CHAPTER VIT 



REMARKS ON THE DISTRIBUTION OF MARINE AND FRESH-WATER 



CRUSTACEA 



A. Marine. 



The great majority of the Crustacea are inhabitants of the 

 sea. From a Zoogeographical point of view we divide the sea 

 into three chief regions, each of which is characterised l)y 

 a special kind of fauna — the littoral, the pelagic, and the 

 abyssal regions. 



The littoral region, which comprises all the shallow coastal 

 waters down to about 100 fathoms, varies very greatly in its 

 physical character according to the nature of the coast, its 

 geological constitution, latitude, etc., but, on the whole, it is 

 characterised by variability of temperature and salinity, by the 

 presence of sunlight, and by the continuous motion of its waves. 

 On the shores of the large oceans this region is also greatly 

 affected by the tides. It is inhabited by a vast assemblage of 

 Crustacea, all of which are dependent upon a solid substratum, 

 either of rock or sand, or of vegetable or animal growth, upon 

 which they may wander in search of food, or in which they may 

 hide themselves. In consequence, the character of the Crustacea 

 on any shore is largely determined by its geological nature. 



Although a certain number of Entomostraca (such as Cope- 

 poda (Harpacticidae and Cyclopidae), Ostracoda (Cypridae and 

 Cytheridae), and a few Operculata are littoral in habit, it is the 

 Malacostraca, from their larger size and ^■ariety of form, which 

 give the character to coastal waters. 



On rocky coasts, especially those affected l>y tides, a great 

 many kinds of Shore-crab are found, which hide at low tide in 



197 



