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 claw, are commonly found in pools on the reef. In the coralshingle formed by abrasion from the reef-edge at a few fathoms depth, Leucosiidae are found, in which, again, respiratory mechanisms for filtering sand from the gills are present. Besides the geological nature of the coast, latitude has a very important bearing upon the distribution of littoral Crustacea. Indeed, the present distribution of littoral Crustacea appears to be far more determined by the temperature of the coastal waters than by the presence of any land-barriers, however formidable. We may distinguish an Arctic, Antarctic, and Circumtropical zone. The =Arctic= zone includes the true Arctic seas, and stretches right down through boreal regions towards the sub-tropical seas. Almost all the truly Arctic forms penetrate fairly far south, the Arctic seas being characterised more by the absence of temperate forms than by the presence of forms peculiar to itself. At the same time it must be noted that the individuals from the coldest regions often grow to an enormous size, a characteristic which is physiologically unexplained. A great many of the Crustacea characteristic of this region are circumpolar, _i.e._ they are not restricted in range to either the Atlantic or Pacific. This is especially true of the extremely northern types, _e.g._ Crangonidae and Hippolytidae, but it is also true of a number of Crustacea which do not now occur as far north as Greenland or Bering Strait, so that there is no longer any free communication for them between Pacific and Atlantic. This gives rise to a discontinuous distribution in the two oceans, exemplified in the common Shrimp, _Crangon vulgaris_, which is found on the temperate European coasts and on the Pacific coasts of Japan and Eastern America. The same is true of _Eupagurus pubescens_ and _E. bernhardus_.