94 THE ORIGIN OF PAEASITES. 



has related forms living in the free state, and that in considerable 

 numbers, namely, the round-worms, or Nematodes. But the free-living 

 Nematodes have only recently become the subject of a close investi- 

 gation. ^ Only a few decades ago, scarcely half a dozen of these forms 

 were known, and these only imperfectly, so that naturalists, mistaking 

 their natural relations, were inclined to class them with the Infusoria 

 rather than with the Nematodes. Under such circumstances it seems 

 easy to understand how the older helminthologists entertained the 

 view that the internal parasites stood isolated, not only biologically 

 but also systematically, from other animals. They united them into 

 a single class (Entozoa), which, although nearly approaching the free- 

 living worms, was understood to have no close relation to them. It 

 will be obvious that such a connection helped greatly to displace the 

 processes of entozootic life from their natural connections. Under its 

 influence parasitism appeared in science as a phenomenon sui generis, 

 which could not be judged according to the laws of ordinary animal 

 life, but, on the contrary, was thought to be opposed to these in many 

 of its relations. On a former occasion (p. 22 et seq.) it has been 

 shown at length how for a long time special and peculiar laws were 

 supposed to govern the existence and origin of the Entozoa, and 

 howthese had been invented, for the most part by systematic 

 helminthologists, until they ultimately learned to judge facts more 

 correctly and more in accordance with nature ; and thus the relations 

 of the Entozoa to the free-living animals have found a more proper 

 recognition. 



As has been mentioned, the relations are most evident among the 

 Nematodes, which are a group of animals whose representatives, far 

 from being exclusively Entozoa, have in the free state such a wide dis- 

 tribution, and under such varying circumstances, that the number of 

 parasitic forms, although also great, is far outbalanced by the former. 

 It would, of course, be impossible here to attempt a full description 

 of these free Nematodes. For our purpose, it will be sufficient to 

 remark that they live in the sea, in fresh water, in mud, and in the 

 earth ; and that sometimes they lead a predatory existence, at other 

 times they live on decaying matters. To the latter belong the best 

 known and most widely distributed forms, tlie species of Dujardin's 

 genus Rhabditis, above mentioned (Leptodera ; Pclodera, Schneider). 

 They are animals of small size, which live everywhere in large num- 

 bers where the earth is imi)regnated with decaying organic substances, 

 and differ from their related forms, especially in the structure of 

 their alimentary and sexual organs. Especially characteristic is the 

 highly muscular oesophageal tube, which encloses in its posterior 



' Especially by Baatian, Ebert, Schneider, Biitsohli, Marion, and de Maan. 



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