DERIVATION OF PARASITES 19 



Parasites are hardly ever inherited amongst animals. 1 According 

 to a few statements, however, Trichinella and Cceniims are supposed 

 to be transmissible from the infected mother to the fcetus. Other- 

 wise most animals acquire their parasites, especially the Entozoa, from 

 without, the parasites penetrating either actively, as in animals living 

 in the water, or passively with food and drink. A particular pre- 

 disposition 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 ecto- 

 parasites 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, Acantho- 

 cephala, Linguatulidce, 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 Linguaiulidce 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 some of them to be regarded as the descendants of the lowest 

 Wiizopoda. We know that the Trematoda, and through these the 

 Cestoda, are closely related to the Turbellaria, from which they may 

 be traced. The Netnatoda, and still more the Acanthocephala, stand 

 apart. This is less evident, however, in the Nematoda, for there 

 are numerous free-living members of these from which it is possible 

 that the parasitic species may be descended. Indeed, this seems 

 more than probable if such examples as Leptodera, Rliabdoncuia and 

 Strongyloides are taken into consideration, 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 comprehensible 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 tran- 



1 However, in the Protozoa there are examples of hereditary transmission of parasites, e.g., 

 in the case of Babesia (Piroplasmd) bovis and Babesia cam's in their invertebrate hosts (ticks) ; 

 in Crithidia melophagia and Crithidia hyalommce ; and in the case of Spiroch^ta duttoni in its 

 invertebrate host (a tick). 



