98 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 tor 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 lo>t 

 about 10 gatherings in the 10 years out of over 5000 samples. Figure 2 

 shows tvpical 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 

 CluBtoceras. 



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 Oopepod Microcalanus pusiUus, Sars, which 

 appeared first in our deeper nets in the summer of 1907, and the Indo-Pacific 

 Diatom BiddulpMa sinensis, Grev., have occurred ; but throughout the series 

 of over 5000 gatherings, extending over nearlj- 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. 

 Thev 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 Tielgolandica (= similis), Pseudo- 

 calanus elongatus, Acartia clausi, 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. 



