580 REPORT OF COMMISSIONER OF FISH AND FISHERIES. 



and young belong in the highest degree to the plankton, but not when 

 mature animals. The copepods, although lively swimmers, are tossed 

 about involuntarily by the water, and, therefore, must be reckoned in 

 the plankton (9, p. 1). If, with Henseu, we thus limit the conception 

 of 2)Ianldon, then we must distinguish the aetirely swimming nelcton 

 from the passivehj driven iilanldon. The teiin thus loses its firm 

 hold, and becomes dependent on quite variable conditions; upon the 

 changing force of the current in which the animal is driven, by the 

 momentaiy energy of voluntary swimming movements, etc. A pehigic 

 fish or copepod, which is borne along by a strong current, belongs to 

 the plankton ; if he can make a little i)rogress across this current, and 

 if, besides this, he can voluntarily and iudependently define his course, 

 then he belongs also to the nekton. It therefore seems to me advisable, 

 as jireliminary, to regard the term plankton in the wider sense, in oi)j)0- 

 sition to benthos. 



Still, for the chief theme which Hensen has set up in his plankton 

 studies, for the physiological investigation of the cycle of matter in 

 the sea {Stofffwechscl des 3Ieeres), this limitation of the plankton con- 

 ception will not hold; for a single large fish which daily devours hun- 

 dreds of i)teropods or thousands of coi)epods exerts a greater intluence 

 on the economy of the sea than the hundreds of small animals which 

 belong to the i)lankton. I will return to this in speaking of the 

 vertebrates of the i)lankton. If with Hensen we could, on practical 

 grounds, separate those animals of the plankton which are carried 

 involuntarily from those following their own voluntary swimming 

 movements (independent of the current), we might distinguish the 

 former as ploteric* the latter as nccferic* 



HALIPLANKTON AND LIMNOPLANKTON. 



Although the swimnung population of fresh water shows far less 

 variety and i)eculiarity than that of the sea, still among the former as 

 among the latter similar conditions are develoi^ed. Already the study 

 begins to take a joyous flight to the pelagic animals of the mountain 

 lakes, etc. Therefore, it will be necessary here also to fix limits, 

 as has been already done for the marine fauna; but since the term 

 "pelagic" should only be used for marine animals, it becomes advis- 

 able to designate as limnetic the so-called "pelagic" animals of fresh 

 w\ater. Among these we can again distinguish auioJimnetic (living only 

 at the surface), zonolimnetic (limited to certain depths), and haihylim- 

 netic (dwellers in the deep waters). The totality of the swimming and 

 floating population of the fresh water may be called Jimnoplanldov, as 

 opposed to the marine liaUpJanMon (9, p. 1), Avhich we here briefly 

 <-A\\\pJ(tnldon. 



* Tl/io>-//p = drifting ; vtiktij^ = swimming, 



