OF WASHINGTON. 3 



but does not entail any disadvantage to the other party (the host) ; 

 thirdly, true parasitism, in which there is a benefit to one of 

 the parties concerned (/". ., the parasite), to the disadvantage of 

 the other party (the host). Naturally, I do not contend that 

 these three grades of parasitism can be separated by sharp lines, 

 any more than we can draw a sharp line between animals and 

 plants. 



Mutualism. We not infrequently find sponges grown fast to 

 the back of a crab. In this symbiosis we can see the first step 

 towards parasitism, but this symbiosis, as Looss and others have 

 already pointed out, is mutually advantageous to both the crab 

 and the sponge, for the former is thereby more or less concealed 

 from view and will thus escape his enemies, while the sponge is 

 carried around from place to place and thus furnished with more 

 nourishment. Hydra viridis presents another case of mutual 

 ism : here the hydra can utilize the oxvgew produced by the 

 zoochlorellae, while the latter can utilize the carbon dioxide pro 

 duced by the hydra. 



Commensalism. The organisms referred to by Germans under 

 the term ^Raum Paris iten" furnish examples under this head. 

 For instance, in the intestinal tract of many aquatic insect larvae, 

 we find numerous rotatoria. These organisms obtain room-rent 

 free ; they do not, however, injure their hosts (so far as we can 

 observe) but they feed upon other microorganisms found in the 

 same place. The numerous infusoria in the first stomach of 

 ruminants would also come under this second grade of parasitism. 



True parasitism. As examples of true parasitism, we can cite 

 the tape-worms, the lumbricoid worms, trichinae, etc., etc. That 

 this symbiosis is of advantage to the parasites will be doubted by 

 no one, for we know that they cannot live and reproduce outside 

 of the body of their hosts. The disadvantage to the host may be 

 of three different kinds: first, the parasites live upon food which 

 should go to the nourishment of the host (cf. the adult tape 

 worms, ascarides, Echinorhynchus, etc.) ; secondly, they may 

 exert pressure upon the various organs and thus cause a me 

 chanical injury, as in the case of echinococcus-hydatids of the 

 liver, brain, etc., or Eustrongylus gig'as in the kidneys ; thirdly, 

 the parasites may form certain chemical products which act in 

 various ways upon the tissues, as in the case of the genus Ascaris 

 and the pathogenic bacteria. 



Rather than define a true parasite, as some persons are inclined 

 to do, as an organism which must necessarily live a parasitic life, 

 I think it better to follow the majority in accepting the term " ob 

 ligatory parasites" for such organisms, and to include those or 

 ganisms which may or may not live a parasitic life under the 

 term " facultative parasites :" 



We can make another division of parasites into " temporary 



