INVESTIGATION OF THE PLANKTON OF THE IRISH SEA. 159 
The poorest years in the Calanus record are 1917, with a maximum of 
only 1440 on July 19th, and 1919, when the top number was 1240 on 
June 5th. 
The swarms of Calanus are generally aceompanied by local mackerel or 
herring fisheries *. 
Mierocalanus pusillus, first described by G. O. Sars in 1903, is a northern, 
deep-water, and very small species, which is found off the west coast of 
Norway only in depths over 150 fathoms (Sars) and in quantity to the 
north of Iceland in deep hauls (Paulsen). It appeared suddenly in numbers 
in our deeper gatherings from mid-channel in the autumn of 1907 (2500 on 
Sept. 12th), and has been present many times since. In 1921, for 
example, over 20,000 were taken in a haul at the mouth of the bay on 
March 28th, and again, in December 1918, there was a decided invasion 
of Port Erin bay by this deep-water Copepod. 
When this minute species was first recognised by Mr. Andrew Scott in 
1907 we supposed that it was probably another case of an invasion of the 
Irish Sea from the north, especially as in that year a northern diatom, 
Thalassiosira Nordenskioldit, was present in great quantity ; but it has been 
present in our records in very irregular numbers nearly every year since, 
generally with its maximum in winter or early spring. Even if not an 
invader from outside the Irish Sea, it is at any rate a deeper-water form 
which only occasionally spreads to the surface and the inshore shallow 
waters, and varies much in its occurrence from year to year. 
Mozzuscan LARVA. 
Lamellibranch larvæ are frequently present in the plankton in large 
numbers, and it is probable that they come from the large scallop bed 
(Pecten opercularis and other species) which lies within a couple of miles to 
the north of Port Erin at a depth of 20 fathoms. In view of the fact that 
some years are known to be much more favourable than others for the deposit 
and further development of molluscan spat, it is important to examine the 
variations in abundance of those larvee during the 15 years. 
In each successive year we find that the swarms of Lamellibranch larvæ 
are most numerous in the early months (January to May) and again in 
autumn (from September to November), their minimum being in the height 
of summer (July and August). Our largest spring records are 112,000 per 
haul in March 1919, 130,000 in April 1918, and 117,000 in April 1909. 
The autumnal records are never so high, averaging about 35,000, and rising 
to 56,000 in October 1912. On the whole, taking both spring and autumn 
* See “ Spolia Runiana,” III. p. 193. 
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