290 GEOGRAPHICAL DISTRIBUTION OF CRUSTACEA. 



sura are altogether wanting beyond the forty-fourth degree of 

 latitude. Again, the Brachyourous and Anomourous Decapods 

 appear to be altogether excluded from some of the most northern 

 regions that have been explored. Of the family of SquillidaB 

 ( 791), so highly characteristic of the Order Stomapoda, it is 

 rare to meet with any members, north of the forty-fifth degree 

 of latitude. And the curious group of Phyllopoda is restricted 

 within a still nearer neighbourhood of the Tropical Region. 



III. Not only are those Crustacea, which are most elevated in 

 the scale, deficient in the Polar Regions ; but their relative num- 

 ber increases rapidly as we pass from the Pole towards the 

 Equator. Thus the Brachyoura, which must be considered as the 

 most elevated of the whole series, are totally absent, as we have 

 just seen, in some parts of the arctic region ; and we find their place 

 taken by the far less complete Edriophthalma, with a small num- 

 ber of Anomourous and Macrourous Decapods. In the Mediterra- 

 nean, however, the Decapods surpass the Edriophthalma in regard 

 to the number of species ; and the Brachyourous division predomi- 

 nates over the Macrourous, in the proportion of two to one. 

 And in the East and West Indies, the short-tailed are to the 

 long-tailed Decapods, as three, four, or even five, to one. Again, 

 the Land-Crabs, which are probably to be regarded as taking 

 the highest rank among the Brachyoura, are only to be met with 

 between the tropics. And of the fiuviatile Decapods (those 

 which inhabit rivers, brooks, and lakes), a large proportion 

 belong in tropical regions to the elevated type of the Brachy- 

 oura ; whilst all those found in the temperate and arctic zones 

 belong to the Macrourous division. 



IV. When we compare together the Crustacea of different 

 parts of the world, we observe that the average size of these 

 animals is considerably greater in tropical regions, than in the 

 temperate or frigid climes. The largest species of the arctic and 

 antarctic seas, are far smaller than those of the tropical ocean ; 

 and they bear a much smaller proportion to the whole number. 

 Further, in almost every group, we find that the largest species 

 belong to the equatorial regions ; and that those which represent 

 them (or take their place, as it were) in temperate regions, are 

 of smaller dimensions. 



