MARINE BIOLOGICAL STATION AT’ PORT ERIN. 75 
extending up to 15 millions and 16 millions in single 
hauls in the case of ARhzzosolenia semirspina—in fact, this 
highest peak in the September maximum of Diatoms is 
mainly composed of this one species of Rhizosolenia, 
whereas in the spring maximum the bulk of the catch is 
made up of Chetoceros contortum and Thalassiosira 
nordenskioldw, species that are rare or altogether absent 
in September gatherings. ‘The genus Vhalassiosira 1s 
mainly a spring form, rarely present after May, and is 
not represented in autumn in this year’s results. 
When a comparison is made between the three similar 
open tow-nets which were worked together for fifteen 
minutes at a time—two at or close to the surface, and the 
other weighted so that it was lowered to a depth of about 
ten fathoms, and gradually rose, as the boat went slowly 
ahead, to a depth of a foot or two below the surface—it is 
almost invariably found that the weighted net, with its 
wider range through the deeper layers of water, gave a 
larger, sometimes a much larger, quantity of organisms. 
The only exceptions to this rule are on some ovcasions in 
April, when the sea was full of Diatoms and the surface 
nets gave very large hauls, equal to, or even exceeding, 
the deeper ones. But even during the Diatom maximum 
in April some days showed more in the weighted than in 
the surface nets. For example, on April 10th, at along- 
shore Station III, the surface gave 1l‘o and the net at 
one fathom 19°5 ¢c.c., and the total Diatoms were 27,000 
im the former and 188,000 in the latter. 
In some cases, as 1 showed last year, the two similar 
surface-nets worked together gave dissimilar results. 
Kiven when the results are very much alike quantitatively, 
they may be very difterent qualitatively, and it is by no 
means always the two hauls that are most alike in bulk 
that agree best in the kind and number of organisms. It 
