446 ANIMAL STUDIES 



fauna or aggregations of species are produced wherever 

 free diffusion is checked by any kind of barrier. 



352. Faunal areas of the sea. In like manner, we may 

 divide the oceans into faunal areas or zones, according to 

 the distribution of its animals. For this purpose the fishes 

 probably furnish the best indications, although results very 

 similar are obtained when we consider the mollusks or the 

 Crustacea. 



The pelagic fishes are those which inhabit the open sea, 

 swimming near the surface, and often in great schools. 

 Such forms are mainly confined to the warmer waters. 

 They are for the most part predatory fishes, strong swim- 

 mers, and many of the species are found in all warm seas. 

 Most species have special homing waters, to which they 

 repair in the spawning season. To the free-swimming forms 

 of classes of animals lower than fishes, found in the open 

 ocean, the name Plankton is applied. 



The lassalian fauna, or deep-sea fauna, is composed of 

 species inhabiting great depths (2,500 feet to 25,000 feet) 

 in the sea. At a short distance below the surface the 

 change in temperature from day to night is no longer felt. 

 At a still lower depth there is no difference between winter 

 and summer, and still lower none between day and night. 

 The bassalian fishes inhabit a region of great cold and inky 

 darkness. Their bodies are subjected to great pressure, 

 and the conditions of life are practically unvarying. There 

 is therefore among them no migration, no seasonal change, 

 no spawning season fixed by outside conditions, and no 

 need of adaptation to varying environment. As a result, all 

 are uniform indigo-black in color, and all show more or 

 less degeneration in those characters associated with ordi- 

 nary environment. Their bodies are elongate, from the 

 lack of specialization in the vertebrae. The flesh, being 

 held in place by the great pressure of the water, is soft and 

 fragile. The organs of touch are often highly developed. 

 The eye is either excessively large, as if to catch the slight- 



