The third section of the Reptantia, the _Anomura_, comprises forms in which the abdomen is variously modified, being either bent upon itself or, if extended, more or less soft and feebly armoured. The last pair of legs are commonly reduced in size, and not used in walking. The members of the four tribes composing the section differ widely in their general appearance. The _Galatheidea_ (Plate VI.) are small, flattened, lobster-like animals which have the abdomen bent under the body. In one family (_Porcellanidæ_) the animals have quite the appearance of little Crabs (see Fig. 41, p. 113), but they may be distinguished from the true Crabs (Brachyura) by the fact that there are only three pairs of walking legs behind the great chelæ, the last pair of legs being very small and carried folded up at the sides of the body, or even within the gill chambers. [Illustration: _PLATE VI_ _Munida rugosa._ BRITISH. (REDUCED)] The _Thalassinidea_ are small lobster-like animals which burrow in sand and mud, and have generally a more or less soft abdomen (see Fig. 38, p. 103). [Illustration: _PLATE VII_ THE COMMON HERMIT-CRAB, _Eupagurus bernhardus_, IN THE SHELL OF A WHELK (REDUCED) (_From Brit. Mus. Guide_)] The tribe _Paguridea_ includes the Hermit Crabs (_Paguridæ_) and their allies. The typical Hermit Crabs (Plate VII.), which are familiar objects in seaside rock-pools, live in the empty shells of Whelks and other Gasteropod Molluscs, which they carry about with them as portable shelters. The structure of the animals is modified in adaptation to this curious habit. The abdomen, which is protected during life by the borrowed shell, is soft and unarmoured, and is spirally twisted. The swimmerets, which have only the function of carrying the eggs in the female, are much reduced, and are usually present only on one side of the body. The uropods no longer form a tail-fan, but are adapted for firmly wedging the hind part of the body into the coils of the shell. One of the chelipeds is much larger than the other, and serves to block up the opening when the animal withdraws into its shelter. In tropical countries certain Hermit Crabs (_Coenobitidæ_) have become adapted to a life on land, and one of these, the well-known Coconut Crab, or Robber Crab (_Birgus latro_), which is the largest species of the tribe, has given up the habit of protecting itself with a shell, and its abdomen has again acquired a strong armour on the upper side. The marine _Lithodidæ_--to which the British Stone Crab, _Lithodes maia_ (Plate VIII.) belongs--seem at first sight to have little resemblance to the Hermit Crabs, for they have the abdomen very small, and tucked up under the body as in the true Crabs. Like the Porcellanidæ, mentioned above, however, the Lithodidæ have only three pairs of walking legs behind the chelipeds, the last pair being feeble and usually folded out of sight within the gill chambers.