The relationship of the Lithodidæ to the Hermit Crabs is shown by the abdomen, which is more or less twisted to one side, and has swimmerets only on one side in the female, and quite wanting in the male. [Illustration: _PLATE VIII_ THE "NORTHERN STONE-CRAB," _Lithodes maia_, MUCH REDUCED. THE LAST PAIR OF LEGS ARE FOLDED OUT OF SIGHT IN THE GILL CHAMBERS (_From Brit. Mus. Guide_)] The _Hippidea_ are curious little Crabs found burrowing in sandy beaches in the warmer seas. They have the abdomen tucked under the body, and the legs flattened for shovelling the sand. The BRACHYURA, or true Crabs, form the fourth section of the Reptantia, and are distinguished by having the abdomen reduced to a tail-flap, which is doubled up under the cephalothorax, and is usually without any trace of the uropods which are present in all the groups already mentioned, with the single exception of the Lithodidæ. At the sides of the head the side-plates of the carapace become firmly soldered to the "epistome," a plate which lies in front of the mouth, and in this way there is formed the "mouth-frame," within which lie the jaws, covered in by a pair of "folding-doors" formed by the flattened third maxillipeds. [Illustration: _PLATE IX_ THE COMMON SHORE-CRAB (_Carcinus mænas_). (REDUCED) _Dromia vulgaris_, CARRYING ON ITS BACK A MASS OF THE SPONGE _Clione celata_. BRITISH. (REDUCED)] The first tribe of the Brachyura, the _Dromiacea_, comprises a number of Crabs that in many points of structure resemble the Lobsters, and are regarded as the most primitive members of the section. _Dromia vulgaris_ (Plate IX.), a furry, clumsy-looking Crab, occasionally found on our southern coasts, has the last two pairs of legs short and carried up over the back, where they are used for holding a mass of living sponge which the Crab uses as a cloak to protect and conceal itself. At the sides of the abdomen, wedged in between the telson and the last somite, a pair of small plates may be seen, which are the last vestiges of the uropods. These are wanting in the other tribes of the Brachyura. [Illustration: _PLATE X_ _Calappa flammea_, BRAZIL. (REDUCED)] The _Oxystomata_ (Plate X.), which form the second tribe of the Brachyura, are distinguished by the form of the mouth-frame, which is narrowed in front so as to be triangular instead of square in outline. The passages through which the water passes out from the gills, which in other Crabs open at the front corners of the mouth-frame, are carried forwards to the front of the head. The Oxystomata are most abundant in tropical seas, but are represented on the British coasts by species of _Ebalia_, small and compact Crabs which are not unlike pebbles of the gravel among which they live.