CHAPTER VIII 



THE DISTRIBUTION OF ANIMAL LIFE 

 IN THE SEA 



Whilb, as we have seen in the previous chapters, 

 the natural regions of the land more or less grade 

 into one another — savana, for example, passing into 

 savana wood, and savana wood into forest, it is very 

 much easier to separate the oceans into natm'al regions, 

 for here the divisions are relatively sharp. The two 

 great conditions which influence the type of animal to 

 be found within any part of the sea are, first, the pres- 

 ence or absence of light, and, second, the presence or 

 absence of a substratum. At the margin of the sea, where 

 the light penetrates to the bottom, we have the littoral 

 area, peopled by littoral organisms. All these organisms 

 agree in that, ia structure or in habit, they show adapta- 

 tions to the two striking features of the area. Thus 

 a sea anemone, fixed to a rock and containing symbiotic 

 algae, is obviously adapted for life in a region where 

 a substratum is present, and where light penetrates. 

 A plaice, so shaped that it is adapted for lying on 

 a sandy surface and bearing well-developed eyes, is 

 similarly fitted. But the dependence of a mud-haunt- 

 ing animal feeding upon minute algae upon Kttoral 

 conditions is no less real, if less apparent, for land- 

 derived mud only accumulates off shores, and the 

 minute algae must have hght before they can Hve. 



In the open water occurs another group of organisms. 



