PELAGIC ZONE— SURFACE LIFE 455 



of both groups that actually live on the floor of the abysmal 

 zone. 



Besides the typical plankton fauna which has just been briefly 

 dealt with, the Pelagic zone is also inhabited by another assem- 

 blage of animals associated with various drifting objects, and 

 particularly with the vast accumulations of sea-weeds that are 

 found in the Sargasso Sea and elsewhere. Details would here 

 be out of place, but it may be remarked that fixed forms of life, 

 such as hydroid zoophytes, by attaching themselves to floating 

 sea-weed, are enabled to maintain a foothold in the Pelagic zone. 

 It is a matter of common knowledge that one of the most curious 

 of fixed Crustaceans, the Ship- Barnacle [Lepas), is commonly 

 found adhering to drifting objects. 



The skins of Whales and Fishes also afford a home to quite 

 a number of attached species, some of which are external parasites 

 belonging to various groups. And many pelagic animals are also 

 the unwilling hosts of numerous internal parasites, from which 

 no zone affords escape. 



Large bodies of fresh water may also be divided into zones, 

 inhabited by characteristic faunas. The plankton of lakes is of 

 particular interest, and also of some economic importance, as its 

 population augments the food -supply available for freshwater 

 fishes. 



