284 GEOGRAPHICAL DISTRIBUTION OF CRUSTACEA. 



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. 



V. It is where the species are most numerous and varied, and 

 where they attain the greatest size, in other words, where the 

 temperature is most elevated, that the peculiarities of structure, 

 which characterize the several groups, are most strongly manifested. 

 Thus the transverse development of the cephalo-thorax, which is 

 so remarkable in the Brachyourous Decapods (the 'breadth of the 

 carapace of the typical Crabs being much greater than its length 

 from back to front), is carried to its greatest extent in certain 

 Crustacea of the Equatorial region; and the same might be 

 stated of the characteristic peculiarities of most other natural 

 groups. Further, it is in this region that we find the greater 

 part of those anomalous forms, which depart most widely from 

 the general structure of the Class. 



VI. Lastly, there is a remarkable coincidence between the 

 temperature of different regions, and the prevalence of certain 

 forms of Crustaceous animals. Thus there are few genera to be 

 met with in the "West Indian seas, which are not represented in 

 the East Indian, the species, however, being usually different. 

 The same may be said of the genera inhabiting the temperate 

 regions of the globe ; similar generic forms being usually met 

 with in the corresponding parts of the Old and New World, and 

 of the Northern and Southern Hemispheres, although the species 

 are almost invariably different. 



