THE HAUNTS OF LIFE 75 



and the sub-surface plankton, both within the light- limit, 

 and the bathy-plankton which extends below that limit, 

 and consists necessarily of animals only. 



Swimmers and Drifters. The open- water animals 

 (Plankton, in the wide sense) are conveniently divided 

 into the active swimmers, such as fishes, which make up 

 the Nekton, and the more passive drifters, with relatively 

 feeble organs of locomotion or none at all, that are swept 

 about at the mercy of tides and currents. Another general 

 distinction should be borne in mind, that between the 

 permanent and the temporary planktonic animals, for 

 while there are many creatures that spend their whole 

 existence in the open sea, such as Ctenophores and Portu- 

 guese Men of War, there are others, which are only there 

 as larvae, e.g. the swimming bells of littoral zoophytes, and 

 the young stages of many worms, echinoderms and molluscs. 



Representative Pelagic Animals. The pelagic fauna 

 is made up of a great variety of types, from the pin-head- 

 like Noctiluca, whose intense luminescence sets the waves 

 aflame in the short summer darkness, to the great whales 

 the giants of the present age. The list includes many 

 Foraminifera (especially the Globigerinids), thousands 

 of different kinds of Radiolarians (so successful perhaps 

 because they have partner Algse living inside them), the 

 active Dinoflagellates (much sought after by small crus- 

 taceans and even by fishes), many other Infusorians, jelly 

 fishes or Medusae, often in great fleets, and the swimming- 

 bells or Medusoids, many of which are the liberated repro- 

 ductive buds of sedentary zoophytes, strange colonies 

 known as Siphonophores such as the Portuguese Man of 

 War and Velella, the delicate Ctenophores which never 

 come to the surface unless it is very calm, not a few free- 



