Handbook of Paleontology 77 



slight variation in chemical conditions, that is, salt 

 content of the water; and except along- the shores, mo- 

 tion affects only the surface waters and is negligible. 

 Animals on the land in general must seek their food. 

 In the sea food substances float everywhere in suspen- 

 sion and are abundant on the bottoms. Some animals 

 go after their food, but some just attach themselves 

 or burrow in the various bottoms and let the sea bring 

 them their food, and still others float surrounded by 

 their food supply. 



Bathymetric or Vertical Range 



Animals and plants of the' sea have a bathymetric 

 (Greek bathos, depth or height; metrein, to measure) 

 or vertical range and a horizontal or geographical 

 range. The horizontal or geographical range is to a 

 greater or less degree a matter of climate, that is, tem- 

 perature. In addition, the character of the shore var- 

 ies horizontally. There are rocky shores, varying ac- 

 cording to the effects of erosion on different types of 

 rocks, sandy shores and muddy shores, each with its 

 characteristic life. Three primary conditions are 

 recognized as profoundly affecting organisms and de- 

 termining their bathymetric distribution : an air or 

 water medium, presence or absence of light, presence 

 or absence of a substratum. The first determines the 

 method of breathing and this usually makes one situa- 

 tion unfit for dwellers in others; the second affects 

 the food supply and indirectly the animal ; the third de- 

 termines the group to which organisms belong, for with- 

 out a substratum organisms must be self-supporting, able 

 to float or swim. The life of the sea is divided into three 

 groups: plankton or floating forms (Greek, planktos, 

 wandering) ; nekton or swimming forms (Greek, nektos, 



