CHAPTER IX 

 DISTRIBUTION 



It is already evident from the preceding chapters that Crustacea 

 are found in the sea, in freshwater, and on land. In dealing with 

 their distribution in more detail it is necessary to discover how the 

 various groups have distributed themselves within these major 

 habitats, and to try to explain how various factors combine to 

 limit or facilitate distribution. 



Numerous references will be made to endemic species; these are 

 species which are peculiar to a particular region, and are not found 

 elsewhere. Such species, and, more particularly, endemic genera, 

 are most important in attempting to define zoogeographical 

 regions. 



MARINE CRUSTACEA 



Most Crustacea live in the sea, and in all parts of the world they 

 form a major component of the marine fauna. This world-wide 

 distribution raises certain questions. Do the crustacean faunas of 

 the various oceans differ one from another? Do the inland seas, 

 such as the Caspian, have different Crustacea from those seas which 

 are in open communication with the great oceans, and if such 

 differences are found can we account for them? 



Animals living in the sea can be roughly divided into those 

 which spend most of their time on the bottom, and those which 

 swim or float freely in the water. The latter are more likely to be 

 widely distributed, because they may come under the influence of 

 the oceanic currents. Differences between the faunas of geographical 

 areas of the seas are more likely to be found among the bottom 

 dwellers. 



The sea floor can be divided into two parts : the shelf, which sur- 

 rounds most land masses to a varying degree, extending down to 

 a depth between 200 and 400 metres, and the deep sea floor which 

 falls away steeply from the shelf and reaches depths of several 

 thousand metres. Depths below a thousand metres are referred to as 



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