60 



LIFE-HISTORY OF PARASITES. 



especially Nematodes, live in damp earth instead of water ; and there 

 are other parasites, but only air-breathing insects, that inhabit drier 



FIG. 39. Free embryo of 

 Distomum fiepaticum. 



FIG. 40. Free embryo of liothrio- 

 cephaltu lattu. 



localities. As a well-known example of this, may be adduced the 

 larva of the flea (Fig. 41), which is found in quantities in retired 

 spots in the neighbourhood of mouldering organic 

 matter, such as dusty corners of rooms, and in the 

 straw of hen-houses, &c. The comparison of a flea- 

 larva to the young of Helminths in this particular 

 does not, however, imply that they agree in all 

 respects. The life of a flea-larva is of long dura- 

 tion, and so noteworthy as regards growth and 

 metamorphosis, that it must be considered quite as 

 important as that of the adult With the Entozoa, 

 however, it is quite different, at least with the 

 greater number ; not merely do the young (except 

 in some cases) take no nourishment during the free 

 stage of their existence, but the period itself is of 

 FIG. 41. Larva of short duration, and serves only as a means to 

 further their distribution and migration. Instead 

 of blind chance, which in other cases directs the fate of the germs 

 of parasites, we have to do with a definite and fixed order of events. 

 This free stage of existence, in spite of its short duration, is long 

 enough, under favourable circumstances, for the parasite to make its 

 way into the body of some host. 



In the first edition of this work I was obliged to leave it 

 uncertain whether any parasites existed in which the free stage 



