312 MANUAL OF ZOOLOGY. 



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 (fig. 160), presenting in this condition a long and 

 well-developed abdomen. It is only after several successive 

 moults that the young Crab assumes its characteristic Brachyur- 

 ous form, and acquires by gradual changes the features which 

 distinguish the adult. The Zoece of the Crabs are usually dis- 

 tinguished 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. Among the Land-crabs, there is no metamorphosis in 

 Gecarrinus; but in some of the Gecarcinidcz the young are 

 Zoea. 



CHAPTER XXXIV. 



DISTRIBUTION OF THE CRUSTACEA. 



DISTRIBUTION OF CRUSTACEA IN SPACE. The Crustacea are 

 distributed over the whole globe, some forms being terrestrial 

 in their habits, but the majority inhabiting the sea or fresh 

 water. As a rule, the development of the Crustacean fauna is 

 in proportion to the temperature, the higher and larger forms 

 being most abundant in warm regions. The groups of the 

 Cirriptdia.) Rhizocephala, Xiphosura, and Lcemodipoda^ are only 

 found in salt water. On the other hand, the Ichthyophthira, 

 Ostracoda, Copepoda, Phyllopoda, Eurypterida (?), Amphipoda, 

 Stomapoda, Isopoda, and Decapoda, are found both in fresh and 

 in salt water. Of these, however, the Phyllopods are princi- 

 pally fresh-water forms, and the Stomapods and Decapods are 

 essentially inhabitants of the sea ; whilst the Eurypterids are 

 certainly mainly a salt-water group, though some forms may 

 perhaps have lived in fresh water as well. The Isopoda and 

 Decapoda also include terrestrial forms. 



DISTRIBUTION OF CRUSTACEA IN TIME. As regards the 

 general distribution of the Crustacea in time, remains of the 

 class are comparatively abundant in all formations except the 

 very oldest ; as might have been expected from the generally 

 chitinous or sub-calcareous nature of their integuments and 

 their aquatic habits. Owing also to their habit of periodically 



