aft direction and the 

 sharp differentiation of heads and tails was in 

 full play. 



The early evolution of a primitive head is also 

 well illustrated in certain flatworms (Fig. 2A), in 

 which the slender nerve threads are drawn together 

 to form the first rudiments of a brain and a very 

 simple type of eyes is attained. In the annelid 

 worms the head is further advanced, since the 

 mouth is now surrounded by various accessory 

 organs for the testing of the food, by horny jaws 

 moved by muscles for the capture of the food, 

 by elaborate eyes and by an extensive fusion of 

 nerve fibers into an incipient brain. The trilo- 

 bites and higher crustaceans (Fig. 2B) carry the 

 story onward, showing us how some of the jointed 

 projections from the sides of the body, which had 

 originally been developed as primitive legs, very 

 early began to serve the mouth by drawing, kick- 



