382 



DEPTHS OF THE OCEAN 



Food of 



Appendicu- 



laria. 



Food of 

 Copepods 



Proportion of 

 plants and 

 animals in 

 the plankton. 



97 most of the forms were diatoms, and to a great extent con- 

 sisted of Rhizosolenia alata. Generally speaking we discovered 

 that salpse do not trouble to make any selection. Lohmann's 

 studies of Appendicular ia have shown us t^t these animals get 

 their nutriment by means of a filter apparatus, which allows only 

 the minutest organisms, coccolithophoridse in particular, and 

 small peridineae, to enter the digestive canal. 



The chief consumers of plants in the sea are undoubtedly 

 copepods. Their conditions of nutriment, however, have so far 

 been principally studied by means of their excrements, which sink 

 down in the shape of small elongated lumps, and are often brought 

 up in numbers by the silk nets. Still, in these excrementa all 

 the softer components have been digested, and the shells that can 

 be identified do not necessarily always belong to species which 

 are an indispensable part of their nutriment. Undoubtedly the 

 calcareous shields of coccolithophoridse occur too frequently for 

 their presence to be ascribed to chance, indicating, moreover, 

 that the digestive juices of copepods cannot have an acid 

 reaction. In addition we very often meet with more or less 

 bent and distorted coverings of peridineae, and in northern 

 waters the excrements contain stiffer forms like the little 

 Dinophysis gi'amilata in a practically unchanged condition. In 

 localities where diatoms predominate, the excrements consist 

 largely of bent and broken bits of species like Rhizosolenia 

 semispina and R. alata. Even if Hensen's view be right 

 that diatoms supply far less nutriment comparatively than the 

 other classes of plants in the plankton, it is at any rate quite 

 certain that the animals do feed on them, and especially when 

 they are plentiful. In the Norwegian Sea I have several times 

 observed that where diatoms abounded there might perhaps be 

 only a few copepods and other plankton animals ; still the 

 copepods were there, and In large numbers too, just below the 

 diatom zone, and their excrements consisted to a great extent 

 of the silicious coverings of diatoms. 



Hensen noticed that the plants in the sea are often 

 so scanty that it is hard to understand how all the animals 

 get enough nourishment, and this is even more difficult to 

 comprehend when we consider that the plants have directly 

 or indirectly to support every single animal from the surface 

 right down to the bottom. In many cases, perhaps, the plants 

 may be more abundant than a cursory examination would seem 

 to indicate ; and the most diminutive forms, which are still 

 practically unknown to us, undoubtedly exist in sufficiently 



