6 PARASITES OF THE DOMESTIC ANIMALS 



exceed twenty feet. After about the six hundredth, each segment is a 

 mature and sexually complete individual, which later, as it is pushed on 

 by new segments formed at the head, becomes filled with fecundated 

 eggs. By the successive detachment of these "ripe" segments and their 

 passage from the body of the host, it has been estimated that Taenia 

 saginata might throw off in a year as many as one hundred and fifty 

 million eggs, of which but an infinitesimal number, as is quite evident, 

 will reach the bodj' of their proper bovine host for larval development. 

 Again, having been so fortunate, it is improbable that the larvae will, 

 while living, reach the intestines of the human host necessary for their 

 further development into adult worms. 



Here, then, is an animal well showing the degree of degeneration 

 which may be reached in extreme parasitism; there are no organs of 

 locomotion, no organs of special sense, no organs of digestion, no organs 

 of respiration, and none of true circulation. The body consists of a long 

 band of connected segments, each, when mature, bisexually complete 

 and in itself a sort of independent reproductive individual, the entire 

 energy of the organism concentrated upon the function of reproduction 

 that the perpetuation of the species may be insured amid the perils with 

 which this process is beset. 



In many forms permanently parasitic there is an early period of 

 development in which organs of locomotion are distinctly present, but, 

 as the animal matures, these fail to develop or become lost. If it is 

 assumed that this gradual loss of organs, change of structure, and protec- 

 tive transmission of the embryo to an intermediate host is due to the 

 parasitic life, it seems reasonable to conclude that all of the parasitic 

 groups have been derived from free-living forms, and that, as parasitism 

 became a more fixed habit, such structural changes were in the course of 

 time brought about as would make this mode of life obhgatory. A re- 

 view of the observed facts, then, in their biologic relationship, leads to 

 the conclusion that symbiosis, of which parasitism is a form, has its 

 causative basis in the struggle for existence, the symbiotic association in 

 more or less measure mitigating the hazards to one or both symbionts. 

 It further follows that, though some forms have undergone an extreme 

 modification, through related contemporary free-living types, their true 

 systematic position may be established. 



