12 OCCURRENCE OF PARASITES. 



connective tissue, the brain and eye, and many other localities; 

 Echinococcus is found in the liver, spleen, kidney, lung, bones, nervous 

 centres, beneath the skin, and, in short, almost everywhere in the 

 human body. Similarly Filaria papillosa of the horse is not only 

 found in the peritoneal membrane, but also in the peripheral connective 

 tissue of various parts of the body, and occasionally in the body-cavity, 

 inside the skull and vertebral column, sometimes even in the eye, 

 either in the outer layers, the anterior chamber, or the vitreous body. 

 The same principle holds good in regard to the relations of a 

 parasite to its host. Some species are limited to a single host ; others, 

 again, are parasitic upon several animals, not merely at different 

 periods of their existence— passing their youth in the body of one 

 animal, and attaining their maturity in that of another — ^but also 

 during the same phase of development. In the first group may be 

 reckoned among human parasites, Pediculus capitis, Bothriocephalus 

 latus, and Oxyuris vermicularis ; also Tmnia crassicoUis of the cat, and 

 Echinorhynchus gigas of the pig. To the second group belong by far 

 the greater number of parasites, such as Strongylus gigas, which is 

 found in many Carnivora — in the genera Canis, Mustda, Naswa, 

 &c., in the horse, the ox, and in man ; Trichina spiralis, which 

 not only infests man, the pig, and the rat, but also the hedgehog, 

 fox, martin, dog, cat, rabbit, ox, and horse, and may be trans- 

 planted even to birds. Bistomum hepaticum has also a very wide 

 distribution among warm-blooded animals, being found in most Eumi- 

 nantia, Perissodactyla, Pachydermata, and Eodentia, and also in the 

 kangaroo, in man, &c. Although it is a general rule that a para- 

 site infests several distinct animals, it is equally true that the 

 distribution of parasites is governed by certain laws. The examples 

 just cited show this clearly. The various animals which are infested 

 by one and the same parasite are always more or less closely related 

 to each other. It is most usual to find that the related species of a 

 given genus, or the genera of a given family, harbour the same para- 

 sites ; there are, indeed, only very few exceptions to this rule, such as 

 Trichina spiralis. But even in these rare cases a certain relation can 

 be observed between the different hosts ; and a parasite which in the 

 same stage of its existence infests sometimes a mammal or a fish, 

 sometimes a mollusc, is quite unknown. This fact becomes more 

 evident when we examine not merely the number of hosts in which 

 a given parasite is met with, but also the statistics of the distribution 

 of parasites, and discover the number of times which it is found in 

 each host ; for instance, in other words, Distomum hepaticum is only 

 rarely found in man, the kangaroo, and rodents, while it is commonly 



met with in ruminants, especially in the sheep. The same holds good 

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