82 LIFE : OUTLINES OF GENERAL BIOLOGY 



must be that lampreys were originally freshwater animals, as most 

 of the species always remain, whereas the Common Eel had origi- 

 nally a deep-water marine home, and took secondarily to an explora- 

 tion and exploitation of the freshwaters. It is interesting to note 

 that in the interior of New York State Petromyzon marinus does not 

 go down to the sea, but passes from river to lake, and from lake to 

 river. Such secondary telescopings of the typical life-history are very 

 significant. 



Invi:rti:brati:s. -Most of the so-called migrations of Inverte- 

 brates are misimderstandings. The mass-movements of butterflies 

 and dragonflies and other insects are very impressive, but the whole 

 trend of the evidence is in favour of regarding them as occasional 

 disjx-rsal movements. On the other hand, there is genuine migration 

 in the movements of land-crabs (Geocarcinus) and robber-crabs 

 (Birgus) from the interior to the shore. In the sea the larvae are 

 hatched out, and in the sea all the youthful stages arc passed. 

 From the s<.>a there is a return of the adults to their inland retreats, 

 and later on they are followed by their hereditarily adventurous 

 offspring. This is migration. 



ANIMALS IN THEIR HAUNTS 



It is part of the pleasant task of ecology to consider living creatures 

 in relation to their environments, and we wish here to think of 

 the major haunts of hfe — the Open Sea, the Shore of the Sea, the 

 Deep Sea, the Freshwaters, the Dry Land, and the Air — with its 

 birds and insects. 



Pelagic. — The pelagic fauna includes all the animals of the open 

 sea, both drifters (Plankton) and swimmers (Nekton). The physical 

 conditions in which they live are very favourable — there is room 

 for all, sunshine without risk of drought, and an evener life through- 

 out the day and throughout the year than is to be found elsewhere 

 except in the abysses of the deep sea. Moreover, the minute pelagic 

 Algic afford an inexhaustible food-supply to the animals. It is not 

 surprising, therefore, to Imd that the open sea has been peopled 

 from the earliest times of which the rocks give us any life record. 



The fauna is representative, exhibiting great variety of types, 

 from the minute Noctiluca. which sets the waves aflame in the short 

 summer darkness, to the giants of modern times— the whales. It 

 includes a few genera of Foraminifera, rich in species, most Radio- 

 larians, Dino-flagellata, many Infusorians, Medusae and Medusoids, 

 Siphonophora and Ctenophora. many "worms," a few Holothurians, 

 a legion of Crustaceans, a few Inst^cts (Halobatida-), .such Molluscs as 

 PteropfKls, Heteropods. and many of the CephalofX)ds, such Tuni- 

 cates as Salpa and Pyrosoma, many fishes, a few turtles and snakes, 



J 



