INVESTIGATION OF THE PLANKTON OF THE IRISH SEA. 169 
diatoms, and green and brown disintegrated material which was doubtless 
of phytoplanktonie origin, along with occasional small copepods (in one case 
I was able to watch through the transparent walls of a young, living, post- 
larva the passage of the remains of a copepod along the intestine until it was 
extruded along with other material as a fæcal pellet); and (2) that the larvæ 
began to take in solid food before the contents of the yolk-sac had been 
completely absorbed. 
CONCLUSIONS. 
The following conclusions may safely be arrived at from our statistics :— 
In reviewing the records of this run of 15 years (1907-1921) we find that 
the spring phytoplankton maximum may range from March to June, and is 
chiefly composed of diatoms which vary from year to year in maximum haul 
from under one million in 1907 to about 206 millions in 1913. 
This immense diatom elevation can be resolved into an earlier crest in 
April or May, chiefly formed of Chetoceras, and a later in June, chiefly 
formed of Rhizosolenta. 
The dinoflagellate maximum follows about a month later than the diatoms, 
and varies in our records from May to July (rarely August). 
The copepod maximum is later again, and ranges from June to October. 
Any one of these three main groups of the plankton or all of them may 
have secondary less conspicuous maxima in late summer or autumn ranging 
from September to November. Rarely, an autumnal maximum may, under 
favourable conditions, attain to large dimensions—for example, in 1912. 
During the time of the diatom maximum in spring the organisms are 
more evenly distributed over the sea and downwards through the layers than 
is the case at other times of year and in the case of other larger organisms. 
As a general rule, with a mixed plankton or a zooplankton, in the daytime 
the largest hauls are obtained not at the surface but a few fathoms below, 
round about 5 fathoms. The precise depth on each occasion probably 
depends upon the meteorological conditions and especially the amount. of 
sun-light. Most marine organisms, perhaps all, seem to find their optimum 
ef sun-light not in the maximum at the surface but in some degree of 
twilight conditions lower down. 
In spring (April, May, or June) the gathering obtained from the tow-net 
is usually a “ monotonic phytoplankton” (mainly diatoms), and in summer 
(say, August) it is frequently a “ monotonic zooplankton ” (Copepoda). On 
other occasions it is a “ mixed plankton,” and may contain large quantities 
of larval stages of coastal animals—such as Echinoderms and molluscs. 
Taking the year round, the Irish Sea plankton is a mixture of “ oceanie ” 
and “neritic” organisms, roughly 50 per cent. of each, or rather more of 
neritic forms, the oceanic ranging from 30 to 60 per cent. Mid-winter and 
mid-summer are more oceanic in character than the intervening months, 
