ANNULOSA : CRUSTACEA. 251 



countries of the Old and New Worlds, as well as Australia. 



They are essentially terrestrial in their habits, and migrate in 



large bodies to the sea, in order to lay 



their eggs. Besides the true Gecar- 



rini, members of other very different 



families live more or less constantly 



on dry land, and have air admitted 



directly into the branchial chamber. 



Amongst these are the Calling-crabs 



(Gelasimus), and the Sand-crabs (Ocy- 



poda). 



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 Fig. 97. Larva (Zoea) of Crab 

 the Brachyura stand higher in the JS$a*ggSJ'-W 1 ! 

 Crustacean scale. The larval Crab 



was originally described as a distinct animal, under the name 

 of Zoea (fig. 97), presenting in this condition a long and 

 well-developed abdomen. It is only after several succes- 

 sive moults that the young Crab assumes its characteristic 

 Brachyurous form, and acquires by gradual changes the features 

 which distinguish the adult. The Zoea of the Crabs are usually 

 distinguished by the possession of long spines developed from 

 the carapace. When first liberated from the egg, the Zoea is 

 enveloped in a larval skin or membrane, which is shed in a 

 few hours. 



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. 



