80 INVERTEBRATE ANIMALS. 



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. 

 *fr Examples of about 6 species. 



Group 187. -Genus XANTHO (Leach) and allies. 



^ Menippe gigas (M. Edwards) is probably the 

 most powerful Crustacean : the pincers are worked by a 

 muscular apparatus enormously developed, and must be 

 capable of crushing with ease the shells of many bivalve 

 Mollusca. Much variety in colour and sculpture is met 

 with amongst the smaller generic forms. 

 Estimated number of species : recent, 50 ; British, 2. 



11^ Examples of Chlovoclius, Etisus, Daira, Actce- 

 odes, &c. 



Group 188.— Genus CAEPILIUS (Leach) and allies. 

 Estimated number of species, 24 ; British, 0. 



^ C. coralinus, C. maculatus, and the rare C. sig- 

 natus afford good examples of the smooth, 

 convex, and highly-coloured shields wdiich pre- 

 vail in this group ; Atergatis, Zozymus, &c. 



Section D. — Crabs ivWi shields more or less triangidar, much 

 narroived in front = Famille des Oxyrhinquees (Milne 

 Edwards). 



