SEA-FISHERIES LABORATORY. 239 
seas this may also occur, and Professor Kofoid has drawn 
my attention to the fact that even at the equator a heavy 
diatom plankton may sometimes be encountered. Thus, 
in the “ Albatross” expedition reports* we find: “It is 
most interesting to note the number of diatoms found in 
this tropical region. They have usually been considered 
as characteristic of more temperate and colder regions. 
On several occasions the surface waters were greatly 
discoloured by their presence.’ A radiolarian plankton 
would also soon stop up the meshes, and this applies also 
to a plankton containing many gelatinous forms—cteno- 
phores, ete. This factor has received far too httle atten- 
tion from the Kiel school. Added to this are two other 
features of some importance. ‘The difference between a 
new silk net and an old one of the same grade silk is 
considerable, and from Kofoid’s experimentst it appears 
that a new silk net, even after having been shrunk bv 
washing and pressing several times, catches at least fifty 
per cent. more than an older one. Furthermore, Hensen’s 
experiments for the filtration coefficients of different 
grades of silk were made with filtered water which left 
no chance of comparison with conditions in seas or lakes. 
These difficulties can, to a certain extent, be overcome 
by determining empirically the coefficient, and by 
determining separate coefficients for the same net, to be 
applied according to the class of plankton and the 
quantity present. The best method is that of parallel 
catches with the net and with pump and tube. A series 
of these should be carried out in the different classes of 
plankton likely to be met with, and tlie coefficients noted 
for future use. The pump tube must be carefully raised 
* “ Reports on Scient. Results of ‘ Albatross’ Exped.,’’ Vol. V, 
Mem. Mus. Comp. Zoology, Harvard, 1306, p. 14, 
+ Kofoid, Loc. cit., p. 263, 
