In _Albunea_, which is found in the Mediterranean, the first antennae[141] are greatly lengthened and apposed to one another, and by means of a system of interlocking hairs they form a tube down which the water is sucked for respiration. The object of this arrangement is to ensure a supply of clear water, filtered from particles of sand, when the crab is buried beneath the surface, on these occasions the tip of the antennal tube being protruded above the surface of the sand. An exactly similar tube is used by the true Crab _Corystes cassivelaunus_, which has similar burrowing habits, but here the tube is formed from the second antennae and not from the first, so that the tubes in the two cases afford beautiful instances of analogous or homoplastic structures between which there is no homology (see p. 189). =Fam. 1. Albuneidae.=—The first legs are subchelate; the carapace is flattened, without expansions covering the legs. _Albunea_ with several species in the Mediterranean, West Indies, and Indo-Pacific. [Illustration: FIG. 117.—_Remipes scutellatus_, dorsal and ventral views, × 1. (From original drawings prepared for Professor Weldon.) ] =Fam. 2. Hippidae.=—The first legs are simple, the carapace is subcylindrical with expansions covering the legs. _Remipes_ (Fig. 117) and _Hippa_ in tropical or sub-tropical seas. =Tribe 3. Paguridea.=[142] The ordinary Hermit-crabs, common on the English as on every coast, are characterised by the fleshy asymmetrical abdomen from which all the hard matter has disappeared, and which is carried tucked away in an empty Gasteropod shell. The abdomen is spirally wound in accordance with the shape of the shell, and a firm attachment is effected by means of the sixth pair of pleopods, especially that of the left side, which is fashioned into the form of a hook and is curled round the columella of the shell; this attachment is so secure that in trying to pull a Hermit-crab out of its shell the body is torn apart before the hold gives way. The other pleopods are in a much reduced condition, being generally altogether absent from the right side of the abdomen, and often greatly reduced on the left side, especially in the male, though in the female they are still used for the attachment of the eggs. The last two pereiopods are much reduced and are concealed inside the shell, which they help to carry. The great chelae are usually asymmetrically developed, that on the right side being much larger than that on the left, and often serving the purpose of shutting the entrance to the shell when the crab is withdrawn inside.