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Show 28 MR. R. GURXEV OX THE [May 12, are six-jointed, one might assume that a seventh is fused with the I »< .ily-wall, in which case the gills might be interpreted as arthro-branchs or podobranchs at will. Still, in the common acceptance of the term, they certainly seem to m e to be properly designated as pleurobranchs. 1 have been unable to detect at this stage the small arthro-branch found in the last larva and in the adult on the third maxillipedes. The first postlarval stage differs from the adult in that there is but a single median tooth on the dorsal surface of the carapace, instead of three teeth transversely arranged. For this reason the young shrimp at this period is hardly distinguishable from C. nanus ; but the latter is somewhat smaller, and the single median tooth of the carapace seems to rise from a short median ridge. Sars figures a small posterior tooth in this ridge, corresponding to the second tooth of the adult, but I have been unable to detect it in m y specimens. The possession of peculiarly distinct larval forms with sudden transition to a uniform postlarval type is one of the most remarkable features in crangonid metamorphosis, and the two forms described are no exception to the rule. Conclusions. Our knowledge of Decapod metamorphosis is for the most part very fragmentary, and for this reason it has been impossible to use the larval stages as evidence of relationship. W e have, however, an exceptionally full knowledge of the metamorphosis of the Crangonida?; and it will be interesting to see if a comparison of the larva? will throw light on the classification of the group. The eleven species of Crangonida? which have been recorded as British have been divided up among five genera :-Crangon Fabr., Cheraphilus Kinahan, sEgeon Risso, Pontophilus Leach, and Sabinea Owen; though Ortmann in his revision of the family joins Cheraphilus, uEgeon, and Pontophilus into one genus Pontophilus. The genus Sabinea may here be left out of account. Looking, then, at the larva? of the species that have been described, we find them falling, as it seems to me, naturally into three groups:- 1. C. vulgaris and C. alhnanni : characterised by a one-jointed maxilla-palp and the absence of an exopodite on the second leg in the Mysis-stage. 2. G. trispinosus, C. nanus, C. echinulatus, and C. fasciatus : characterised by their two-jointed maxilla-palp, possession of five pairs of exopodites in the Mysis-stage, form of the rostrum, and arrangement of the abdominal spines. 3. C. spinosits and C. norvegicus : distinguished from the second group by their extremely elongated body form, shape of the rostrum, possession of a single long median spine on the third abdominal segment, and by the form of the tail-plate. |