OUR ANCIENT RELATIVES 



between the inner and the outer layer and is con- 

 centrated into a ring around the mouth. This 

 mouth is far from being homologous with our own. 

 It represents at most the "primitive streak" of 

 the early embryos of vertebrate animals. Never- 



^.,mt/i 



Fig. 1. The First Mouths. 



Slipper animalcule (A) with gash-like mouth; Jellyfish (B), a two-layered 



sac with primitive mouth. (Both after Parker and Haswell.) 



For details, see p. xiii. 



theless it was the starting-point for further de- 

 velopments. 



The direct line of ascent toward the vertebrates 

 is not yet definitely known and we can only sur- 

 mise what the next few steps may have been. 

 The flatworms appear to represent highly devel- 

 oped descendants of the jellyfish group, which 

 had abandoned the drifting habits of their remote 



