1. Collections of phyto- and zooplankton made off the Isle of Man were examined for vitamin content, and for chemical and biological characteristics.

2. The important species entering into the composition of the plankton are noted, and a new method of estimating plankton catches is described. Figures which represent the numbers of a species present in a given quantity of plankton are translated into figures representing the volume occupied by that species. Comparisons are drawn between the numerical and volumetric estimations of the various zooplankton species.

3. The phytoplankton catches taken in May showed a fairly uniform composition throughout the collecting period, species of the genera Chaetoceros and Lauderia forming more than 90 per cent. of the material. The zooplankton taken in July consisted mainly of Copepoda. The catches were of two types: those having a higher percentage of Acartia, Calanus copepodid stages IV and III, and the Peridinian Ceratium; and those containing less of these but rich in the adult and other stages of Calanus finmarchicus.

4. By the use of curves correlating length and volume, measurements of important species are translated into factors representing numbers per c.c.; application of these factors converts the numerical estimation of a species into an estimation in terms of volume. The numerical and volumetric estimations of various species in four separate groups of plankton are compared and the yield of oil from each group is also recorded. The volumetric method conveys a significantly different picture of the plankton than is to be gained from the numerical method alone. Calanus finmarchicus is shown to constitute a relatively larger part of the plankton than other more numerous but smaller species and likewise its adult or sub-adult stages than its earliest copepodid stages. The correspondence of a good oil yield with those groups having a high Calanus content is held to be suggestive of a possible correlation between the two.

5. Chemical and biochemical analyses of planktonic organisms by other investigators are reviewed and their results compared with those obtained during the present work. The rôle of plankton organisms in the nutrition of various other organisms is discussed.

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