METABOLISM IN THE SEA 179 



Worms, too, are abundantly represented. All these 

 groups of animals, with the unicellular plants and 

 an incredible number of bacteria, form what we 

 may call the permanent portion of the plankton, 

 which, throughout their lives, pursue this drifting, 

 pelagic existence, which is characteristic of the 

 assemblage of organisms we are considering. But 

 there are a vast number of other animals which 

 have a planktonic phase ; which, when young, drift 

 freely about in the sea, but which, when they enter 

 on their adult career, belong either to the benthos 

 or to the nekton. This transitory plankton consists 

 chiefly of the eggs and larvae of molluscs which 

 live resting on, or burrowing in, the mud or sand 

 at the sea-bottom, the larvae of sedentary worms, 

 of Crustacea, and of zoophytes, star-fishes, etc., and 

 the eggs and larvae of ascidians and fishes. For 

 a variable time these and other animals belong to 

 the plankton, and it is in this stage that they 

 become distributed far and wide in the sea by the 

 mechanical agencies of the winds and currents. 



Of all the departments of marine investigation 

 the study of the plankton is the most attractive, 

 because of the astonishing variety and abundance 

 of life which is contained in it, and because of the 

 surpassing interest of the problems which we 

 encounter in this kind of work. Its study is com- 

 paratively recent, and may be said to date from 

 1845, when Johannes Mtiller, the great anatomist 

 and physiologist, began to use the " tow-net " in 



