ANNULOSA: CRUSTACEA. 195 



Reproduction in the Crabs is the same as in the Macrura, 

 but the larva is exceedingly unlike the adult, and approximates 

 closely to the type of the Macrura, another proof that the 

 Brachyura stand higher in the Crustacean scale. The larval 

 Crab was originally described as a distinct animal, under the 

 name of Zoea, presenting in this condition a long and well 

 developed abdomen. 



CHAPTER XXXY. 



DISTRIBUTION OF THE CRUSTACEA. 



DISTRIBUTION OF CRUSTACEA IN SPACE. The following general 

 principles have been laid down by Milne-Edwards with regard 

 to the geographical distribution of the Crustacea : 



1. The different forms and modes of organisation of the 

 Crustacea are more varied and numerous, in proportion as we 

 pass from the Polar Regions towards the Equator. 



2. The number of different species is not only greater, but 

 the number of types is greater in warm regions as compared 

 with cold. 



3. The higher Crustacea are either entirely wanting or are 

 sparingly represented in the colder regions of the globe, but 

 increase rapidly in relative numbers as the Equator is ap- 

 proached. 



4. The size attained by the Crustacea is greater on the 

 average in warm regions than in colder climates. 



5. The special points of structure which are characteristic 

 of the different groups of Crustacea are more strongly manifested 

 in the warmer regions of the globe. 



6. There exists a decided relation between the temperature 

 of any given region and the character of its Crustacean fauna ; 

 similar generic forms being usually found occupying regions of 

 the same climatal character. 



DISTRIBUTION OF CRUSTACEA IN TIME. The class Crustacea is 

 largely represented in past time, ranging from the Cambrian 

 Rocks up to the present day. The oldest families of the 

 Crustacea are the Trilobita and the Eurypterida, both of which 

 are exclusively Palaeozoic, and died out at the close of the 

 Carboniferous epoch. It is worthy of notice how larval are 

 the characters of these ancient groups when compared with 

 their modern successors. Of the remaining orders the Cirri- 

 pedia, Ostracoda and Phyllopoda are the three which are most 

 largely represented. 



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