GEOGRAPHICAL DISTRIBUTION OF CRUSTACEA. 



The extensive study of this large and important class, by 

 M. Milne-Edwards, has enabled him to arrive at some very in- 

 teresting conclusions in regard to its Geographical Distribution. 

 An outline of these will be here introduced, because they would 

 probably serve, with so"me modifications, to represent the general 

 facts relating to the distribution of other Classes. 



900. It has been pointed out, in the preceding sketch of 

 the principal forms of the Crustacea, that different species have 

 different localities, or residences assigned to them (as it were), 

 on the surface of the globe. We have seen that some are exclu- 

 sively confined to fresh water, that others are inhabitants of the 

 brackish water of estuaries, that others take up their abode on 

 the shore, where they are periodically covered and left dry by 

 the tide, that others frequent the shallow waters in the neigh- 

 bourhood of the shore, that others are found near the bottom 

 of the deeper waters, at no great distance from land, that others, 

 again, roam freely through the open sea, and finally, that others 

 are only to be met with on the- dry land, at a considerable dis- 

 tance from the shore. Thus we see that each species has an 

 appropriate kind of residence, for which it is peculiarly adapted 

 by its organisation and habits ; but it may be further stated, that 

 each species has an appropriate place of residence, which is very 

 much determined by the temperature of the region. It is true 

 that there are many species very extensively distributed ; but 

 this results from their being adapted, by some peculiarities of 

 structure and habit, which we cannot detect, to sustain life 

 under a considerable variety of external conditions ( 13). The 

 extent to which even these species are distributed, however, will 



