210 THE CONDITIONS OF LIFE IN THE SEA [PART III 



The plankton as food-stuff. By far the greater number of 

 the nektic animals are carnivores, and we find that there are also 

 a considerable number of benthic animals which feed in a similar 

 manner, while among the plankton there is also a proportion of 

 flesh-eaters. Even when a nektic or benthic animal eats some 

 other animal we find that the food of the food is, in the long run, 

 an inhabitant of the plankton ; while many fishes, molluscs, 

 Crustacea, &c., devour the drifting microscopic life of the sea 

 directly. We then regard the latter as a permanent source of 

 food for other marine animals; and it is important for such an 

 enquiry as this that we should be in possession of reliable estimates 

 of the density of the microscopic life of the sea. But the material 

 for the construction of such estimates hardly as yet exists in the 

 literature. The reader will remember that the catches made with 

 nets of MuUergaze No. 20 do not represent the total contents 

 of the sea in plankton organisms, for a considerable proportion 

 of the latter are so small as to escape through the meshes of nets 

 made of this material. Therefore we cannot use the figures 

 obtained by means of such apparatus for the purpose of making 

 estimates of the amount of organised food-stuffs in the unit volume 

 of sea water, or at the best we can only employ these figures for 

 the construction of minimum values. But, as an illustration of 

 what may be regarded as maximum and ordinary values, we may 

 take the results of two estimations made by Brandt and Lohmann. 

 The former is based on a catch made in Kiel Bay, and the latter 

 on a series of estimations of the total plankton contained in the 

 open Mediterranean, off S3'racuse. 



The haul described by Brandt^ is one of the richest ever made 

 in northern seas and we may perhaps regard it as giving us a 

 value for the organised food contents of the sea water, which is 

 very considerably above the average. In it were found : 



3173 millions of diatoms (chiefly Ghaetoceros) ; 

 500,000 peridinians ; 

 15,000 copepods. 



Its volume was 1385 c.c. and the weight of the contained organ- 

 isms, dried at 100° C, was 1'06 gramme. Now considering only 



1 " Stoffwechsel im Meeres," Wiss. Meeresunt. Kiel Komm. Bd. vi. Abth. Kiel, 

 1902, p. 71. I refer to this haul, and its " reduction " in Chap. VIII. p. 163. 



