PLANKTON 263 



or alter it without too much expenditure of muscular effort, 

 many free-swimming or floating animals, from Fishes down 

 to Protozoa, have some form of hydrostatic apparatus, such 

 as the swim-bladders of Fishes, the gas-containing floats 

 or pneumatophores of Siphonophora, the oil-globules of 

 Radiolaria and of some fish-eggs, or have the tissues so 

 reduced in bulk and so permeated with water, as in Medusae, 

 Salpse, etc., that the specific gravity of the body becomes 

 much the same as that of the surrounding sea. In some 

 cases the gas in the float can be secreted or absorbed as 

 required, so as to compensate for increased or diminished 

 pressure when changing to a different level. 



Another device has been adopted in many cases in order 

 to take advantage of the varying viscosity of the water in 

 accordance with depth and temperature, viz., an increase 

 of the surface of the body in relation to its bulk by means of 

 changes of shape and formation of outgrowths, such as 

 flat expansions, long spines, and branched or plume -like 

 setae. Many examples of such remarkable devices, leading 

 to extraordinary and very ornamental appearances, are 

 seen in Copepoda, Foraminifera, Radiolaria, etc., especially 

 in warmer seas, where the viscosity is low. 



One of the most striking phenomena of the plankton, 

 in temperate seas at least, is the way in which it differs both 

 in quantity and quaUty, in the same locaHty, at different 

 times of year. In British seas, for example, a typical haul 

 of the plankton-net in spring (say March or April) will 

 consist almost wholly of Diatoms and allied organisms (Plate 

 XIX, Fig. I, and Plate XXII, Fig. 1) ; it is a phyto -plankton ; 

 while a corresponding haul in summer (say July or August) 

 will have few Diatoms, if any, but will show a large number of 

 Copepoda (Plate XIX, Figs. 3 and 4), and many other kinds of 

 minute animals, making up a tjrpical zoo -plankton. At the 

 time of the spring Diatom maximum a small silk tow-net 

 hauled for about fifteen minutes through about half a mile 

 of the surface water of the Irish Sea will usually catch some 



