ANNULOSA: CRUSTACEA. 2IQ 



CHAPTER XXXV. 



DISTRIBUTION OF THE CRUSTACEA. 



DISTRIBUTION OF CRUSTACEA IN SPACE. The following gene- 

 ral 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 aver- 

 age 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 mani- 

 fested 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 Crus- 

 tacea are the Trtlobita and the Eurypterida, both of which are 

 exclusively Palaeozoic, and died out at the close of the Car- 

 boniferous 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 Cirripedia, 

 Ostracoda, and Phyllopoda are the three which are most 

 largely represented. 



i. Cirripedia. The Cirripedes are hardly known as Palaeo- 

 zoic fossils, but valves of a singular member of this order (2ur- 

 rilepas] have been found in the Silurian Rocks of Scotland. 

 With this exception, the Cirripedes are entirely confined in 

 past time to the Secondary and Tertiary epochs. The Bala- 

 nidce are the most common, commencing, as far as is yet 

 known, in the Eocene period, and attaining their maximum in 

 recent seas. The Verrucidcs, commence in the Chalk, and the 



