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Show 780 PROF. T. H. HUXLEY ON THE CLASSIFICATION [June 4, I am inclined to suspect the existence of a rudimentary anterior arthrobranchia in VII.; but I could not make sure of it. By the structure of its branchia? Stenopus is sharply separated from Peneeus, with which it has hitherto been associated, although it approaches Peneeus in the almost complete abortion of the branchial element of the podobranchiae. In the Penaeida?, in fact, each branchia consists of a stem which is pointed at both ends and gives off two series of opposite lateral branches. This stem is attached by a pedicle near its lower end. Near each end of the stem the lateral branches are very short, and stand straight out ; but they rapidly become longer; and as they do so they curve outwards towards one another, and eventually meet in the middle line. The middle of the branchia consequently assumes the form of a hollow cylinder. The outer face of each lateral branch gives off a close-set series of secondary branches, which diminish in size towards the free end of the lateral branch, and at the free end are simple undivided filaments. But towards the attached end of the branch the secondary branches are themselves dichotomously subdivided in the direction of their length ; so that the most complicated of these secondary branches presents a short stem whence two branches proceed, each of these again gives off two, and these may terminate in yet other two. Hence each secondary branch is like a flat triangular plate slit by fissures of vaning depth?, and attached by its apex to a lateral branch. All these secondary branches are directed upwards and outwards. A detached lateral branch closely resembles one of the branchiae of Thysanopoda ; while, so far as I can judge from the figures given by Kroyer1 and Sars2, the branchia? of Sergestes and Lophogaster still more closely approach those of Peneeus. A similar structure was described by Duvernoy in Aristeeus, and was supposed by him to be characteristic of that genus; but Dana has already justly expressed a doubt whether, in this respect, Aristeeus differs from the other Penaeida?. In Peneeus, the last and the penultimate thoracic limbs present no trace of podobranchia?; and in all the rest the podobranchia is reduced to a small epipodite, which, in the middle of the series, is bifurcated at its free end. The latter lies between the arthrobranchia? of its own somite and the next following. There are seven pleurobranchiae, of which the hindermost is the largest, while the most anterior is very small. 1 Forsog til en monographisk Fremstilling af Kraebsdyrslagten Sergestes: 1855. a Beskrivelse over Lophogaster typicus: 1862. |