366 FRESH-WATER BIOLOGY 



certain sense they are not inhabitants of fresh water, they infest 

 aquatic animals and their hfe histories form a part of aquatic 

 biology. To be sure some species of parasites never come into 

 contact with the external world but are transferred from host to 

 host with the material in which they are Kving and others are en- 

 tirely dependent upon terrestrial animals as hosts. Such parasites 

 have no direct relation to fresh-water life and will be entirely 

 omitted in the present discussion. However, in the large majority 

 of parasitic forms the parasitic stage alternates with a longer or 

 shorter non-parasitic period. During this period of free existence 

 the species is a dweller in fresh waters alongside of their normal 

 inhabitants, possessed of similar organs of locomotion and other 

 adaptations to a free existence, often unrecognized in their true 

 nature, and properly regarded as members of the shore or bottom 

 fauna or plankton. This fact alone compels their consideration 

 in any discussion of aquatic life. 



Contrasted with this stage is the parasitic period which is more 

 extended, usually embracing almost all of the life history. In it 

 the worm remains with its host, dependent upon the latter for 

 protection, locomotion, and subsistence, showing structural modi- 

 fications which aid in maintaining this dependence and indicating 

 by the absence of organs calculated to provide for successful inde- 

 pendent activity the changes which the parasitic habit has induced 

 in its original structure. 



As already indicated most parasites show distinct adaptations 

 to the conditions under which they Hve. To be sure some, such 

 as certain small parasitic nematodes, are indistinguishable from 

 their free-living relatives, but such instances are rare. The large 

 majority have lost organs usually found in free forms and have 

 gained structures of significance only for a parasitic existence. 

 Furthermore, both loss and gain are relative and graded, rather 

 than absolute and unrelated. Thus in some flukes the ahmentary 

 system is about as well developed as in the free-living Turbellaria, 

 and of much the same type {cf. Figs. 678 and 639^); in other 

 flukes the system is greatly reduced {cf. Microphallus, Fig. 697); 

 and finally in the cestodes it is entirely lacking. The same condi- 

 tions prevail in the threadworms. Most of the true Nematoda 



