GENERAL BIOLOGY 719 



of nutriment derived from the shore ? These questions must 

 be left to future research. 



In Chapter VI. Gran has described the vertical distribution Pelagic 

 of pelagic plants. In the open Atlantic he found that the p^^^^^^- 

 great majority of the plants occur in depths between 10 and 50 

 metres ; at 75 metres the numbers decrease to about one-half, 

 and at 100 metres to one-tenth, of the numbers found in the 

 upper layers. The whole of the animal life in the oceans, 

 5000 or 6000 metres deep, thus mainly depends on the pelagic 

 plants suspended in the uppermost 100 metres of water. The 

 animals frequenting this upper layer feed partly on plants, 

 partly on other animals, while in deeper water only animal 

 food is available, besides the dead plants and animals sinking 

 from the surface. Nutrition in the upper " plant "-region must 

 therefore be different from that in the deeper layers. 



Many animals of the plant-region are typical plant-eaters, 

 and their bodies are organised for this purpose. This is 

 especially the case as regards appendicularians and salpse, the 

 foremost part of their digestive tract, the so-called branchial sac, 

 being provided with a grating of the finest and most delicate 

 structure, retaining even the most minute plants (the cocco-litho- 

 phoridse). Many of these minute plant- forms were indeed first 

 discovered by examining the stomach-contents of salpse (Stein, 

 Sir John Murray, Lohmann), and during the Atlantic cruise- of 

 the " Michael Sars " Gran also collected salpae in order to secure 

 material for comparison with our tow-net captures of minute plants. 

 The coelenterates (medusae, ctenophores, siphonophores) are 

 well adapted to capture minute plants by the aid of their tentacles, 

 and so are the unicellular animals (foraminifera and radio- 

 laria) by the aid of their long thin plasm threads (pseudopodia). 

 The most important of all plant-eaters are, however, the small 

 crustaceans, particularly copepoda, which seem specially adapted 

 for feeding on the microplankton of the ocean. Gran has 

 examined the excrements of copepoda, which sink through the 

 water in the shape of minute sausage-like lumps, and are very 

 often taken in considerable quantities in the silk nets. All the 

 soft parts have been digested, but the shells of the plants eaten, 

 the calcareous shells of the coccolithophoridse, the armour of 

 peridinese and the silicious shells of diatoms, can be identified. 

 In the Norwegian Sea Gran observed that the copepoda were 

 present in enormous numbers just below the layers containing a 

 wealth of diatom plant-life, but nevertheless the excrements of 

 these copepoda consisted of the frustules of the diatoms. The 



