MARINE BIOLOGICAL STATION AT PORT ERIN. 87 
The Irish Sea contains a surprising number of what 
are sometimes regarded as “ oceanic” species—not merely 
as occasional visitants, but as normal and continuous 
constituents of the plankton during a great part of the 
year. Amongst these may be mentioned Chatoceros 
densum, Coscinodiscus radiatus, Rhizosolenia semispina, 
Cerateum tripos, Pertdinium sp., Tomopteris onisciformas, 
Pleurobrachia pileus, Calanus helgolandicus, Anomalocera 
pattersoni, Acartia clausi, Oithona similis, and Orkopleura 
dioica. Some of these oceanic species seem, so far as we 
can judge from the published records, to be more abundant 
and more continuously present round the Isle of Man than 
they are even in the western part of the English Channel. 
We have evidence from our closing vertical nets that 
the zone of most abundant life is not on the surface but is 
generally a few fathoms below—say, usually, between 5 
and 10 fathoms. Samples of water from 5, 10 and 20 
fathoms obtained with the ‘“ Mill” water-bottle support 
the above statement. But this conclusion was arrived at 
and could be established, quite apart from the evidence of 
the vertical nets, from a comparison of the results obtained 
by the weighted and surface open horizontal tow-nets. 
At the time of the Diatom maximum in spring, however, 
our closing vertical nets showed that these Protophyta are 
more abundant in still deeper zones, and increase in 
density downwards to at least 20 fathoms. 
In the cases of some groups, e.g., Cladocera and 
Oikopleura, the distribution is sometimes remarkably 
regular, the same numbers being taken simultaneously by 
comparable nets at localities up to ten miles apart; but on 
the other hand even with these same groups there may, on 
other dates, be very diverse hauls indicating an uneven 
distribution. Some species, and some groups of neritic 
larve markedly congregate in shoals, and this also adds 
to the unevenness of the distribution, 
