88 SEA FISHERIES 



Although the specific weight of pelagic creatures is 

 always a little higher than that of sea-water, they remain 

 at the surface or at an inconsiderable depth. This is 

 because most of them possess vibratile cilia or flagellae 

 or locomotive appendages. Moreover, as the water is 

 never calm, it holds passive bodies, such as eggs, in 

 suspension. Then we must not forget that we have to 

 do with living bodies, which assimilate food, breathe, and 

 perform work. Plankton is often more abundant on the 

 surface in the night than in the day. " The experimental 

 studies of Brandt," writes Professor Pruvot, "have 

 explained the mechanism of these movements among 

 creatures devoid of motive power, such as the Radiolaria. 

 Among the Radiolaria of which we have experience, the 

 floating equilibrium is maintained by the fact that the 

 excessive weight of the central capsule is corrected by 

 the lesser density of the gelatine which constitutes the 

 extra-capsular portions, and of the liquid which fills its 

 vacuoles. Any stimulus or excitation causes the con- 

 traction of the vacuoles and the expulsion of their liquid ; 

 hence an increase in density and a descent to the bottom. 

 Then, the perturbing action having ceased, the vacuoles 

 become filled with fresh liquid, and the animal re- 

 ascends. In the open sea these creatures are driven from 

 the surface by the mechanical stimulus due to the move- 

 ment of the waves, or by a thermal stimulus such as the 

 daily warming of the surface of the sea ; they descend 

 slowly, refill their vacuoles in the deeper layers of the sea 

 which are at rest and are not warmed by the sun, when 

 they once more float to the surface, only to sink again, 

 and so forth, until the exciting cause has disappeared, 

 when they remain at rest upon the surface." In the case 

 of the Copepoda the oscillatory movements are due to 

 the temperature alone. 



