CHAPTER V. 



PARASITISM. 



It is a well-recognized biological principle that degeneration is the 

 inevitable outcome of continued parasitism (Lankester Degeneration). 

 A certain crustacean parasite begins life with the same number of 

 appendages as other crustaceans, but when it becomes attached to a 

 crab host, its appendages atrophy, evidences of other structures dis- 

 appear, and it becomes a mere bag — sacculina — on the abdomen of 

 its victim. Ascaris, trichina, and their allies similarly have lost most 

 of the dermal musculature and the power to move as most worms do. 

 Tenia and other tapeworms in like manner have lost not only the 

 body musculature, but digestive organs as well. Such parasites, living 

 in the digestive tracts of their hosts, are surrounded by digested and 

 partly digested food which passes by osmosis through the body wall; 

 mouth and digestive organs are unnecessary, and their disappearance 

 is to be accounted for on the theory of disuse. 



While degeneration of the usual vegetative organs is the inevitable 

 outcome of parasitism, the restricted mode of life of the parasite may 

 require certain accommodations which may lead to structural adapta- 

 tions on its part. The internal parasites of the digestive tract, for 

 example, might easily be dislodged and carried out of the intestine 

 with the muscular contraction and currents of that organ, while external 

 parasites would be readily detached and swept away, were they not 

 provided with some means of holding on, hence sucking disks, hooks, 

 and spines are characteristic of internal and external parasites. In 

 addition to increased development of certain attaching organs and 

 degeneration of vegetative organs of digestion, etc., there is an enor- 

 mous increase in the power of reproduction. It is a biological fact that 

 the number of offspring of an animal is in inverse proportion to the 

 chances of reaching maturity, and the number is always great enough 

 to maintain the species. It is quite apparent that a parasite living in a 

 certain portion of a given host would experience no little difficulty in 

 reaching that spot, hence every parasite has acquired the power of 

 reproducing immense numbers of progeny; a tapeworm, for example, 

 produces many hundred thousands of eggs, and yet the frequency of 

 infection by tapeworms is not great enough to cause any apprehension 

 among people who live with ordinary decency. 



With a ubiquitous group of organisms like the protozoa, it is to be 



