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).
PLATE VII
THE COMMON HERMIT-CRAB, Eupagurus bernhardus, IN THE SHELL OF A WHELK (REDUCED)
(From Brit. Mus. Guide)
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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 (Cœnobitidæ) 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. 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.
