82 PHYSIOLOGY CHAP. 



from the ocean bed. By this method a certain amount of exact 

 knowledge has been obtained in regard to the fauna and flora of 

 the seas. As regards the vertical distribution of these pelagic 

 organisms, it is interesting to note that the forms which live at 

 different heights in the same region of the sea vary enormously 

 among themselves, so that we ought to speak of so many special 

 biological zones in relation to the different depths of water. 



The main factor which determines this diversity in the forms 

 of life at various depths, is light ; then come temperature, and 

 movement of the water, which are of secondary importance ; while 

 pressure of water is, save for the Teleosteans provided with a swim- 

 bladder, of no importance, as we have already pointed out. 



In regard to light, the following zones can be distinguished 

 in a vertical section of the water of the ocean : 



(a) A first zone, highly illuminated, which extends from the 

 surface to about 30 metres down. 



(ft) The shaded zone, from about 30 metres below the surface 

 to the farthest limits to which light penetrates (some 500 metres 

 deep). 



(c) The dark zone, which commences at 500 metres, and extends 

 to the greatest depth known to be inhabited, i.e. some thousands 

 of metres (in the Mediterranean the Puritan dredged to a depth 

 of some 1500 metres). 



It agrees with this, and with the fact that light is an indis- 

 pensable condition of plant life (chlorophyll function), that no 

 vegetable organisms (algae) have so far been dredged at a lower 

 level than the shaded zone, i.e. below 500 metres. On the other 

 hand, numerous animal organisms have been found, and described, 

 below this level, in accordance with the fact that light is not 

 an indispensable vital condition to animals. It is, however, 

 interesting in those animals which live entirely in the dark, to 

 observe the morphological changes in the sense organs destined 

 to receive luminous stimuli (eyes). In some they atrophy com- 

 pletely, as in terrestrial creatures living in caves; in others, on 

 the contrary, they develop enormously ; while in order to furnish 

 the stimuli required to make them perform their functions, they 

 develop numerous and powerful luminous organs in different parts 

 of the body. 



Hensen was the first to propose the collective name of Plankton 

 (ir\avKTo<s, wandering), which is now universally accepted, to indicate 

 the world of living organisms (fauna and flora) in mid-ocean ; 

 while the name of Benthos (ftcvOos, bottom) is applied to the 

 aquatic organisms that live at the bottom of the sea. 



Lo Bianco (1903), on the strength of the facts already discussed, 

 to the effect that light is the factor determining the varying dis- 

 tribution of plankton, proposed to term the biological stratum 

 which corresponds with the first of the above zones phao-plankton, 



