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.

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]]

The Zoaea (Fig. [123], A) differs from that of the Galatheidea mainly in the absence of the long spines. It possesses the usual appendages characteristic of the Zoaea, namely, the first and second antennae, mandibles, first and second maxillae, and two pairs of biramous swimming maxillipedes and small third maxillipedes. In the Metazoaea (B), as in the Anomura generally, the third maxillipedes develop into biramous swimming organs, a thing they never do in the Brachyura, and the rudiments of the thoracic segments put in a first appearance. The abdominal segments are already fully formed in the Zoaea stage, so that here as in all other Zoaeas, the order of development from in front backwards is disturbed by the precocious differentiation of the abdominal segments. The next stage is the “Glaucothoe” (Fig. [123], C), which corresponds to the Megalopa of Brachyura (Fig. [125], p. 183). It differs from the adult Hermit-crab in the perfect symmetry of its body, the segmented abdomen, and the presence of five pairs of normal biramous pleopods. At this stage, which lasts four or five days, it resembles closely a little Galatheid. The asymmetry of the adult (Fig. [123], D) is now imposed upon this larva by the migration of the liver, gonads, and green glands into the abdomen, and by the shifting of the posterior lobes of the liver on to the left side of the intestine, which is displaced dorsally and to the right. The gonad lies entirely on the left side. The pleopods of the right side now degenerate, more completely in the male than in the female, and this degeneration is not completed until the little crab has found a shell and lived in it for some time. If a shell is withheld from it, the degeneration of the pleopods is much retarded, so that although the Hermit-crab assumes its asymmetry without the stimulus of the spiral shell, yet this stimulus is necessary for the normal completion of the later stages.

Fam. 1. Pylochelidae.—The abdomen is macrurous and symmetrical, with all the limbs present. Pylocheles (Fig. [118], p. 173).

Fam. 2. Paguridae.—The abdomen is asymmetrical, with some of the limbs lost. The antennal scale is well developed, and the flagella of the first antennae end in a filament.

Sub-Fam. 1. Eupagurinae.—The third maxillipedes are wide apart at the base, and the right chelipedes are much larger than the left. Parapagurus from deep-sea, Eupagurus from temperate, especially north temperate seas. Pylopagurus.

Sub-Fam. 2. Pagurinae.—The third maxillipedes are approximated at the base; the chelipedes are equal or subequal, or the left is much larger. Chiefly in the warm and tropical seas, but Clibanarius and Diogenes also in the Mediterranean.

Fam. 3. Cenobitidae.—The abdomen is as in Paguridae. The antennal scale is reduced, the flagella of the first antennae end bluntly. The members of this family are characteristic of tropical beaches, where they live on the land. Cenobita, with about six species, in the West Indies and Indo-Pacific, living in Mollusc shells; Birgus (Fig. [119]) on Indo-Pacific coral islands.