198 CRUSTACEA CHAP. 
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 Palinurus) are dependent on rocks, but they 
rarely come close in-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, eg. Hippidea or Mole-crabs, 
Corystes, Matuta, Calappa, ete. 
Characteristic of sandy bottoms are also the Thalassinidea, 
such as Callianassa, which excavate galleries in the sand. 
On tropical sandy shores various species of Ocypoda and Cela- 
simus are conspicuous, which have deserted the sea, and live in 
burrows which they excavate on the shore. Gelasimus is 
especially abundant im 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, Xantho, 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 
fo} 
claw, are commonly found in pools on the reef. In the coral- 
