VII DISTRIBUTION—THE ARCTIC ZONE 199 
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, z.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 Hupagurus pubescens and E. bernhardus. 
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. 
