VII DISTRIBUTION THE ARCTIC ZONE I 99 



shingle formed by abrasion from the reef-edge at a few fathoms 

 depth, Leucosiidae are found, in which, again, respiratory- 

 mechanisms for filtering sand from the gills are present. 



Besides the geological nature of the coast, latitude has a 

 very important bearing upon the distribution of littoral 

 Crustacea. Indeed, the present distribution of littoral Crustacea 

 appears to be far more determined by the temperature of the 

 coastal waters than by the presence of any land - barriers, 

 however formidable. We may distinguish an Arctic, Antarctic, 

 and Circumtropical zone. 



The Arctic zone includes the true Arctic seas, and stretches 

 right down through boreal regions towards the sub-tropical seas. 

 Almost all the truly Arctic forms penetrate fairly far south, the 

 Arctic seas being characterised more by the absence of temperate 

 forms than by the presence of forms peculiar to itself At the 

 same time it must be noted that the individuals from the 

 coldest regions often grow to an enormous size, a characteristic 

 which is physiologically unexplained. 



A great many of the Crustacea characteristic of this region 

 are circumpolar, i.e. they are not restricted in range to either the 

 Atlantic or Pacific. This is especially true of the extremely 

 northern types, e.g. Crangonidae and Hippolytidae, but it is also 

 true of a number of Crustacea which do not now occur as far 

 north as Greenland or Bering Strait, so that there is no 

 longer any free communication for them between Pacific and 

 Atlantic. This gives rise to a discontinuous distribution in the 

 two oceans, exemplified in the common Shrimp, Crangon vulgaris, 

 which is found on the temperate European coasts and on the 

 Pacific coasts of Japan and Eastern America. The same is true 

 of Eupagurus pxibescens and E. hernliardus. 



At the same time the boreal Atlantic and Pacific have their 

 peculiar forms. Thus the European and American Lobsters are 

 confined to the Atlantic, while the North Pacific possesses a very 

 rich array of Lithodinae, which cannot be paralleled in the 

 Atlantic. 



We may explain the community of many littoral forms to 

 both the North Atlantic and Pacific coasts by the continuous 

 coast-line uniting them, which in former times possibly did not 

 lie so far north, or else was not subjected to so rigorous a climate 

 as now. 



