20 THE ANIMAL PARASITES OF MAN 



sition to true parasitism. The great advantages attached to a parasitic 

 life consist not only in protection, but also in the supply of suitable 

 food, and consequently in the easier and greater production of eggs, 

 and thus fully account for the gradual passage of facultative parasitism 

 into true parasitism, in many forms the young stages live free for 

 some time (Strongylidce), in others, as is the case in Rhabdoneuw, 

 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, by taking their manner of life into account, this 

 matter presents greater difficulties in regard to other helminthes. It 

 is true that the segmented Cestoda may be connected with and traced 

 from the less known and interesting single-jointed Cestoda (Auiphilijia, 

 Archigetes, CaryopJiyllczits, Gyrocotyle). Trematodes are all parasites, 

 with the exception of one group, Teinnocephalidce, several genera and 

 species of which live on the surface of the bodies of Crustacea and 

 turtles of tropical and sub-tropical fresh waters. Ternnocephalidcv are, 

 nevertheless, predacious. They feed on Infusoria, the larvae of small 

 insects and Crustacea. So far as is known they do not nourish them- 

 selves on part of the host. They belong 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 

 its food. However, space parasitism may still be regarded as the 

 first stage of commensalism, which is again to be regarded as a sort 

 of transition to true parasitism. 



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

 trematodes, 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 the few parasitic turbellaria there are some that possess 

 clinging discs or suctorial pores, and these are only differentiated 

 from 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, 

 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 opinion 

 that the present intermediate hosts, which belong principally to the 

 lower animals, were the original hosts of the parasites, and fostered 



