THE OPEN SEA 



185 



The pelagic fauna is very representative. It includes many 

 Infusorians, such as the " phosphorescent " Noctiluca ; a few genera, 

 but many species of Foramini- 

 fera or chalk-forming Protozoa ; 

 most Radiolarians several 

 thousand different kinds with 

 which unicellular Algae live in 

 intimate internal partnership 

 (symbiosis}] jelly-fishes (e.g. our 

 common Aurelia and Cyanea) ; 

 swimming-bells or Medusoids, 

 most of which are the liberated 

 reproductive buds of zoophyte 

 colonies ; Siphonophora, like 

 the Portuguese man-of-war 

 (Fig. 107), and Velella (Fig. 

 106); Ctenophores, like the 

 common Beroe andPleurobrachia-, 

 many ' ' worms ' ' of diverse classes ; 

 two or three Holothurians or 

 sea-cucumbers ; a legion of Crus- 

 taceans ; a few insects belong- 

 ing to one family (Halobatidae) ; 

 sea - butterflies, many cuttle- 

 fishes, and some other Moll- 

 uscs ; the larvae of Ascidians and 

 a few adult forms ; many fishes, 

 and the floating eggs of many 

 more ; a few turtles and snakes, 

 besides such birds as the petrels 

 and such mammals as the whales. 



Pelagic animals often occur 

 in enormous swarms, which is 

 doubtless indicative of relatively 

 easy conditions. As one would 

 expect, they tend to be lightly built, delicate, and translucent. 

 Many of the drifting forms, e.g. small Crustaceans, are remark- 

 able for the length of their appendages and outgrowths, which 



FIG. 107. The Portuguese man-of-war 

 (Physalia), one of the Siphonophora, a 

 characteristic pelagic animal in warm 

 seas. It is a colony of individuals or 

 zooids, showing much division of labour. 

 There is a large bladder or float (Fl), 

 sometimes six inches long, and from 

 that are suspended nutritive zooids (N), 

 reproductive zooids (R), tentacle - like 

 sensitive zooids (T), and long filaments 

 bearing batteries of stinging cells (St). 

 colour is a fine translucent blue. 





