If we look at the dorsal surface of the abdomen we find that, unlike that of the Hermit-crabs, it is completely protected by a number of hard plates (Fig. 120, B). Beneath the carapace can be seen a number of small plates belonging to the last thoracic segment; following these there are four large plates (1–4) representing the terga of the first four abdominal segments; the fifth, sixth, and the telson are, as has been stated, carried on the under side of the abdomen, but they are represented diagrammatically (5, 6, _T_) in the dorsal view. Besides the large terga, there are a number of small plates laterally, usually two to each segment, but they show a tendency to subdivide and increase in the largest specimens. This condition of affairs is very different to that in the naked fleshy abdomen of an ordinary Pagurid, but it can easily be deduced from that of the genus _Cenobita_, ordinary Hermit-crabs found in the Indo-Pacific Oceans, from which the Robber-crab has evidently descended. In _Cenobita_ (Fig. 120, A) we see the same system of plates upon the dorsal surface of the abdomen, but they are much smaller, and the lateral plates are not so numerous; indeed, the greater part of the abdomen remains fleshy and uncalcified. The under surface of the abdomen shows the same rugosity as is found in _Birgus_, and from a number of other anatomical characters it is evident that the Robber-crab is a highly modified _Cenobita_ that has deserted its shell and developed a symmetrical abdomen protected by expanded and hardened plates which represent those found in a reduced condition in _Cenobita_. The species of _Cenobita_ although they inhabit shells and have normal branchiae, live on the shore, and have not been seen to descend actually into the sea. The Lithodidae, which are found in temperate seas, especially on the Northern Pacific coasts (though _Lithodes maia_ occurs in the North Sea, and certain species inhabit deep water in the Indian Ocean), have a deceptively Brachyuran appearance, the thorax being much shortened and the abdomen being much reduced and carried tightly flexed on to the ventral surface of the thorax. They live a free, unprotected existence, and are highly calcified. They are, however, certainly Pagurids, as is evidenced by a number of anatomical characters, but most clearly by the asymmetry of the abdomen, especially in the female, which is not only markedly asymmetrical in the arrangement of its dorsal plates (Fig. 121), but also in the presence of three pleopods upon the left side only, as in _Birgus_. The male is without these appendages, and the sixth pair of pleopods is absent in both sexes. The remarkable calcified plates upon the abdomen bear a superficial resemblance to those in _Birgus_, but their evolution is traced, not from a Cenobite, but from an Eupagurine stock.[144] [Illustration: