■P?iOUf/7 



eye 

 Tey-jau/s 





^•(rm 





Fig. 2. Two Early Stages in the Evolution of a Head 

 (after Parker and Haswell). 



Flatworm (A), showing a head-and-tail differentiation, inrluding the 

 beginnings of a brain and of eyes; Sand-flea (B), showing the interrelations 

 of eye, brain, mouth, leg-jaws and nerve cord. 



For details see p. xiii. 



