1892.] The Problem of Marine Biology. 803 
larly certain coelenterates, echinoderms and worms, which 
pass only part of their life as free swimming animals; for the 
remainder they are bottom dwellers. Such species are not 
usually found far from the coast, and hence the true oceanic 
Plankton is made up of forms which pass their entire life as 
free swimming organisms. By the presence or absence of — 
these bottom dwelling species the Planktologist can determine 
approximately the region where the forms were captured. 
mere list of the genera, not to mention the species of 
plants and animals up to the present found to take part in the 
constitution of the Plankton would be very formidable. The 
range in size is enormous; from the exceedingly minute uni- 
cellular algae ,,;4;, of an inch in diameter to the huge bulk of 
many fishes and cetaceans. The microscopic forms constitute 
the fundamental food supply in the cycle of marine life. 
They are capable of exceedingly rapid multiplication, and 
furnish nourishment for the myriads of large animals, which 
in time are preyed upon by the still higher forms. The incon- 
ceivable number of individuals of the smaller species is dem- 
onstrated by Prof. Hensen’s determination of the number of 
individuals in about two cubic yards of Baltic Sea water. 
This was found to contain 5,700,000 distinct organisms; of 
these only about 150,000 were visible to the unaided eye. 
But very often microscopic forms become so numerous as. to 
form a slime upon the surface of the water for a considerable 
_area. Ships frequently sail for miles through water colored 
by these microscopic organisms, e. g: the so-called “black 
water ” of the Arctic and Antarctic Seas, is aslime of diatoms, 
which serve as food for the shoals of minute crustacea and 
-mollusca (Pteropods, sea butterflies, and Cephalopods, squid, 
cuttlefish) upon which the walrus and whales feed. In the 
„warm regions the inconceivably enormous quantity of dia- 
toms are replaced by another kind of alge, the Oscillatorie, 
which often for an area many miles in extent color the sea a 
dark red or yellowish brown. The Red Sea received its name 
from the abundance of one of these alge, Trichodesmium 
_erythreum, which, according to Ehrenberg, colored the water 
along the shore a blood red. In the warm region also are 
