THE TUBE-WITHIN-A-TUBE BODY PLAN 



183 



an infection has gotten under way. It is 

 also well to remember that meat sold on the 

 market is not inspected for trichina, pri- 

 marily because it is too difficult to detect 

 light infections. One poorly cooked ("pink") 

 pork chop can contain billons of worms, 

 which are adequate to kill a person. It is 

 true that there are fewer and fewer cases of 

 trichinosis reported, probably because the 

 practice of feeding meat scraps to hogs is 

 less prevalent and also because a general 

 war on rats has cut down the rat-hog cycle 

 which normally keeps the worms going. 



There are numerous parasitic nematodes 

 that cause bizarre diseases in the tropics, 

 diseases which are normally known only to 

 parasitologists and medical men who are 

 experts in the field of tropical medicine. 

 However, during World War II the tropics 

 became the battleground for many Ameri- 

 can men and consequently tropical diseases 

 suddenly loomed as a significant health 

 problem. Among the numerous roundworm 

 parasites the one that causes elephantiasis 

 ( Wiichereria hancrofti ) is perhaps the most 

 important. The life cycle of this worm dif- 

 fers from that of other nematodes in that it 

 requires two hosts. The larvae, called micro- 

 filariae, circulate in the blood of the in- 

 fected person (Fig. 10-7). An interesting 

 adaptation is that these tiny worms come 

 out in the peripheral blood vessels in the 

 evening, a time when the mosquitoes which 

 are the carriers (intermediate host) are 

 active. The mosquito picks up the micro- 

 filaria with the blood as it feeds; inside the 

 mosquito the worm grows and eventually 

 makes its way out through the proboscis 

 of the host. During the biting process the 

 microfilaria slips off the proboscis onto the 

 skin of the next host and immediately bores 

 its way in. Once in the blood stream of 

 the final host it moves into the lymph 

 glands where it becomes mature. The 

 worms become so numerous that they can 

 effectively clog the lymph passages, which 

 results in huge swellings, usually in the ex- 



tremities. A leg may grow to weigh 100 

 pounds, hence the name elephantiasis. 



Preventive measures consist in keeping 

 from being bitten by infected mosquitoes. 

 Light infections are not serious because 

 eventually the body forms new lymph chan- 

 nels so the swelling is reduced to normal. 

 The danger lies in continual infection where 

 the same individual is bitten again and 

 again by infected mosquitoes. 



PHYLUM NEMERTINEA 



The second step to higher complexity, 

 namely, the development of a circulatory 

 system, first appears in the phylum Nemer- 

 tinea, representatives of which are some- 

 times called "band worms" because of their 

 long ciliated flat bodies. The nemertines 

 live primarily in the ocean where they crawl 

 among the rocks. They are highly colored 

 and greatly elongated, sometimes measur- 

 ing as much as 80 feet in length. If cap- 

 tured, their bodies stretch so that they often 

 break into two parts under their own 

 weight, but regeneration occurs as readily 

 as it does in planaria. They are also able to 

 break their bodies into many parts, a proc- 

 ess called autotomy (self-cutting), which 

 is not an uncommon characteristic among 

 invertebrates. 



In addition to a complete digestive tract, 

 the nemertine possesses a very primitive 

 circulatory system (Fig. 10-8). It consists of 

 three blood vessels that run throughout the 

 length of the body, but which do not break 

 up into tiny capillaries as in higher animals. 

 In these forms oxygen and food still diffuse 

 some distance through fluid before arriving 

 at the cells. Although still inefficient, this 

 method is a considerable improvement over 

 that found in lower forms, where diffu- 

 sion of digested material and oxygen from 

 sources of intake to cell must pass a greater 

 distance. The circulating fluid ( blood ) con- 

 tains red cells in some species, much like 

 those in higher animals. The red color is 



