The Life-history of Dromia vulgaris.




We now have a good idea of the larval stages of Dromia vulgaris. It is evident from the above descriptions that the larvæ differ in almost every important point from those of the Brachyura, although the brachyuran larva has certain characters which may be derived from the Galatheidæ. Firstly, it differs in the armature of the pre-zoea; afterwards, in the form of the carapace and spines of the zoea, antennule, antenna, in the presence of a functional swimming exopodite on the third maxillipede and in the number of setæ on the expodites of the maxillipedes, in the presence of a functional exopodite on the first leg and traces of exopodites on the second and third, in the shape and armature of the telson, and in the gills. On the other hand, it has certain features which resemble Upogebia and its relatives, particularly the antennal endopodite, the functional exopodite on the third maxillipede and the basal endopodite, the functional exopodite on the first leg, and the exopodites on legs 2 and 3. The telson also resembles that of the members of the Thalas-sinidea except for the fact that there is no median spine. In the gills it differs much. In the armature of the pre-zoea, the zoeal telson, carapace, functional third maxillipede, and basal endopodite, and in some other minor characters it resembles the Anomura, especially in the gills resembling the Galatheidea, but differs from the Anomura in having exopodites on the first three legs. Thus it combines characters belonging to the anomuran section of the Thalassinidea and to the Anomura. Dromia, however, with other members of the Dromiacea, has characters of its own, the antennal scale being surrounded with setæ which is unlike the Anomura and Thalassinidea, the third maxillipede being functional in the first zoea, the megalopa being spiny and shaped like the Brachyura, but the fifth legs unlike any brachyuran known, and the gills being quite different.

It seems that we must remove the Dromiacea from the Brachyura, but, as Gurney rightly says, it is difficult to place it in the Anomura, and it cannot be placed in the Thalassinidea. Apparently the best position for it in a natural classification is in a separate group and not in the direct line of descent of the Brachyura. Possibly, again referring to Gurney, the Brachyura are derived from a stem common to the Galatheidea and Dromiace.