All authorities are agreed that these[149] are the most primitive of the Brachyura. In them the abdomen is much less reduced in both sexes than in other Brachyura; there is a common orbitoantennary fossa, into which eyes and antennae are withdrawn, instead of a separate one on each side for each organ; the carapace is often much elongated as in the Macrura and Anomura, and a number of other anatomical characters might be mentioned which characterise the Dromiacea as intermediate between the true Brachyura and the lower forms. There are, however, two views as to the relationship of the Dromiacea; Claus held that they proceeded from a Galatheid stock, and hence that the development of the Brachyura ran through an Anomurous strain; but Huxley, and latterly Bouvier,[150] adopt the view that the Dromiacea are descended, not from the Galatheidae, but direct from the Macrura, and especially from the Nephropsidea. Special resemblances are found between the Jurassic Nephropsidae and certain present day Dromiacea, _e.g._ _Homolodromia paradoxa_, the detailed form of the carapace in the two cases being very similar. It is, however, a little strange that in the Dromiacea we meet with the same reduction and dorsal position of the last, or last two pairs of thoracic limbs which we saw to be such a characteristic feature of the Anomura, especially of the Galatheidae. In the Dromiacea these limbs may be chelate, and they are used for attaching shells and other bodies temporarily to the back. Must we suppose that this resemblance to the Anomura is due to convergence, or that the Nephropsidae, which gave rise to perhaps both Galatheidae and Dromiacea, had this character, and that it has been subsequently lost in the Macruran stock? We have already mentioned that the Metazoaea of _Dromia_ has not only a well-developed swimming third maxillipede, but also a biramous first pereiopod, a character which speaks strongly for Macruran affinities. [Illustration: FIG. 126.—_Dromia vulgaris_, × 1. (After Milne Edwards and Bouvier.) ] =Fam. 1. Dromiidae.=—The eyes and antennules are retractile into orbits. The last two pairs of thoracic limbs are small, and held dorsally. The sixth pair of pleopods are rudimentary or absent. _Homolodromia_ from West Indies, deep-sea. _Dromia_, widely dispersed. _D. vulgaris_ (Fig. 126) occurs on the English coasts. =Fam. 2. Dynomenidae.=—Similar to the preceding family, but only the last pair of thoracic limbs is small, and held dorsally. The sixth pair of pleopods are reduced, but always present. _Dynomene_ in the Indo-Pacific.