FIG. 121.—_Lithodes maia_, ♀, in ventral view, × ¼. The abdomen is flexed on the thorax, so that its dorsal surface is seen. _l.3_, Lateral plates of third abdominal segment; _l.5_, left lateral plate of fifth abdominal segment; _m_, marginal plate; _T_, brush-like last pereiopod; _Te.6_, telson and sixth abdominal segment. ] In some of the Eupagurinae, e.g. _Pylopagurus_, feebly calcified plates are present upon the segments of the abdomen (Fig. 122, A). In the most primitive of the Lithodidae we witness the reduction (Fig. 122, B) and disappearance (C) of these original plates, their place being taken first by a number of irregularly situated small spines and warts, which, however, subsequently fuse up to form definite segmental plates. In _Lithodes maia_, ♂ (D), there are a series of lateral and marginal plates, while in _Acantholithus_ (E) a number of median plates appear, presumably by the fusion of the small spines present in the median line in _Lithodes maia_; finally, a fusion of the marginal and lateral plates may take place, so that each abdominal segment is covered by a median and two paired lateral plates. [Illustration: FIG. 122.—Diagrams of abdomen: =A=, of _Pylopagurus_, sp.; =B=, of _Hapalogaster cavicauda_; =C=, of _Dermaturus hispidus_; =D=, of _Lithodes maia_, ♂; =E=, of _Acantholithus hystrix_. _c_, Central plates; _l_, lateral plates; _m_, marginal plates; _T_, telson; 1–6, 1st–6th abdominal segments. (After Bouvier.) ] It is to be noted that the males and females of the various species do not follow a parallel course of development, the plates in the male being symmetrical, while those of the female are often highly asymmetrical (compare Figs. 122, D, and 121), thus giving the strongest evidence of a Pagurid ancestry. _Birgus_ and the Lithodidae, then, are Pagurids which have given up living in shells, and have become adapted to a free existence, protecting their soft parts by the development of hard plates, and re-acquiring, to a greater or less degree, a secondary symmetry of form. But the story of Pagurid evolution does not apparently stop here. The genus _Paralomis_, from the West Coast of America, superficially resembles _Porcellana_, and is held to be descended from such forms as _Pylocheles_, while isolated species are known (though not well known), such as _Tylaspis_, described in the _Challenger Reports_,[145] which appear to be Pagurids that have deserted their shells. [Illustration: FIG. 123.—Four stages in the development of _Eupagurus longicarpus_ or _E. annulipes_, × 20. =A=, Ventral view of Zoaea; =B=, lateral view of Metazoaea; =C=, dorsal view of Glaucothoe; =D=, dorsal view of adolescent stage. _Ab.6_, 6th abdominal appendage; _Mxp.1_, _Mxp.3_, 1st and 3rd maxillipedes. (After M. T. Thompson.) ] The metamorphosis of the Hermit-crabs has recently been studied by M. T. Thompson.[146]