SURFACE-SWIMMING FAUNA (INVERTEBRATES). 91 



together are lighter than water and float con- 

 tinuously without any action of the muscles. 

 If the life-belt were considerably smaller than 

 usual the man and belt would sink, but much 

 less rapidly than the man alone ; and the muscu- 

 lar energy required to keep himself afloat would 

 be far less with the belt than without it, con- 

 sequently he would be able to keep afloat much 

 longer with the same expenditure of muscular 

 energy. The bodies of many of these surface- 

 swimming animals may then be best compared 

 with a man assisted by a small life-belt. When 

 dead or still they slowly sink, but a slight 

 amount of muscular energy expended in swim- 

 ming is sufficient to keep them afloat. In what has 

 been said above about the body of the Copepod, 

 reference has been made to certain bright red 

 granules. These are in all probability little 

 globules of some oily or fatty substance lighter 

 in weight than the sea-water, which serve to 

 buoy up the body of the little creature. It is 

 difficult to say why they should have such bright 

 colours. We have no record .of observations 

 that show that the colours can be of any use 

 to them as a protection from their enemies, 

 nor is there any physical explanation of the 

 colours of these granules any more than of the 

 blood, the bile and other products of animal 

 and vegetable vital processes. The eggs con- 

 tained in the egg-sacks of the Copepods also 

 bear a certain amount of oily substance very 

 frequently different in colour from that of the 

 other parts of the body, and this probably 

 acts in the same manner upon the body of the 



