20 THE ANIMAL PARASITES OF MAN 



otherwise all animals acquire their parasites, especially the entozoa y 

 from without, the parasites penetrating either actively, as in 

 animals living in the water, or passively with food and drink. 

 A particular predisposition to worms is not more likely than a 

 spontaneous origin of parasites. 



Derivation of Parasites. Doubt now no longer exists as to the 

 derivation of the temporary and of many of the stationary 

 ectoparasites from free-living forms. This conclusion is founded 

 on the circumstance that not only are there numerous 

 intermediate degrees in the manner of living and feeding 

 between predacious and parasitic animals, but that there is 

 more or less . uniformity in their structure. The differences that 

 exist are easily explained as consequences of altered conditions of 

 life. The case is more difficult in regard to groups that are 

 exclusively parasitic (Cestoda, Trematoda, Acanthocephala, Lingua- 

 tulidcz, and Sporozoa), or groups that are chiefly parasitic (Nematoda), 

 because in these cases the gulf that divides these forms from 

 free-living animals is wider. It is true that we know that the 

 nearest relatives of the Linguatulidce are found amongst the 

 Arachnoidea, and indeed in the Acarina ; that, moreover, the 

 structure and development of the Sporozoa refers them to the 

 Protozoa, and allows them to be regarded as the descendants of 

 the lowest Rhizopodce. We know that the Trematoda, and through 

 these the Cestoda, are closely related to the Turbellaria, from 

 which they may be traced ; the Nematoda, however, and still 

 more the Acanthocephala, stand quite apart. This is less evident 

 however, in the Nematoda, for there are numerous free-living 

 kinds of these from which it is possible that the parasitic 

 species may have descended. Indeed, this seems more than 

 probable if one takes into consideration such examples as Lepto- 

 dera, Rhabdonema and Strongyloides, as well as the conditions of 

 life of free-living nematodes. These mostly, if not exclusively, 

 spend their lives in places where decomposing organic substances 

 are present in quantities ; some species attain maturity only in 

 such localities, and there propagate very rapidly. Should the 

 favourable conditions for feeding be changed the animals seek 

 out other- localities, or they remain in the larval form for 

 some time until more favourable conditions set in. It is com- 

 prehensible that such forms are very likely to adopt a parasitic 

 manner of life which at first is facultative (Leptodera, Anguillula), 

 but may be regarded as the transition to true parasitism. The 

 great advantages attached to a parasitic life consisting not only 



