Februaby 8, 1907] 



SCIENCE 



211 



safely be affirmed that this substance, 

 which in the case of a resting parasite must 

 be discharged directly into the tissue of its 

 host, is to that tissue poisonous. If such 

 material is given off in small amount it will 

 evidently act as a stimulant upon the sur- 

 rounding cells and be the factor in bring- 

 ing together the leucocytes which accumu- 

 late about the invading organism. An 

 inert body of the same size, if introduced 

 into the same position, would not induce 

 the formation of a cyst or certainly the 

 tissue proliferations which accompany in 

 some eases the attacks of parasites. It may 

 even be doubted whether under the condi- 

 tions for pearl formation, grains of sand, 

 as formerly believed, will give the proper 

 stimulus for the formation of the pearl. 

 The important factor then must be one 

 which is associated with the organism, not 

 as an object occupying space, but as a liv- 

 ing thing, and the most evident char- 

 acteristic of this is the giving off of waste 

 matter. This leads naturally to the next 

 subdivision of the subject. 



As physiological effects may be grouped 

 together those influences of the parasite on 

 the host which express themselves in the 

 limitation or modification of the normal 

 physiological processes of the latter. In 

 some cases it is often true here that the 

 primary effect is hidden and that the 

 secondary result alone can be seen. It is 

 sometimes possible to determine the pri- 

 mary result by study of the secondary, and 

 to see the way in which it has been brought 

 about. In other eases we merely know the 

 secondary effects without being able to dis- 

 entangle from the complicated series of 

 phenomena the primary cause. Further- 

 more, those interferences with normal func- 

 tion which are grouped under this heading 

 are not such as might be traced to the 

 action of the parasite as a foreign body, 

 but such as are related to its own activity 

 as a living organism. It will be seen that 



the distinction here is not perfectly clear, 

 and perhaps somewhat artificial, for the 

 mechanical disturbances of parasitism in- 

 terfere with the normal activities of the 

 host in the same way that other foreign 

 bodies may modify or limit the working of 

 the same organism. However, it is my in- 

 tention under this heading to consider the 

 results which come from the contact of life 

 with life, the interaction of function with 

 function. 



Perhaps the most evident factor and the 

 most frequently mentioned, certainly the 

 first to be noted in this connection, is the 

 absorption of nutriment. The parasite de- 

 mands a certain amount of food matter to 

 carry on its own vital processes. This food 

 matter is furnished, so far as endoparasites 

 are concerned at least, in a partially or 

 fully digested condition, by the host ani- 

 mal. Many observers have maintained 

 that the actual loss to the host in this way 

 is so slight as to be negligible. 



Leuckart says that a Dibothriocephalus 

 latus 7 meters in length weighs 27.5 grams, 

 and gives off in a year a total of 15 to 20 

 meters of proglottids of a weight of ap- 

 proximately 140 grams, and that Tcenia 

 saginata produces daily on the average 11 

 proglottids, an amount equal to about 550 

 grams. These figures have been taken by 

 some to indicate the actual loss of food 

 material on the part of the host ; this would 

 be evidently insignificant in comparison 

 with the amount consumed by a man within 

 the year. It has even been suggested that 

 the increased appetite induced by the pres- 

 ence of the parasite more than compensates 

 for this slight loss. It would appear, how- 

 ever, that such a method of statement is 

 exceedingly inadequate. It is very diffi- 

 cult to estimate the amovmt of food con- 

 sumed by an animal in proportion to its 

 weight, but it is certainly grossly insuffi- 

 cient to indicate this in any way as com- 

 mensurate with the amount of growth 



