CHAPTER II. 

 OCCURRENCE OF PARASITES. 



JUST as there is hardly a single creature which is not preyed upon 

 by some carnivorous foe, so it seems probable that every animal gives 

 shelter at one time or another to some parasite. We are even 

 acquainted with cases where the parasite itself is subject to the 

 attacks of other parasites. Many of the parasitic Crustacea, for 

 example, give shelter to mites or thread- worms ; the parasitic larva of 

 the ichneumon-fly again is inhabited by other minute parasitic 

 larvae (Pteromalinse.) In the case of the Nematode, Trickosomum 

 crassicauda, which infests the rat, we find three or four males living 

 parasitically within the uterus of the female. 1 Neither small dimen- 

 sions nor a concealed mode of life offer the slightest protection against 

 these enemies. 



Nevertheless, every animal is by no means equally subject to the 

 attacks of parasites ; on the contrary, we find the greatest differences 

 in this respect. In certain animals, the presence of parasites appears 

 to be the rule, since they may be found in great abundance 2 in every 

 individual examined ; in certain others hundreds of specimens may 

 be searched without finding a single parasite. On the whole, it may 

 be safely stated that the Vertebrata are far more generally infested by 

 parasites than the Invertebrata, and indeed it was thought for a long 

 time that their occurrence in the latter was a mere accident. Which- 



1 See Vol. II. The recent researches of Biitschli and von Linstow have fully de- 

 monstrated this fact. Moreover, there is another free-living worm (Boncttia), the male 

 of which in a similar fashion lives within the genital duct of its own female. See 

 Kowalewsky, " Du male planariforme de la Bonellia," Revue dcs Sci. Nat., pL iv., 1875 

 (translated from the Russian); Vejdovsky, Zeitochr. f. wiss. Zool., Bd. xxx., p. 487, 

 1878 ; Spengel, MittlteU. zool. Stat. Xeapel, Bd. i., p. 357 et seq. 



3 The snipe, the goose so long as it lives in meadows the turbot, have their 

 intestines almost always filled with numerous Helminths, generally Cestodes. How vast 

 is occasionally the number of parasites is shown by certain cases of trichinosis and the 

 Cochin-China diarrhoea. Even the larger intestinal worms are sometimes found in great 

 numbers. Bloch (" Abhandlung von der Erzeugungder EingeweidewUrmer," Berlin, 1782, 

 p. 12) found in a male bustard at least a thousand specimens of Tcenia rillosa, some of 

 which were no less than 4 feet in length. Goze also (" Versuch einer Naturgeschichte 

 der Eingeweidewurmer," 1782, p. 32, note) found the alimentary canal of a parrot so full of 

 Cestodes, 20 ells in length and about the thickness of a straw, "that it (intestine) was 

 almost ready to burst." When the whole mass was placed in water, Goze was astonished 



