86 THE STORY OF LIFE IN THE SEAS. 



or roughness of the sea, the conditions of the 

 tides and winds which cause changes in the 

 surface temperature of the water, independently 

 of the direct heat of the sun, all influence the 

 delicate tissues of which these animals' bodies 

 are composed, and cause them to change their 

 position. 



The animals which compose the surface Plank- 

 ton may be considered under two heads those 

 that are adult, and those that are the larvse of 

 sessile and crawling forms of life which in the 

 adult stage live at the bottom. 



Those belonging to the former group frequently 

 occur far out in the open ocean as well as in the 

 neighbourhood of the land, and have as a rule a 

 wide geographical distribution. Those belonging 

 to the latter group are more usually found within 

 a few miles of the coast line, although winds and 

 tides may occasionally drift them far out into the 

 sea, where their larval existence is prolonged for 

 an abnormally long time. Leaving out of con- 

 sideration for the moment the many interesting 

 exceptions, we may say that the Plankton of the 

 open oceans differs from that of the neighbour- 

 hood of the coasts, by the larger proportion of 

 adult forms that it bears. 



A great variety of animals pass the whole of 

 their lives in the surface waters of the sea, but the 

 commonest and most widely distributed of all 

 probably are the Copepods belonging to the class 

 Crustacea. 



The Copepods are minute creatures, rarely 

 exceeding a quarter of an inch in length, which 

 row themselves through the water by a pair of 



