47^ SCOTT'S LAST EXPEDITION 
These floating organisms, which include representatives 
of all the larger divisions of the animal kingdom, are spoken 
of collectively as the plankton. On some occasions the 
net was towed behind the ship for about half an hour to 
catch the floating population of the surface waters. Some- 
times the ship was kept stationary and the net sent down 
by* a sinker to 500 fathoms or less and hauled up again ; 
by this means samples of those forms which live below the 
surface were obtained. 
About 70 samples of the plankton were collected. 
They vary greatly in size ; one catch hardly covers the 
bottom of a half-pound honey jar, while another requires 
two seven-pound fruit jars to contain it. 
The size of a catch of course depends upon various 
factors, such as the size of the net, the time it was fishing, or 
the amount of water passing through it, and the quantity 
of plankton in the sea at the place where the haul was 
obtained. 
A small number of sea-water samples were collected 
from various depths by means of the Nansen-Pettersson 
water-bottle. These were generally taken from the areas 
in which plankton samples were obtained. The object of 
these w^ater samples is to ascertain the salinity of the sea 
at different points and at different depths. 
Any change in the salinity means a marked change in 
the character of the plankton. 
The plankton catches, when sorted, will doubtless be 
found to contain many new genera and species to add to 
the list of the known forms of living things. The vertical 
hauls, which were generally made for quantitative pur_ 
