294 



PHYL UM A R THROPODA. 



Sub-order 2. Brachyura. Abdomen short, and bent under the 

 thorax. It is narrow in the male, and does not usually bear 

 more than two pairs of appendages ; it is broader in the 

 female, and bears four paired appendages. The ventral 

 ganglia have fused into an oval mass. Cancer (edible crab) ; 

 Carcinus manas (shore crab) ; Portumts (swimming crab) ; 

 Dromia (often covered by a sponge) ; Pinnotheres (living 

 inside bivalves) ; J^elpJmsa (a fresh-water crab) ; Gecarcinus 

 (land-crabs, only visiting the sea at the breeding season). 



FIG. 141. Hermit-crab (Pagurus) in shell of Fitsus. On the shell are 

 acorn-shells (Balanns] and tubes of Serpula, a sedentary worm. 



History. Fossil Crustaceans are found in Cambrian strata, but the 

 highest forms (Decapoda) were not firmly established till the Tertiary 

 period. Some of the genera, e.g. the Branchiopod Estheria, living from 

 Devonian ages till now, are remarkably persistent and successful. How 

 the class arose we do not know ; it is probable that types like Nebalia 

 give us trustworthy hints as to the ancestors of the higher Crustaceans ; 

 it is likely that the Phyllopods, e.g. Apus, bear a similar relation to the 

 whole series ; the Copepods also retain some primitive characteristics ; 



