176 . PROF. W. A. HERDMAN ON THE DISTRIBUTION 
5 per cent. formol, and is re-measured it may be weeks later when 
submitted to detailed examination. It is this last more accurate measure- 
ment that has been made use of for calculations and curves. The 
first estimation of the volume is only taken in case some accident happens 
later ; but we have been very fortunate in that respect: we have only lost 
about 10 gatherings in the 10 years out of over 5000 samples. Figure 2 
shows typical plankton curves for the years 1913 (dotted line) and 1914, 
taken from the recorded monthly averages of the plankton hauls. As an 
example of the height to which individual hauls may rise in spring, I may 
quote from our records of this year that on April 18th, 1917, the standard haul 
measured 165 c.c, and on April 19th 150 c.c., and consisted mainly of 
Chetoceras. 
Our confidence that these samples are adequate and representative receives 
support from the fact that the same organisms are recorded in much the 
same quantities year after year, and that practically no new forms turn up. 
Mr. Andrew Scott, A.L.S., who has made a detailed microscopic examination 
of all the material for the purpose of determining the species, has met with 
none new to science. Some rare species previously unknown in British 
seas, such as the Norwegian Copepod Mierocalanus pusillus, Sars, which 
appeared first in our deeper nets in the summer of 1907, and the Indo-Pacific 
Diatom Biddulphia sinensis, Grev., have occurred ; but throughout the series 
of over 5000 gatherings, extending over nearly every week of 10 years, 
no species actually new to science has been determined from the macro- 
plankton. The various new Copepoda which have been described from time 
to time from our work at Port Erin have all been bottom-living forms 
obtained by dredging. This is, so far, a satisfactory result of our work, as 
it seems to indicate that probably all the pelagic species of Copepoda in our 
sea are now known. It is not the rare species that are of most interest. 
They may have an interest of their own—morphological or distributional— 
but for my present purpose it is the common species that are of most import- 
ance, those species which by their abundance in nature play their part in 
providing fish-food for man or in affecting the public health either by 
keeping the sea clean or by causing plagues. 
For the purpose then of arriving at some conclusion as to the distribution 
throughout the year of these really significant organisms, I have picked out 
from our records the following six species of Copepoda as being undoubtedly 
the most abundant and economically the most important representatives of 
that section of the plankton :— Oithona helgolandica (= similis), Pseudo- 
calanus elongatus, Acartia elausi, Temora longicornis, Paracalanus parvus, and 
Calanus jinmarchicus. These are all cases of genera where there is only one 
species in our seas (e. g., Calanus) or only one common species (e. g., Oithona), 
so that we are dealing with half-a-dozen very distinct forms, and there can 
be no doubt as to what is in question even if the genus only is referred to, 
