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 Hensen, we thus limit the conception | 
of plankton, then we must distinguish the actively swimming nekton 
from the passively driven plankton. 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 pelagic 
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 tothe nekton. It therefore seems to me advisable, 
as preliminary, to regard the term plankton in the wider 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 (Stoffwechsel des Meeres), 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 intinence 
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 as ploteric,* the latter as necteric.* 
HALIPLANKTON AND LIMNOPLANKTON. 
Although the swimming population of fresh water shows far less 
variety and peculiarity 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 
‘““nelagic” should only be used for marine animals, it becomes advis- 
able to designate as limnetic the so-called “pelagic” animals of fresh 
water. Among these we can again distinguish autolimnetic (living only 
at the surface), zonolimnetic (limited to certain depths), and bathylim- 
netic (dwellers in the deep waters). The totality of the swimming and 
floating population of the fresh water may be called limnoplankton, as 
opposed to the marine haliplankton (9, p. 1), which we here briefly 
call plankton. 
*TAorip = drifting; vy«ti¢ =Swimming. 
