Febbuaey 8, 1907] 



SCIENCE 



203 



traceable to the fact that it neither bur- 

 rows into nor feeds upon the mucous lining 

 of the canal, but contents itself with taking 

 its food from the partially digested con- 

 tents of the intestine. Inasmuch as the 

 organism is very small, this is evidently a 

 negligible factor in the economy of the 

 host; but even here, as I shall show later, 

 there is the possibility that under some cir- 

 cumstances the organism may become a 

 menace. Again, Filaria loa, the African 

 eye worm, lives for many years in the con- 

 nective tissue of the human body, wander- 

 ing from point to point, often not far be- 

 low the skin. In the course of its migra- 

 tions it does apparently no injury to the 

 host, who is indeed unconscious of its pres- 

 ence until it happens to come into the con- 

 nective tissue over the surface of the eye- 

 ball. Here it appeals to the sense of sight, 

 and from here it has most frequently been 

 extracted. But in this ease again there 

 are swellings which appear from time to 

 time on the surface of the body and which 

 are believed by some to be due in one way 

 or other to this parasite. 



In the group of indifferent bodies we 

 must also include many, if not all, of the 

 resting forms, those which, like the bladder 

 worms, or the young trichinffi, are encysted 

 at a given point and which consequently 

 are not to any appreciable extent absorb- 

 ing nutrition or producing toxic materials. 

 The encapsulated trichina is but a grain 

 of sand, the encysted bladder worm no 

 more than a globule of fat in the tissue. 

 During its entire life in the body the 

 guinea worm does not seem to exercise any 

 influence whatever upon the host organism 

 until the female appears at the surface and 

 is desirous of securing an opening through 

 which it may discharge the young into the 

 outer world, where they will find conditions 

 for further existence. 



These few preliminary statements have 

 paved the way for a somewhat clearer gen- 



eral idea of the factors which determine 

 the degree of influence exerted by the para- 

 site on its host, and the first is evidently 

 the numerical factor. Commonly, the 

 single parasite leaves no effect; it is the 

 multiplication of parasites which is to be 

 feared. Evidently this multiplication will 

 be most serious when it takes place within 

 the host and leads directly to a multiple 

 infection. This is known among the pro- 

 tozoa, where, in at least one host, there is 

 usually a recurrence of generations of the 

 same type, and this feature is so character- 

 istic that Doflein designates this cycle of 

 the life history as the multiplicative cycle, 

 defining its purpose as being to achieve the 

 multiple infection of the host. There are 

 also some worms among the Nematodes in 

 which the same thing takes place and by 

 virtue of which the infection of the ali- 

 mentary canal is enormously increased in 

 direct fashion. 



Among most metazoan parasites, how- 

 ever, including all Trematodes, Cestodes 

 and some Nematodes, such is not the case. 

 The eggs of the individual must be trans- 

 ported to the outer world before they can 

 carry out their development, and they reach 

 a new host after a more or less complicated 

 life history which may involve alternation 

 of generations and parasitism in one or 

 more intermediate hosts, so that when the 

 stage comes in which the infection of the 

 first species is possible, it is very unlikely 

 that the original individual will be rein- 

 fected. Among such parasites the effects 

 from a single individual are not serious 

 and the real danger lies in a multiple infec- 

 tion through the increase in numbers which 

 such a species often experiences in the in- 

 termediate host, or within a limited area 

 in the outer world, so that by the taking in 

 of a single external object a large number 

 of parasites may be introduced. Where 

 alternations of generations exist the dan- 

 gers of the parasitic existence are so great 



