THE TUBE-WITHIN-A-TUBE BODY PLAN 



179 



Fiq 10-4 Life cycle of the common hookworm (Necafor americanus). The adult worms are shown attached to the 

 lining of the intestine. The fertilized eggs begin development while still in the digestive tract. They pass out with 

 the feces and eventually hatch in the soil where they lie in wait for their next host. 



power of the enzymes. Oxygen must be ob- 

 tained from carbohydrate breakdown within 

 its own body, since there is very httle oxy- 

 gen in the gut. The only hope for survival 

 is to produce a great many eggs, which it 

 does most effectively. A large female has 

 been known to contain 27,000,000 eggs, 

 200,000 of which she lays every day. As in 

 all parasites, the chance for any one egg to 



produce a mature worm is extremely re- 

 mote, but by this colossal effort to bring 

 forth potential offspring ascaris has been 

 very successful in the world, as attested by 

 its universal occurrence. 



A notorious relative of ascaris is the hook- 

 worm {Nccator americanus), which is di- 

 rectly responsible for untold misery and 

 indirectly for the death of millions of peo- 



