562 DEPTHS OF THE OCEAN 



When Chun in 1898 fitted out the "Valdivia" Expedition, 

 special arrangements were made for the purpose of obtaining 

 an accurate knowledge of the animal life in " mid-water." 

 Hundreds of hauls with closing-nets and with other large nets 

 were taken at various depths, the material procured proving 

 that the main conclusions drawn from the " Challenger" Expedi- 

 tion were quite correct. Even in hauls between 5000 and 4000 

 metres living crustaceans as well as larvae of the same animals 

 were captured — a sufficient proof that these organisms not only 

 live but also breed at these depths. 



The conception of a " pelagic " mode of life, originally 

 associated with the animal-life of the ocean-surface, thus 

 gradually proved to hold true for life in mid-water also, and to 

 apply to floating or drifting organisms as well as free-swimming 

 animals. The main characteristic of pelagic life is its independ- 

 ence of the bottom. The term " bottom- animals " is applied 

 not only to the animals fixed to or creeping along the bottom, 

 but also to those animals which, like certain crustaceans and 

 bottom-fishes, swim and feed along the bottom. But it is im- 

 possible to draw a perfectly sharp limit between these migrating 

 bottom-dwellers and some of the deep-living pelagic animals, 

 which have been called " bathypelagic." In accordance with the 

 varying conditions in deep and shallow water and in different 

 parts of the ocean, the pelagic animals have been subdivided into 

 groups : thus Ernst Haeckel ^ introduced the idea of " Holo- 

 pelagic " (wholly pelagic) to distinguish those forms leading an 

 entirely pelagic life from those forms having a bottom-stage 

 like the Hydromedusae, which he called " Meropelagic " (partly 

 pelagic) ; he further distinguished those forms found only in 

 coastal waters by the term " Neritic " from those found only in 

 the open sea, which he called "Oceanic." 



As in all geographical comparisons of animals we may 

 divide the pelagic organisms into tropical, subtropical, boreal, 

 arctic, and antarctic forms. It has also been proposed to 

 arrange the pelagic fauna in certain bathymetrical zones, 

 distinguishing between those forms living in profuse light, or 

 in the region of twilight, or in the dark abyssal waters, but 

 such distinctions are arbitrary, because our knowledge of the 

 bathymetrical distribution of animals is limited, because 

 the laws of distribution are imperfectly understood (for 

 instance, the effects of light), and because the bathymetrical 



1 Ernst Haeckel, Plankton-Siudien, ]Qndi, 1S90. Haeckel used the words " holoplanktonic " 

 and " meroplanktonic," but I prefer " holopelagic " and "meropelagic," as the word 

 " plankton" is not so clearly defined, and is used in different ways (see Chapter X). 



