52 THE FAUNA OF THE DEEP SEA 



It is the function of every true naturalist to con- 

 sider animals from every possible point of view. Not 

 only must he regard them as members of a certain 

 species belonging to a genus, a family, an order, and 

 so on, presenting certain peculiarities of structure 

 and development ; not only must he regard them as 

 inhabitants of a certain locality or zone of depth, but 

 he must also pay attention to their habits and mode 

 of life. 



Now amongst marine animals we can recognise 

 three principal modes of life. Some animals simply 

 float or drift about with the currents of the sea and 

 are unable to determine for themselves, excepting, 

 perhaps, within very small limits, the direction in 

 which they travel. Such are the countless forms of 

 protozoa, the jelly-fishes and medusse, numerous 

 pelagic worms and Crustacea, the pyrosomas and 

 salps, and many other forms well known to those 

 who are in the habit of using the tow-net. This 

 portion of the fauna has recently been called the 

 Plankton. 



Then there are the animals that are capable of 

 very considerable swimming movements, animals 

 that are able to stem the tide and migrate at will 

 from one part of the sea to another, such as the 



