=A. Marine.= The great majority of the Crustacea are inhabitants of the sea. From a Zoogeographical point of view we divide the sea into three chief regions, each of which is characterised by a special kind of fauna—the littoral, the pelagic, and the abyssal regions. The =littoral= region, which comprises all the shallow coastal waters down to about 100 fathoms, varies very greatly in its physical character according to the nature of the coast, its geological constitution, latitude, etc., but, on the whole, it is characterised by variability of temperature and salinity, by the presence of sunlight, and by the continuous motion of its waves. On the shores of the large oceans this region is also greatly affected by the tides. It is inhabited by a vast assemblage of Crustacea, all of which are dependent upon a solid substratum, either of rock or sand, or of vegetable or animal growth, upon which they may wander in search of food, or in which they may hide themselves. In consequence, the character of the Crustacea on any shore is largely determined by its geological nature. Although a certain number of Entomostraca such as Copepoda (Harpacticidae and Cyclopidae), Ostracoda (Cypridae and Cytheridae), and a few Operculata are littoral in habit, it is the Malacostraca, from their larger size and variety of form, which give the character to coastal waters. On rocky coasts, especially those affected by tides, a great many kinds of Shore-crab are found, which hide at low tide in the rock-pools and under stones. _Carcinus maenas_ is characteristic 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, _e.g._ Hippidea or Mole-crabs, _Corystes_, _Matuta_, _Calappa_, etc. 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 _Gelasimus_ 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.