PELAGIC ANIMAL LIFE 631 



variations apparently occur, for at a depth of 200 metres the 

 temperature exceeds 1 7" C. in the Sargasso Sea, in the 

 Mexican Gulf it is above 20'' C, in the Indian Ocean it varies 

 between 13° and 20° C., while in the southern Atlantic it is 

 only a little above 10' or 12" C. The fauna living at this 

 depth is thus subject to temperatures varying between 10" 

 and 20"^ C, corresponding with what we found in the case of 

 the fishes of the Atlantic coast banks from south of the Canaries 

 to the south-western coast of Britain. 



All the silvery fishes of the region between 150 and 500 

 metres are small, and the same remark applies to all the other 

 organisms of the community. They consist almost exclusively 

 of small crustaceans (copepoda, ostracoda, amphipoda), sagittidse, 

 pteropoda, and small medusae. Besides these we commence 

 to find the larvae of squids and fishes, which, however, become 

 more numerous in the layer above 150 metres. 



Pelagic Comm^utities in Depths less than 150 Metres. — In 

 reviewing the pelagic oceanic forms I mentioned that they 

 belong mainly to the warm belt on both sides of the equator 

 between lat. 40° N. and 40° S., where both species and 

 individuals are most numerous. Foraminifera, radiolaria 

 (acantharia), copepoda, medusae, siphonophora, pteropoda, 

 and salpae all occur in abundance, and the number of species 

 rapidly decreases as soon as we leave tropical waters. This 

 is also the case with the typical and most abundant surface 

 fishes, the scopelidae, which occur in numerous tropical and sub- 

 tropical forms, while only a few species are found in the 

 northern part of the North Atlantic. 



The beautiful siphonophores Physalia and Velella were first Distribution 

 seen by us during our short visit to the Mediterranean and in pj^^^P °"°" 

 the Spanish Bay. On the way from the Canaries to the Azores 

 and thence westward to Station 64 they were frequently seen, 

 sometimes accompanied by Agalmopsis and Cestus veneris, 

 besides various surface mollusca. On the other hand, none of 

 these forms were observed on our northern track between 

 Newfoundland and Ireland. 



The shelled pteropods (Thecosomata) are vertically limited Distribution 

 to a comparatively thin layer, extending in our northern section ° *^''°p° ^' 

 down to only 50 or 100 metres, and in the southern section to 

 250 metres, four-fifths of all the individuals taken occurring 

 within these limits. No less than 3500 individuals comprising 

 22 species were preserved by us, and of these only about 500 

 specimens comprising 16 species came from the northern section. 



