INVERTEBRATE ANIMALS. 77 



are not first-rate swimmers, but are active and voracious, 

 and from their omnivorous propensities have been termed 

 the scavengers of the sea. 

 Estimated number of species : recent, 30 ; British, 12. 



H Examples of Scylla, Polyhius, Portumnus, &c. 



Group 185.— Family ERIPHID^ (Dana). 



Mr. Dana has established this family to include 

 certain crabs distinguished by some important peculi- 

 arities connected with the respiratory apparatus. The 

 small but pretty species of Trapezia, with bright porcel- 

 lanous shields, are found amongst the branches and 

 folia of living corals. 

 Estimated number of species : recent, 60 ; British, 2. 



H Examples of Eudora, Pilumnus, Galene, Domcetia, 

 Geryon tridens, &c. 



Family CANCERIDiE. 



Group 186. — Genus CANCER (Linnjeus). The generic 

 name Cancer has been retained to include the common 

 edible crab, Cancer pagurus, and its allies. Several 

 of the species grow to a large size. The gills of crabs 

 are attached to the bases of the legs, and are contained 

 in cavities protected by the sides of the carapace. An 

 apparatus of fan-like appendages sweeps the water out 

 of the gill-cavities by orifices in front, efferent canals, 

 whilst a fresh supply of water rushes in from behind 

 through orifices near the base of the fourth pair of legs, 

 afferent canals, thus keeping up a constant circulation. 

 Estimated number of species : recent, 10 ; British, 1. 

 II Examples of about 6 species. 



