PARASITES AND THEIR DEVELOPMENT. 



555 



large along with the pork in which they are con- 

 tained, and such persons as partake of this comes- 

 tible in an imperfectly cooked condition thereby 

 qualify themselves for becoming the " hosts " of 

 tapeworms ; since, when a cystic worm from the 

 muscles of the pig is introduced into the human 

 stomach, the little bladder or sac which the worm 

 possesses drops off, and the minute head of the 

 worm becomes attached to the lining membrane 

 of the digestive system. Once fixed in this posi- 

 tion, the circle of development may be said to be 

 completed. A process of budding sets in, and 

 joint after joint is produced, until the adult tape- 

 worm, measuring, it may be, many feet in length, 

 is developed ; while each egg of this full-grown 

 being, if surrounded by the requisite conditions, 

 and if provided with a pig-host to begin with, 

 will repeat the marvelous and complicated life- 

 history of its parent. 



The history of the tapeworms, like that of the 

 flukes, therefore, exhibits a very complex series 

 of conditions, and, unless these conditions are ful- 

 filled by the young parasite, development is either 

 cut short or is altogether suspended. The fact 

 of a double host having to be provided for the 

 due development of tapeworms is not peculiar to 

 the production of the species inhabiting man. 

 All of these parasites pass through an essentially 

 similar series of developments. The cystic worms 

 which cause the " measles " in the pig become, as 

 we have seen, and when eaten by man, the com- 

 mon species of human tapeworm. The cystic 

 worms man obtains from underdone beef are de- 

 veloped within his economy into a tapeworm of 

 another kind. The young parasites which reside 

 in the liver of the rabbit, and which attain no 

 higher development than that seen in the pig, 

 become, when swallowed by the dog or fox, the 

 special tapeworm-tenant of these animals. The 

 cystic worm of the mouse develops into the tape- 

 worm of the cat ; so that the dog, fox, and cat, 

 do not enjoy an immunity from enemies, but act- 

 ually acquire disease from the victims they so 

 ruthlessly pursue. The chances of destruction 

 which beset the young parasite on its way through 

 the world are so multifarious when compared 

 with its chances of favorable development, that, 

 practically, the immense number of eggs pro- 

 duced by these animals are of small account. 

 Of the thousands of eggs developed, the merest 

 fraction attain development, and the presence of 

 a complex life-history in parasites must be re- 

 garded as in reality forming a saving clause, as 

 far as man is concerned, when we consider our 

 comparative immunity from their attack. 



Even more extraordinary than the phases of 

 development which have just been detailed, are 

 those undergone by a special form of tapeworm 

 inhabiting the dog. The egg of this latter para- 

 site gains admittance to the body of the dog- 

 louse, and therein becomes the cystic worm, simi- 

 lar to that formed within the muscles of the pig 

 in the case of the human tapeworm. Tlie dog, in 

 the process of cleaning his skin, swallows the 

 skin-parasite with its contained but immature 

 tapeworm ; and, once introduced to the dog's 

 digestive system, the latter form liberates itself 

 from the louse and becomes the mature tapeworm. 

 Anything more extraordinary than this peculiar 

 circle of development can hardly be imagined in 

 the life-histories of animals. Nor are the condi- 

 tions which have determined, and which continue 

 the development, rendered clear to us by the 

 most careful study of the subject. Why it is that 

 the tapeworm should not attain its full develop- 

 ment within the pig, rabbit, mouse, or dog-louse, 

 as its first host, we do not know ; nor can it be 

 rendered plain what conditions have so sharply 

 divided the life of these parasites into two periods 

 of such well-marked kind. 



The whole question of parasitism, however, 

 exhibits a striking illustration of the influence of 

 habit and of surrounding conditions on the life of 

 animals. No one may doubt that the habit of 

 one animal attaching itself to another is an ac- 

 quired one. The most ardent advocate of the 

 doctrine of special creation would never dream 

 of maintaining that parasites were created as we 

 find them in relationship with their hosts. Even 

 were this argument advanced as a mere matter of 

 unsupported belief, the order and succession of 

 life upon the globe would present facts which 

 would at once veto the belief. The lowest ani- 

 mals appeared first, and were succeeded by forms 

 of gradually increasing complexity. Hence the 

 parasites must have been developed before their 

 hosts. Man appeared long after the tapeworms 

 or their ancestors were produced ; and the intri- 

 cate relationship between man and his neighbor- 

 animals and the parasites must have been ac- 

 quired in a gradual fashion. Best of all, this 

 opinion is supported by the information to be 

 gained from a survey of parasitic life at large. 

 We may begin such a survey by noting animals 

 which attach themselves to other animals as mere 

 "lodgers." Such are external parasites. Next 

 may be traced parasites which depend for house- 

 room upon other animals, but which do not re- 

 quire board and sustenance from their hosts. 

 Such " messmates " are presented by the little 



