CHAPTEK V. 

 THE ORIGIN OF PARASITES, 



AND THE GRADUAL DEVELOPMENT OF PARASITIC LIFE. 



IF we endeavour to summarise our knowledge of parasitic life as we 

 have delineated it above, and to express its principal modifications in 

 a few words, we shall arrive at some such scheme as the following : 



I. Temporary Parasitism. To this category belong almost ex- 

 clusively ectoparasites, which differ from their free-living relations in 

 respect only of the quality and source of their nutriment. 



II. Stationary Ectoparasitism. The animals in this group show, 

 on the whole, only slight peculiarities. They either pass through all 

 their developmental stages (from the egg onwards) on the host, or at 

 first lead an independent existence under a form more or less different 

 from the adult. 



III. Entoparasitism. The entoparasite is always stationary, but, 

 with the exception of Rhabditis, which is apparently only occasionally 

 parasitic, never passes through all its developmental stages in one host. 

 The young brood is expelled, either in the form of free embryos or of 

 more or less developed (perhaps wholly undeveloped) eggs. In the 

 latter case the embryonic development occurs in the free state. But 

 from this stage onwards the fate of the embryos varies in different 

 directions. 



(1.) The embryos of Entozoa lead a free life for some time under aii 

 altered form (Nematodes, with Mhabdih's-like young stages). 

 They are not only capable of free motion, but take in food just 

 in the same way as other creatures. 



(a.) In the course of this free life the young form arrives at sexual 

 maturity, and thus only the sexually produced descendants re- 

 turn again to parasitism (Rhabdonema (Ascaris) nigrovenosa). l 

 (b.) The young form itself becomes again a sexually mature parasite 

 at a certain stage of its development. From the commence- 

 ment it enters its definitive host, and its development ends 

 there, although this happens occasionally, as in the case of 



1 Since the name Ascaris is quite incorrect to apply to these animals, I shall adopt 

 henceforth the new generic name Rhabdonema to designate them. 



