INTRODUCTION 5 



dwelling continuously upon or within the bodies of their hosts, we have 

 in the ffistridse, among the dipterous insects, a cycle involving internal 

 parasitism duruig the larval stage, a familiar example being the common 

 horse botfly (Gastrophilus intestinalis) , the development of which is 

 given on page 54. It is plain that a very small percentage of the eggs 

 deposited by this fly can reach the horse's mouth, and that, having got 

 thus far, many of the larvae must be destroyed or pass entirely through 

 the intestinal tract without having succeeded in becoming fixed to the 

 mucous membrane. For this there seems to be compensation in the 

 large number of eggs deposited by the persistent female. 



While in some cases the complete life cycle of a parasite requires but 

 one host, often, for reasons stated in the foregoing, two successive and 

 generally specifically different hosts are required. A rather compli- 

 cated example of the latter case is the life history of the common liver 

 fluke (Fasciola hepatica), one of the flat worms infesting in its adult 

 state the livers of Herbivora. It will be noted in referring to the cycle of 

 this parasite, given in detail elsewhere (page 160), that it is a very 

 hazardous one, and that its completion must depend upon the co- 

 operation of numerous favorable conditions. The eggs, of which each 

 individual fluke is capable of producing in the neighborhood of one 

 hundred thousand, must reach the exterior amid surroundings favorable 

 to their hatching. If hatched, the larva must escape its many aquatic 

 enemies and within a few hours find a suitable snail host. Providing the 

 snail is not eaten by a duck, or does not otherwise perish during this 

 phase of the cycle, it issues from its host as the free-swimming cercaria, 

 when it is again liable to fall prey to various small aquatic animals. 

 Escaping this and })ecoming encysted, the chance of any herbivorous 

 animal coming along and swallowing it is very small. The relation of 

 the enormous number of eggs, and the number of individuals which one 

 egg may produce, to the survival of the species amid conditions fraught 

 with such dangers seems quite evident. 



In general it may be said as to the propagation of parasites that their 

 prodigious fecundit}^ and the great vital resistance with which most of 

 them are endowed enables species to survive and perpetuate their kind 

 amid varied destroying influences which otherwise would bring about 

 their extermination. The tapeworms inhabiting the intestines of man 

 and other animals, afford another example of extreme parasitism accom- 

 panied by this remarkable development of the reproductive function. 

 Here is a creature so altered to its degenerate existence that it has be- 

 come devoid of mouth and intestine, the body consisting of a scolex, 

 usually referred to as the head, from which are give off segments which 

 remain united until there is formed, as in Tcenia saginata of man, a 

 band-shaped colony of from twelve hundred to thirteen hundred or 

 more, passing back from the worm's attachment to a length which may 



