The Zoaeae, or young larval stages of the Galatheidea, are characterised by the immense length of the spines upon the carapace (Fig. [115]). The young Zoaea which hatches out from the egg resembles in other respects that of the Brachyura. The Metazoaea, however, differs from that of the Brachyura in the fact that the third maxillipede is first present as a biramous swimming organ, and at its first appearance is not developed in its definitive form. The other thoracic limbs are not schizopodous when they appear, and indeed in nearly all respects the development proceeds as in the Brachyura.
Fam. 1. Aegleidae.—The gills are trichobranchiae, and there are eight arthrobranchs. There are no limbs on the second abdominal segment of the male. The abdomen is not carried folded on to the thorax. The first two characteristics separate this family from all the other Galatheidea. Aeglea laevis, a fresh-water species from the rivers of temperate S. America, is the sole representative.
Fam. 2. Galatheidae.—The abdomen is not folded against the thorax. The members of this family are often littoral in habit (Galathea, Fig. [116]), but often go down into great depths (Munidopsis, Fig. [114]).
Fig. [116].—Dorsal view of Galathea strigosa, × ½. (From an original figure prepared for Professor Weldon.)
Fam. 3. Porcellanidae.—The abdomen is folded against the thorax, and the body has a crab-like form. These are always littoral in habit, never descending into the depths. Pachycheles in the tropics, Porcellana with numerous species in all seas, P. platycheles being a common British species.
Tribe 2. Hippidea.
The Mole-crabs have the habit of burrowing in sand, and their limbs are peculiarly modified into digging organs for this purpose (see Fig. [117]). In other respects they are seen to be closely related to the Galatheidea by the form of the carapace, the condition of the abdomen, and the reduced last thoracic limbs.
In Albunea, which is found in the Mediterranean, the first antennae[[141]] are greatly lengthened and apposed to one another, and by means of a system of interlocking hairs they form a tube down which the water is sucked for respiration. The object of this arrangement is to ensure a supply of clear water, filtered from particles of sand, when the crab is buried beneath the surface, on these occasions the tip of the antennal tube being protruded above the surface of the sand. An exactly similar tube is used by the true Crab Corystes cassivelaunus, which has similar burrowing habits, but here the tube is formed from the second antennae and not from the first, so that the tubes in the two cases afford beautiful instances of analogous or homoplastic structures between which there is no homology (see p. [189]).
Fam. 1. Albuneidae.—The first legs are subchelate; the carapace is flattened, without expansions covering the legs. Albunea with several species in the Mediterranean, West Indies, and Indo-Pacific.