Some Crustacea construct habitations for themselves. On turning over a flat stone between tide-marks, one often finds a little mass of bits of weed and rubbish attached to it, and if this be torn open a greenish-brown, shrimp-like animal, about three-quarters of an inch long, is seen slithering away on its side. This is an Amphipod (_Amphithoë rubricata_) which builds the shelter for itself, sticking the fragments together with threads of a cementing material produced by glands on the surface of its body and legs. Other Amphipods construct more neatly finished tubular dwellings of mud, or even of small stones, which are attached to sea-weeds and the like; and some make portable shelters of the same kind, which they carry about with them like the caddis-worms of fresh-water streams. Some of the true Crabs also employ portable shields for purposes of defence or of concealment. The species of _Dorippe_ which are found in tropical seas have the last two pairs of legs short, elevated on the back so that they cannot be used for walking, and ending in a kind of grasping claw. By means of these claws the Crab holds over its back some object, generally one valve of a molluscan shell, sometimes even a mangrove-leaf, to supplement the protection afforded by its carapace. The "Sponge Crabs" (_Dromiidæ_), of which one species, _Dromia vulgaris_ (Plate IX.), occurs on the southern coasts of Britain, have also the last two pairs of legs elevated on the back and used in a similar way; but in this case the covering is usually a mass of living sponge, one of the Sea-squirts (Tunicata), or some similar organism. [Illustration: _PLATE XIII_ A SWIMMING CRAB, _Portunus depurator_. BRITISH. (REDUCED) A SPIDER-CRAB, _Maia squinado_, DRESSED IN FRAGMENTS OF WEED. BRITISH (REDUCED)]