THE INVERTEBRATE BODY 109 



primitive ganglia of the segments which have united to form the 

 head. (Fig. 66.) 



We have now reviewed and emphasized the body plan of Hydra, 

 Earthworm, and Crayfish as representative Invertebrate types 

 that illustrate several of the fundamental structural principles 

 which are to be found in the Vertebrate body. But it will be re- 

 called that certain other Invertebrates exhibit body plans that 

 superficially at least depart very widely from the types described, 

 although it is believed that these forms do not break the general 

 evolutionary continuity of the Animal Kingdom, 



