THE TUBE-WITHIN-A-TUBE BODY PLAN 



199 



pierces with its teeth. An enzyme ( hirudin ) 

 is secreted by the sahvary glands, which 

 prevents the blood from clotting. Blood 

 is sucked by the pumping action of the 

 pharynx and stored in the large sacculated 

 crop to be passed on into the intestine for 

 digestion a little at a time. 



In review, we have seen in this chapter 

 the advent of characteristics of key impor- 

 tance to the development of more complex 

 animal bodies. The acquisition of a coelom 



and segmentation, together with the "tube- 

 within-a-tube" body plan, have laid the 

 foundation for higher forms. Perhaps the 

 features that have appeared so far in 

 the animal kingdom are more important than 

 any that follow, because later groups, while 

 adding some new characteristics, actually 

 become complex by elaborating features 

 that are already established in the annelids. 

 Let us keep these fundamental character- 

 istics in mind as we examine the next 

 groups of animals. 



