GEOGRAPHICAL DISTRIBUTION OF ANIMALS 305 



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 in- 

 habit a region of great 

 cold and inky darkness. 

 Their bodies are subjected 

 to great pressure, and the 

 conditions of life are prac- 

 tically unvarying. There 

 is therefore among them 

 no migration, no seasonal 

 change, no spawning sea- 

 son fixed by outside con- 

 ditions, and no need of 

 adaptation to varying en- 

 vironment. As a result, all 

 are uniform indigo-black 

 in color, and all show more 

 or less degeneration in 

 those characters associated 

 with ordinary environ- 

 ment. Their bodies are 

 elongate, from the lack of 

 specialization in the ver- 

 tebrae. 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 exces- 

 sively large, as if to catch the slightest ray of light, or else 

 it is undeveloped, as if the fish had abandoned the effort 

 to see. In many cases luminous spots or lanterns are de- 

 veloped by which the fish may see to guide his way in the 

 21 



FIG. 180. A crinoid (Rhtzocrmiis loxoten- 

 sis). A deep-sea animal which lives, 

 fixed plant like, at the bottom of the 

 ocean. 



