DERIVATION OF PARASITES 21 



in protection, but also in the supply of suitable food, and con- 

 sequently in the easier and greater production of eggs, fully 

 account for the gradual passage of facultative parasitism into 

 true parasitism. In many forms the young stages live free for 

 some time (Strongyloides), in others, as is the case in Rhabdunema, 

 parasitic and free-living generations alternate ; in others, again, 

 the free period is limited to the egg stage or entirely suppressed. 



Though it is possible thus to connect the parasitic with the 

 free-living nematodes, also by taking their manner of life into 

 account, this matter presents greater difficulties in regard to other 

 helminthes. It is true that the jointed cestoda may be connected 

 with and traced from the less known and interesting single- 

 jointed cestoda (Amphilina, Archigetes, Caryophyll&us, Gyrocotyle), 

 but trematodes are all parasites, with the exception of one form, 

 Temnocephala, a peculiarly shaped creature, several species of which 

 live on the surface of the body of crustaceans and freshwater 

 turtles. Temnocephala is, nevertheless, a predacious animal ; it 

 feeds on infusoria, the larvae of small insects and crustaceans. 

 So far as is known it does not nourish itself on parts of its host; 

 it belongs to the group of commensals, or more correctly, to that 

 of the SPACE PARASITES, which simply dwell with their host and 

 do not even take a portion of the - superfluity of his food. How- 

 ever, space parasitism may still be regarded as the first stage of 

 commensalism, which is again to be regarded as a sort of transi- 

 tion to true parasitism. 



It is possible that parasitism came about in this way in the trema- 

 todes, in which connection we must first consider the turbellaria-like 

 ancestors of the trematodes. Much can be said in favour of such a 

 genetic relationship between turbellaria and trematodes and hardly 

 anything against it ; it should also be remembered that amongst 

 discs or suctorial pores, and which are only differentiated from 

 the few .parasitic turbellaria there are some that possess clinging 

 ectoparasitic trematodes by the possession of a ciliated integu- 

 ment, which is found only in the larval stages of the latter. 



The ACANTHOCEPHALA occupy an isolated position. Most authors 

 certainly regard them as related to the nematodes ; in any case, 

 however, the connection is not a close one, and the far-reaching 

 alterations which must have occurred prevent a clear view. Perhaps 

 the free original forms of acanthocephala are no longer in existence, 

 but that such must have existed is a foregone conclusion. 



An explanation of the CHANGE OF HOST so frequent in parasites 

 is more difficult than that of their descent. R. Leuckart is of I 



