26 
BIOLOGICAL INVESTIGATIONS. 
The following table exhibits the general character of the 
organisms caught by surface and ground nets at the excursions. In 
the case of Plankton the approximate quantity of the material, after 
being allowed to settle, is given and the time during which the net 
was down. Unless otherwise stated, the steamer was trawling at 
the time the net was being used, and the speed would therefore be 
about 2 knots. The net used on all these occasions was made of 
bunting and the diameter of the hoop would be about one foot. 
It will be seen that from 1 to 9 c.c. are obtained in about one 
hour, occasionally bulking when Medusoids are present. The 
Crustacean life practically always obtained in Plankton determina- 
tions includes Copepoda and Cladocera, the former being conspicuously 
prominent. We have had to record instances, however, of the 
Cladocera being predominant. Occasionally, also, in-shore a quantity 
of dinoflagellate protozoa are met with, and these appear to quite 
predominate in mid North Sea. If we take it that 5 c.c. is the usual 
catch and 5,000 cubic feet the quantity of water which passes through 
the net, this would give 1 c.c. to every 1,000 cubic feet near the 
surface. That this figure is much too small, however, is evidenced 
by the fact that a short gentle row with a similar net at Cullercoats 
will give 1-3 c.c. in some 300 feet. The column of water in the 
net prevents the ingress of much fresh material unless the speed be 
slow enough to equalize the passage of the water through the 
material of which the net is made. 
It is fairly constant in quantity — at all events it is so in our 
ordinary in-shore experiments. On every occasion we obtain much 
the same volume of material. In June, 1899, however, the 
quantity was very much above the average. 
To the above Crustacean and Protozoan life higher Crustacea are 
added occasionally even at the surface, as Hippolyte varians, Mysids, 
Amphipods, especially Hyperoche and Euthemisto, and Isopods. Of 
the latter it is noteworthy that in 1896-7 Eurydice achatus was very 
commonly obtained at the surface, but since then it has only rarely 
been observed. It was often taken in company with Idotea 
tricuspid ata, Gammarus locusta and lumpsucker embryos. In the 
early part of the summer we procure certain fish ova and larval fish 
and during the year a succession of larval forms representing nearly 
all the groups of invertebrate life. Fairly constantly present, like- 
wise, are Sagitta and Pleurobrachia. Beroe is sometimes very 
common, and the ordinary jelly fishes occupy naturally a conspicuous 
place in our Plankton. 
