DISPERSAL OF PARASITIC EGGS 197 



present inv^estigation is not concerned. The conveyance of the 

 eggs to the outside is only a short stage in the life of the parasite. 

 Thereafter a more or less lengthy and complicated career awaits 

 them before they are suited to re-infect their original host. This 

 portion of their life-history follows one of two broad lines : 

 I. The egg develops and in time produces a larva, which may 

 be retained in the egg-shell or set free, but in either case the 

 larva is ready to re-infect. 2. The egg gives rise to a larva 

 which enters an animal (intermediate host) different from that 

 in which the egg was produced. In this second animal it passes 

 a short part of its life and, in the event of this animal being 

 devoured by the first, the parasite is enabled to complete its 

 life-cycle and cause infection again. 



" The avenue by which the first animal is infected is in the 

 great majority of cases its food ; in a few instances it may be 

 infected by the larva penetrating its skin. It is evident, 

 therefore, that there must be some means by which the food 

 is contaminated with excremental matter in the first of the 

 above-mentioned categories, and, in the second, some means 

 by which the eggs of the parasite are conveyed to the second 

 animal. Water is by far the most important vehicle of transit. 

 A certain amount of moisture is necessary for the development 

 of the eggs although many of them can resist drying for long 

 periods. In closely-associated communities, however, mechanical 

 transit on the feet of individual animals is a common mode of 

 dispersing the eggs. It is here that the agency of flies has to 

 be reckoned with, especially such flies as divide their attention 

 between excremental matter and food-stuffs. The common 

 house-fly is well known to have such habits and it is thus with 

 reason that it has been suspected of conveying the eggs of 

 parasites from faecal material to food. There are many flies, 

 other than Musca domestica, which display like habits, and 

 though they do not bear such a close relationship to man 

 several of them are commonly associated with the domesticated 

 animals. 



" With regard to their life-histories, the parasites of man may 

 be divided into two classes, excluding the Filaria worms, and 

 mentioning only the best known species, as follows : 



