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THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT 



strength to sweep away the greatest obstacles 

 man or Nature may oppose to its fury. Invest 

 the idea of the single rain-drop with time, and 

 the action which appears feeble, if viewed for a 

 single moment, becomes of mighty extent when 

 multiplied into years and centuries. And simi- 

 larly with the case of the fluke and its neighbor- 

 parasites. A single fluke is of itself an unimpor- 

 tant quantity, but when this quantity becomes 

 multiplied by hundreds, the proverb that " union 

 is strength" receives a new and very decided ap- 

 plication. Existing in large numbers within the 

 liver-ducts of the sheep, the flukes cause irrita- 

 tion, and a whole train of symptoms which end 

 usually in. starvation and death. Hence the ex- 

 treme fertility of parasites might well afford a 

 text whereon a sophist might inveigh against the 

 wise regulation of the domain of living Nature, 

 were it not that in reality these animals are 

 checked and controlled through the actual com- 

 plexity of their own development. Strange as 

 the statement may seem, it is nevertheless true 

 that Nature appears to offer a premium against 

 the development and increase of these and other 

 parasites, through their having to undergo a series 

 of very striking changes on the way to maturity. 

 The parasite's path to adult life may truly be de- 

 scribed as checkered in the highest degree. There 

 are numerous pitfalls and snares laid for its re- 

 ception, and for the extinction of its young life ; 

 and the " struggle for existence " in the present 

 case is not only fierce, but, in the case of a very 

 large majority of the combatants, utterly hopeless. 

 Let us briefly trace the life-history of a fluke 

 by way of practical illustration of these latter 

 remarks. From each individual fluke residing 

 within the body of its sheep-host, hundreds of 

 eggs are discharged. Each egg undergoes a pre- 

 liminary process of development, and, from the 

 eggs which escape into water, little free-swimming 

 bodies are liberated. These minute living parti- 

 cles are young or embryo flukes. Each resem- 

 bles an inverted cone in shape, and swims rapidly 

 through the water by aid of the microscopic fila- 

 ments which fringe its body. It is clear that such 

 eggs as do not reach water will not undergo 

 development, and hence a first check to the in- 

 crease of the flukes exists in the fact that many 

 eggs must perish from the absence of appropriate 

 surroundings. Sooner or later, the young fluke 

 loses its power of swimming, and becomes of oval 

 shape ; crawling inelegantly, by contractions of 

 its body, over the muddy bottom of its pool or 

 river. Thereafter it appears to seek an entrance 

 to the body of some co-tenant of its pool, such a 



creature being usually found in the shape of a 

 water-snail. Buried within the tissues of this 

 first "host," the young fluke becomes trans- 

 formed into a sac or bag, within which other 

 young may arise by a veritable process of bud- 

 ding. This rising generation appears in the form 

 of small bodies, each provided with a vibratile 

 tail From the body of the snail, these " second- 

 ary young" soon make their escape ; and, while 

 existing in the water, are readily conveyed into 

 the stomach of the sheep in the act of drinking. 

 Thence these young flukes penetrate to the liver 

 of the animal, and become transformed into the 

 mature and flattened adult. 



The unexplained necessity for such a com- 

 plicated series of changes in development, and 

 for the varied circumstances which mark the 

 career of the young fluke, presents us with condi- 

 tions which operate powerfully against the undue 

 increase of the race. An exactly analogous series 

 of changes is to be perceived in the development 

 of many other parasites, and among others in 

 that of the various groups of tapeworms which 

 reside within the digestive system of man and 

 other quadrupeds. But for the complexity of 

 their development, and for the consequent limita- 

 tion of their increase, these parasites would over- 

 run and exterminate their hosts in a short period 

 of time. A common tapeworm begins life as a 

 minute body, set free from its coverings and in- 

 vestments, and provided with a special boring 

 apparatus, consisting of six hooks. This little 

 creature will perish unless it can gain access to 

 the body of some warm-blooded quadruped, and 

 the pig accordingly appears on the scene as the 

 most convenient host for the reception of the lit- 

 tle embryo. But within the body of the pig, 

 there is not the slightest possibility of the little 

 embryo becoming a tapeworm. The pig has mere- 

 ly to perform the part of unconscious " nurse," 

 and to prepare its " guest " for a yet higher stage 

 of existence. Being swallowed by the pig, the 

 young parasite bores its way through the tissues 

 from the digestive system to the muscles of the 

 animal, and there develops around its body a 

 kind of bag or sac. In this state it represents 

 the " cystic worm " of old writers ; and occasion- 

 ally it may prefer the liver, brain, or even the eye, 

 of its first host to the muscles in which it usually 

 resides. Here, however, it can attain no further 

 development. If the pig dies a natural death, 

 there can be no possibility of the tapeworm stage 

 being evolved. But if, as is most likely, the p : g 

 suffers death at the butcher's hands, the little 

 " cystic worms " may be bought by mankind at 



