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Show 1900.] POLYNOID PROM NEW ZEALAND. 977 narrow transverse grooves, which lead inwards to a median dorsal groove or channel lying between the ridges of the right and left sides. This channel is incompletely divided into two, longitudinally, by a series of firm, truncated tubercles, which posteriorly form a single median row, but anteriorly a double row, enclosing a spindle-shaped raised area, the channel passing forwards outside the tubercles (Plate L X . fig. 3). The result is that the channel bifurcates anteriorly; but on the second segment there are again a couple of median tubercles (Plate L X . fig. 3 & Plate L X I . fig. 4), and the channel is thus carried forwards right to the base of the prostomium (Plate L X I . fig. 4). Posteriorly, the tubercles cease on segment xix, while the channel continues backwards to segment xxii. The tubercles serve to support the mesial moieties of the elytra, thus leaving a clear subelytral channel, which, from its function, may be termed the dorsal " respiratory channel." Towards the posterior end the channel becomes deeper, and is closed by the transverse union of the parapodial ridges of the last elytriferous segment. The channel thus ends just under the aperture mentioned above, formed between the last pair of elytra. Looking more closely at the parapodial ridges, a number of small processes are observable, at their outer ends and especially towards the margins of the transverse canals. These processes may be as many as twenty in number on a cirriferous segment, rather fewer on an elytriferous segment (Plate L X I . figs. 7 & 8). They are little finger-shaped evaginations of the body-wall, rarely branched, and evidently serve as gillsl. Unfortunately there were no living specimens obtainable, so that w e could not verify the following inferences, but the appearances seem to justify them :-currents of water are brought by ciliary action 2 into the transverse canals ; the water is filtered by the bunches of notopodial hairs, and passes thence over the branchiae, oxygenating the blood ; the water then passes into the dorsal median canal and finds its way out by the posterior aperture. Haswell, in his Monograph of Australian Aphroditea [2], mentions that both Williams and Quatrefages record a respiratory current in Aphrodita and Hermione, caused by the rhythmical movements of the elytra under the felty coating of the back. But he adds :-" In species in which the felt-like dorsal covering does not exist, this function would appear to be in abeyance, and in Polynoe and allied genera, so far as I have observed, the elytra remain perfectly motionless while the animal as a whole is at rest." It is suggested that, in L. giganteus, while probably no actual movement of the elytra is necessary to produce the stream, yet, by means of the "respiratory channel" and the dorsal tubercles 1 Compare Buchanan's figure 4 in illustration of Eupolyodontus cornishii, in Q. J. Micr. Sc. vol. xxxv. 2 Cilia have been observed by several naturalists on the sides of the parapodia in Polynoids; but Mr. Thomson did not examine the histological structure of the body-wall in the present annelid.-W. B. B. |
