64 LIFE-HISTORY OF PARASITES. 



the Ephemera hindered this process to some extent by pushing away 

 the Gordius-larvae, and rendering their attempts ineffectual. I found 

 one specimen in the fat body endeavouring, but without success, to 

 make its way between two huge drops of fat : every time that the larva 

 pushed them asunder, they flowed together again directly. The longer 

 the Ephemerid larvae remained in the infected water, the greater was 

 the number of Gordius-embryos, which penetrated into their bodies ; 

 I found them in all the organs, the legs, palpi, fat body, and 

 especially in the body-cavity, and even in the dorsal vessel, lying some- 

 times close to one of the valves, and moved to and fro by its pulsations. 

 The number of parasites in one larva was sometimes so great (as 

 many as forty) that I am inclined to attribute to this helminthiasis 

 the sudden mortality which took place among the Ephemerce" 



For a considerable time it was believed that the parasitism of these 

 free embryos was always brought about by their own active migration 

 into the body of a host ; of course, it was possible that it might be 

 effected in other ways, but there was no proof of this. At present 

 we know that many larval parasites find their way into the body of a 

 host by means of drinking water. I transferred a quantity of muddy 

 water containing embryos of Dochmius trigonocephahis (Fig. 43, a, b,) 

 to the alimentary canal of a dog, and saw them grow into the 

 parasite after the lapse of a few days. 1 Man is infected in a simi- 

 lar way by Dochmins diwdenalis, and the horse by Sclerostomum 

 equinum. It is probably only the free young stages of Nematodes 

 which select the natural passages in order to become Entozoa ; at any 

 rate they are the only forms that can, by the thickness of their 

 skin, withstand the action of the digestive juice. Meissner, however, 

 and others have shown that this is not a complete protection ; the 

 former observed numerous Gordius-embvyos destroyed by the digestive 

 fluids of Ephemerid larvae, and I have observed the same in Monosto- 

 mum. In a similar fashion the often numerous specimens of Filaria 

 sanguinis, which the mosquito sucks up with the blood of man, shortly 

 perish almost without exception in its alimentary canal. 2 



1 See Vol. II. 



2 From the observations of Manson (Trans. Linn. Soc. Land., pp. 367-8, 1884) there 

 can no longer be any doubt that the few embryos which can pass without danger to 

 themselves through the intestine of the mosquito undergo further development in the 

 body-cavity, in consequence of which they now differ in size and in the structure of the 

 mouth parts from the embryo at an earlier stage. Manson is of opinion that embryos, 

 having thus reached a certain stage in the body-cavity, get into water only on the 

 death of the host, and that they are taken into the human body with the water. This 

 statement still requires demonstration, but even were this proof forthcoming, there would 

 yet remain a possibility that the embryos evacuated with the urine (which probably no 

 more represent a useless production than the eggs of intestinal worms which pass out with 

 the faeces) may be transported to certain small hosts, and by these means human beings 

 may perhaps be infected more commonly than in the way pointed out by Manson. R. L. 



