436 THE POPULAR SCIENCE MONTHLY 



of parasitism over the whole organic world, I have not only distorted it 

 beyond recognition, but have deprived it of any usefulness which it 

 may have had. To such accusations I can only reply that I gladly 

 concede that it is admissible for practical purposes to circumscribe para- 

 sitism by arbitrary names and definitions in special fields of biological, 

 sociological and psychological study, but I must insist, nevertheless, 

 that it is a very fundamental and far-reaching phenomenon, which, for 

 theoretic and heuristic purposes, may properly be said to include any 

 complex of vital processes, which maintain themselves at the expense of 

 other vital processes, in the same or in other organisms, without react- 

 ing on these processes in a similarly sustentative manner. But let us 

 return to the more conventional conception of the subject. 



When an organism becomes parasitic it, of course, undergoes 

 structural and physiological changes. These express themselves in the 

 loss or modification of previously existing characters and in the acquisi- 

 tion of new characters. The amount of this loss, modification and 

 acquisition depends, first, on the intimacy of relationship of the para- 

 site to the host ; second, on the nature of this relationship ; third, on the 

 time in the parasite's ontogeny when this relationship is established, 

 and fourth, on the portion of the ontogeny which it covers. Ectopara- 

 sites, as we all know, are, as a rule, less modified than entoparasites, but 

 each of these categories includes very different degrees of modification, 

 according as the parasite is confined to a particular organ of the host 

 or is capable of moving more freely over its surface or through its 

 tissues. The habitus of a parasite is most profoundly influenced and 

 characterized by the moment in its ontogeny when it joins its host, and 

 especially by the length of the period during which this association is 

 maintained. According as this association is coextensive with the para- 

 site's life or merely for a briefer period, we may distinguish permanent 

 and temporary parasites. The latter, again, may be divided into those 

 that are free from their hosts only during larval or early life and those 

 that are free as adults. To these three types practically all animal 

 parasites can be referred. They are best represented by such forms as 

 certain tape-worms and flukes, by such crustaceans as Sacculina, and 

 by such insects as the Ichneumonidae and Chalcididas among Hymen- 

 optera and the Tachinidas among Diptera. Permit me to describe very 

 briefly the salient peculiarities of a typical example of each of these 

 groups. 



The tape-worm is an excellent example of a permanent parasite. 

 It produces an enormous number of very minute eggs, and either these 

 or the singular embryos, known as onchospheres, which they contain, 

 are passively swallowed by the host. The onchosphere passes from the 

 alimentary tract into the tissues of the host and there becomes a bladder- 

 worm. This, in order to become a sexually mature tape-worm, must 

 enter the alimentary tract of a second host. The transfer is effected 



