5H0 KEPOKT OF COMMISSIONER OF FISH ASD FiSIIFHlES. 



and young' bcluiig in the highest degree to the phxnkton, 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 (0, p. 1). If, with Ilensen, we thus limit the conception 

 of planlio)!, then we must distinguish the actively swimming ncJiton 

 from the j^as.s/rc/^ driven pUnikion. The term 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 

 momentary energy of voluntary swimming movements, etc. A pelngio 

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

 the plankton; if he can make a little progress across this current, and 

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

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

 as i)reliminary, to regard the term plankton in the Avider sense, in oppo- 

 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 {>Sto,f\vechsel dcs Mceres), this limitation of the plankton con- 

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

 dreds of pteropods or thousands of copepods exerts a greater inliuence 

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

 belong to the plankton. I will return to this in speaking of the 

 vertebrates of the plankton. 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 Sisploteric;* the latter as nccteric* 



HALIPLANKTON AND LIMNOPLANKTON. 



Although the swimming population of fresh water shows far less 

 variety and j>eculiarity than that of the sea, still among the former as 

 among the latter similar conditions are developed. 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 

 "])elagi('" should only be used for marine animals, it becomes advis- 

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

 water. Among these we can again distinguish auioHmnciic (living only 

 at the surface), zonoHtitnetic (limited to certain depths), and bathylim- 

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

 floating ])0])Tdation of the fresh water may be called Jinin(>]>l<(nlto)i, as 

 o})pose(l to the marine Jmliplanlton (1>, p. ]), Ai'hich we here briefly 

 v\\\\pJ((n]:fon. 



' WkuTiip = drifting ; vijxTTjg = swimmingi 



