Fam. 4. Lithodidae.—The abdomen is bent under the thorax, and the body is crab-like and calcified. The rostrum is spiniform, and the sixth abdominal appendages are lost.
Sub-Fam. 1. Hapalogasterinae.—Abdomen not fully calcified, and without complicated plates. Hapalogaster and Dermaturus in the North Pacific littoral.
Sub-Fam. 2. Lithodinae.—Abdomen fully calcified, with a complicated arrangement of plates. Lithodes (Fig. [121]) practically universal distribution, littoral and deep sea. Acantholithus, deep littoral of Japan; Paralomis, west coast of America. This last genus should probably be placed in a separate family.
Sub-Order 3. Brachyura.[[147]]
The abdomen is much reduced, especially in the male, and is carried completely flexed on to the ventral face of the thorax so as to be invisible from the dorsal surface. The pleopods in the male are only present on the two anterior segments, and are highly modified as copulatory organs; the pleopods in the female are four in number and are used simply for carrying the eggs; the pleopods of the sixth pair are always absent in both sexes. The first antennae and the stalked eyes can be retracted into special pits excavated in the carapace.
Fig. [124].—A, Zoaea, × 24, and B, Metazoaea, × 13, of Corystes cassivelaunus. Ab, 3rd abdominal segment; An, 1st antenna; E, eye; G, gills; M, 1st maxillipede; T.8, last thoracic appendage. (After Gurney.)
Fig. [125].—Later stage (Megalopa) in the development of Corystes cassivelaunus, × 10. A, Antenna; Ab, 3rd abdominal segment; C, great chela; T.8, last thoracic appendage. (After Gurney.)
The larva hatches out as a Zoaea[[148]] (Fig. [124], A) very similar to that of the Anomura; it is furnished with an anterior and posterior spine on the carapace. It is characteristic of the Brachyuran Zoaea that the third maxillipede is fashioned from the beginning in its definitive expanded form, and is never a biramous swimming organ as in the Anomura. The only exception to this rule is found in the Dromiacea, the most primitive of the Brachyura, to be soon considered, in which not only the third maxillipede, but also the first pair of pereiopods may be developed as biramous oars, a condition taking one back to the Mysis stage of the Macrura. The Metazoaea (Fig. [124], B) has the rudiments of the thoracic limbs developed and crowded together at the back of the carapace; they are all laid down in their definitive forms, and the abdomen has the pleopods precociously developed. These Zoaeal stages are of course pelagic, but the Metazoaea next passes into the Megalopa stage (Fig. [125]), in which the little crab forsakes its pelagic life and assumes the ground-habits of the adult; the Megalopa, which corresponds exactly to the Glaucothoe of the Pagurids, resembles a small Galathea or Porcellana, the abdomen being still large and unflexed and furnished with normal pleopods. From this stage the adult structure is soon achieved, though, owing to the continued growth of the Crustacea even after maturity is reached, there is often a slight progressive change in structure, especially in the male, at each successive moult of the individual. The Megalopa of Corystes cassivelaunus is peculiar in the immense production of the second antennae, which act as a respiratory tube (Fig. [125]).