212 



SCIENCE 



[N. S. Vol. XXV. No. 632 



which the animal achieves within a given 

 period. 



There is also another factor which must 

 be taken into account, viz., that the para- 

 site consists almost exclusively of actively 

 functioning organs, and that there is a 

 minimum of such inert parts as skeletal 

 structures which add greatly to the weight 

 of an animal, but do not involve the con- 

 sumption of food material for their main- 

 tenance. It has been shown recently that 

 parasites contain a large amount of gly- 

 cogen; in analyses from one fourth to one 

 third of the dry weight consists of this 

 material. The highest value previously 

 known was 14 per cent, in Gardium (a 

 moUusk) and mammals apart from the 

 liver have only about 3 per cent. Later 

 the same author (Weinland) demonstrated 

 that the vital processes of Ascaris proceed 

 without oxygen and involve a fermentation 

 process, resembling that of bacteria and 

 yeast in producing butyric acid and al- 

 cohol. The possibility of such a wasteful 

 process, measured in calories actually used, 

 is found in the especially favorable condi- 

 tions of existence of the parasite, which 

 afford it carbohydrates in superabundance 

 and both protection and warmth from the 

 host. But naturally the waste of the proc- 

 ess means draft on the host. 



Furthermore, the presence of the gly- 

 cogen, which is unquestionably reserve ma- 

 terial, indicates unmistakably that meta- 

 bolic processes are exceedingly active in the 

 organism. Another indication of the same 

 thing is to be found in the reproductive 

 activity. The tapeworm is producing and 

 maturing an enormous number of eggs, 

 each of which is supplied with a consider- 

 able yolk dowry. It is safe to afSrm that 

 during reproductive periods the draft of 

 any animal upon its food supply is at a 

 maximum. All of these three items, then, 

 the rapid growth of the individual, the 

 production of an unlimited supply of re- 



serve material and the extreme reproduc- 

 tive activity, point to a heavy draft upon 

 the host. I have been unable to find any 

 calculations which might be applied to such 

 organisms as parasites with any likelihood 

 that they would yield even approximate 

 estimates of the material actually con- 

 sumed. It would seem clear that previous 

 calculations are incomplete and that the 

 draft on the host is far greater than has 

 been imagined heretofore. 



An important effect on the host is trace- 

 able to the increase in the size of the 

 parasite. This normal accompaniment of 

 growth is most significant when the para- 

 site occupies a limited space or when the 

 increase in size is marked. The brain cys- 

 ticercus, ordinarily found in a ventricle, 

 grows until the cavity is occupied and then 

 pressure iipon the nervous tissue brings a 

 sudden end to the life of the host. A liver 

 hydatid may continue to groAV almost with- 

 out limit and only when important struc- 

 tures become involved is the pressure of 

 serious moment. Most bladder worms do 

 not exceed insignificant limits in growth 

 and consequently exert little or no influ- 

 ence on the host. The echinococcus, 

 through its indefinite growth, is sure to 

 reach a size which will interfere with the 

 activity of the host, and from its usual 

 location in the liver is likely to include 

 important vessels, or nerves, and lead to 

 fatal interference with normal functions. 



Another general infiuence which para- 

 sites are thought to exercise on the host is 

 explained by the hypothesis of reflex nerv- 

 ous action. According to this view intes- 

 tinal parasites affect the host by irritating 

 the nerve terminations and provoking in 

 reflex fashion the varied troubles of hel- 

 minthiasis Avhich the clinician recognizes. 

 This is a role which has been regularly 

 ascribed to them and yet, as Guiart says, 

 this view is in fact a pure hypothesis. It 

 has been invoked as an easy way to explain 



