280 
Birge—The Crustacea of the Plankton. 
tered. As a matter of fact, the net caught about twice as many 
Crustacea as the tube, thus indicating that its coefficient is about 
two. 
In this method of determining the coefficient the quantities 
compared -are by no means uniform; indeed, it is known that 
the number of Crustacea caught in a given haul of the tube may 
be only one-half the number caught in a second haul within a 
few seconds. A single comparison has therefore very little 
value and accuracy in the determination of the coefficient by 
this method can be reached only by a considerable number of ob¬ 
servations. In my own work I made use of six sets of obser¬ 
vations, taken on May 14th, October 12th and 25th, 1895, Feb¬ 
ruary 25, May 18th, and July 11th, 1896. By distributing 
the observations over so long a time it was possible to get at 
the coefficient of the net at different times in its life and under 
different conditions of plankton. In May the number of Crus¬ 
tacea is at a maximum, and the amount of algae is small. In 
October the number of Crustacea is considerable, but the veget¬ 
able life is at a maximum; while in February the amount both 
of animal and vegetable life is of course small. From four to 
six pairs of observations were taken in each set. The ratio of 
the catch of the tube to that of the net was computed for each 
observation in the set, and the average of these ratios was com¬ 
puted, using the method of least squares. As a result of 
these determinations, the following ratio was established: 
Tube : net : : 49.85 : 100. The probable error of the deter¬ 
mination is ± 1. The appended table shows the general results 
Several facts appear from the table. It will be noticed 
that the amount of difference between the maximum and mini¬ 
mum numbers caught varies greatly on different occasions. It 
is plain also that the net shows no greater amount of variation 
on the whole than does the tube. On the contrary, on those 
occasions where the numbers are approximately constant in the 
tube, they are similarly constant in the case of the net; and 
where the numbers vary considerably in the case of the net, 
they vary to much the same degree in the case of the tube. 
There is therefore no reason to suspect any considerable irregu¬ 
larity on the part of the net due to the stoppage of its openings, 
or to any other cause. 
