198 CRUSTACEA 



the rock-pools and under stones. Carcinus maenas is character- 

 istic of the rocky coasts of the North Sea, while it is replaced 

 in warmer seas and all round the tropics by Crabs of the family 

 Grapsidae, which are typical rock-livers, and exceedingly agile in 

 clambering over tide-washed rocks. Porcellanidae are also very 

 common under stones at low tide on rocky beaches. Such 

 typical Shore-crabs as these are remarkably resistant to 

 desiccation, and can live out of water for an astonishing time ; 

 nor do they require a change of water provided they have 

 access to the air. The edible crab (Cancer pagurus) and the 

 lobsters (Homarus and Pcdinurus) are dependent on rocks, but they 

 rarely come close iu-shore, preferring depths of a few fathoms. 



Sandy coasts are preferred by Shrimps and Prawns, which 

 haunt the shallow coastal waters in shoals ; and in the sand are 

 found all the Crabs whose respiratory mechanism is specially 

 adapted for life in these regions, e.g. Hippidea or Mole-crabs, 

 Corystes, Matuta, Calappa, etc. 



Characteristic of sandy bottoms are also the Thalassinidea, 

 such as Ccdlianassa, which excavate galleries in the sand. 

 On tropical sandy shores various species of Ocypoda and Gela- 

 simus are conspicuous, which have deserted the sea, and live in 

 burrows which they excavate on the shore. Gelasimus is 

 especially abundant in the muddy sand of tropical mangrove 

 swamps. ' 



Besides the rocky and sandy coasts we must distinguish the 

 muddy shores and bottoms which support a large amount of 

 vegetable and animal growth. These, besides harbouring the 

 greater number of Amphipods and Isopods, are also the natural 

 home of the Dromiacea and Oxyrhyncha, or Spider-crabs, among 

 which the habit is common of decking themselves out with 

 pieces of weed or animal growth in order to harmonise better 

 with their surroundings. Pagurids are also especially abundant 

 in the deeper waters of these coasts. 



Coral-reefs support a characteristic Crustacean fauna. In 

 the growing coral at the reef-edge a number of small Cyclometopa 

 are found, e.g. Chlorodius, Actaea, Xantlio, which are finely 

 sculptured and often coloured so as to harmonise with the coral. 

 Alpheidae also, Shrimp-like Macrura with highly asymmetrical 

 claws, which can emit a sharp cracking sound with the larger 

 claw, are commonly found in pools on the reef. In the coral- 



