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 proteids 

 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 

 particular 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 ancestry 

 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 

 Cestode seems bound up with this ^particular activity. The 

 young worm attached to the intestinal wall grows by absorption 

 of food digested and prepared for assimilation by the functioning 

 digestive cells of the host. The first trace of reproduction is 

 the formation of a somite-like bud at the posterior end of one 

 parasite. Continued growth involves continued new bud-for- 

 mation with enlargement of the older buds until a long chain of 



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