INVESTIGATION OF THE PLANKTON OF THE IRISH SEA. 161 
of mouth and a shearing plate of the same size, made of coarse meshed 
canvas, is very effective for towing horizontally at various depths, such as 
5 or 10 fathoms, in order to catch the larger, organisms of the macroplankton 
such as Sagitta, the larger Crustacean larvæ, medusæ, and young fishes, and 
its catching power may be estimated at about 10 times that of our standard 
horizontal tow-nets of 14 inches diameter of mouth. Of these standard tow- 
nets, those made of No. 20 (now No. 25) Dufour’s bolting silk caught more 
of the smaller forms of the plankton (microplankton), such as diatoms and 
dinoflagellates, and those made of the coarser No. 9 bolting cloth caught a 
larger number of the Copepoda and the larger larval forms and fewer of the 
small diatoms, and consequently in a zooplankton or a mixed plankton 
usually gave the larger catches, When, however, there was much micro- 
plankton in the water the finer-meshed net caught most. 
One of the first objects of our experiments was to determine whether 
simultaneous hauls of two or more similar nets gave identical samples of the 
plankton, and as the result of many observations, under various conditions, 
year after year, the evidence was that they do not. Even when alike in 
quantity, the samples are generally unlike in quality, as we have shown 
above. These observations lead to the conclusion that the plankton is not 
evenly distributed through the water, but is in most cases disposed in zones, 
tracts, or swarms according to the nature of the organisms. This is 
especially the case with Copepoda and the larval stages of the higher 
Crustacea, Sagitta, Tomopteris, Medusæ, and other forms with some slight 
power of locomotion which may enable them to get out of one zone or tract 
of water into another. We have recorded various instances showing that 
Copepoda and other larger animals of the plankton are very markedly in 
swarms. 
The phytoplankton is more evenly distributed, but even diatoms are usually 
more abundant in soine zones of the water than in others. Asa rule, during 
daylight the most abundant plankton of all kinds is found a few fathoms 
below the surface—say, from 5 to 10 fathoms. Our “weight” net, which was 
exactly the same as the surface-nets with the addition of a heavy weight 
attached to the rope so as to cause the net to tow some fathoms deep, 
invariably caught more than the nets above, and in many cases obtained 
about double the quantity of some organisms. 
One of the difficulties in working the open-mouthed tow-net is that it is 
almost impossible to go fast enough to keep the net working near the surface 
and at the same time slow enough to prevent the water from “ banking-up ” 
in front, causing currents across the mouth. Consequently we found it 
necessary to prolong the mouth of the net into a funnel of canvas with the 
narrow end forwards so as greatly to reduce the amount of water entering 
the mouth. Another useful device we adopted, in order to avoid any possible 
