CHAPTER VIII 

 PARASITISM: PHYSIOLOGICAL ADAPTATION 



A. THE TAPEWORM, TAENIA SP. 



Many types of adaptation can be traced back directly to the 

 effects of the environment. These may be either structural or 

 functional or both. A tapeworm has no mouth or digestive 

 tract, but obtains its food by absorption of dissolved proteins 

 from the host. It has little need for movement if it were 

 necessary for it to move, it could not do so easily, for the body 

 musculature is inadequately developed. It might be inferred 

 from its position in the digestive tract that such a parasite would 

 need some apparatus of attachment. Suckers and hooks are 

 developed for this purpose. Absence of muscular develop- 

 ment indicates lack of need for nervous system. The nervous 

 system is most primitive. So, too, are organs of excretion. All 

 of these structural features indicate an adaptation for the par- 

 ticular mode of life of an intestinal parasite. Physiological 

 adaptations must also have been developed with the change 

 from independent individualism to dependent association. 

 The loss of digestive tract could not have occurred in the an- 

 cestry of our cestode, so long as there was need of it for life of 

 the worm (Fig. 83). 



The greatest physiological adaptation, however, is apparent in 

 the reproductive system. The entire construction of the tape- 

 worm seems bound up with this particular activity. The 

 young worm, attached to the intestinal wall, grows by ab- 

 sorption of food digested and prepared for assimilation by the 

 functioning digestive cells of the host. The first trace of repro- 

 duction is the formation of a somite-like bud at the posterior end 

 of the parasite. Continued growth involves continued new bud- 

 formation with enlargement of the older buds, until a long chain 



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