LIFE IN THE SEA 475 



eludes those creatures which are more or less passively 

 carried by the movement of the water, and are not far 

 from the surface. At first sight it might seem that 

 such forms were few and of little interest. From the 

 deck of a vessel one sees an occasional jellyfish, frag- 

 ment of seaweed, or, if fortunate, a "Portuguese man- 

 o'-war" (Physalia). Better indications of the abun- 

 dance of the plankton are obtained at night, when the 

 wake of the ship glows with the phosphorescent light 

 of small organisms. If, however, the fine-meshed 

 plankton net is used, dragged not* too rapidly along in 

 the surface waters, it is found to contain a whole popu- 

 lation of animals and microscopic plants. These, on 

 being sorted out, are found to belong to many different 

 phyla, classes, orders, and families. Some are very 

 young forms of littoral or bottom species, others are 

 permanent inhabitants of the plankton layers. Many 

 are transparent, or delicately tinted with blue, so as to 

 be almost invisible, though of fair size. 



5. At various levels between the surface and the bed The nekton 

 of the ocean will be found a number of free-swimming 

 animals, mostly fishes. These are collectively known 

 as the nekton. Unlike the typical plankton, they 

 move freely through the water, and are not drifted 

 hither and thither by the currents and tides. Much 

 of this nekton fauna is hard to catch, and our knowl- 

 edge of it is correspondingly imperfect. We know, 

 however, that there are great diurnal and seasonal 

 migrations of many species, so that they may appear 

 near the surface at certain hours, and at others be far 

 below ; or they may travel from one locality to another. 

 These movements are of the greatest importance for 

 the fishing industry, and in Europe an international 

 organization was investigating these and other prac- 



