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Show 978 MR. W. M. THOMSON ON A BRANCHIATE [Dec. 18, supporting the mesial regions of the elytra, a continuous stream of water may, nevertheless, be passed over the back, thus producing the same result as the rhythmical movements observed in Aphrodita. This is interesting as showing how the same need has been met in animals of different structure by adaptations of different kinds. The apparatus in this species seems to have attained a considerable degree of perfection, and in the literature at our disposal I can find no reference to any similar arrangement. In this dorsal view further points of interest may be noted. W h e n the elytra were removed in a female, a pale pinkish substance was observed, especially above the bases of the notopodial bunches of hair. This, when examined uuder the microscope, was found to consist of numerous ova which did not seem to have been fertilized ; for neither were polar bodies to be seen, nor were any eggs segmented, while they all presented the large vesicular nucleus characteristic of the unfertilized ovum. It would seem, therefore, that the ova are all passed up under the elytra (the female contained a great number of eggs in the coelom), and are there fertilized by sperms carried in by the respiratory current. According to a statement made by Haswell, development must also take place there to a certain extent. The elytriferous segments are 2, 4, 5, 7 & c , 21-23, the elytro-phore being a special part of the parapodial ridge. The alternate parapodia carry dorsal cirri with bulbous tips, long enough to protrude freely (as much as 3 mm.) beyond the edges of the elytra. The base of tbe cirrus is extremely muscular (Plate L X . fig. 3 and Plate L X I . fig. 7), and they are evidently capable of considerable movement. The dorsal cirri of the first, i. e. the peristomial segment, are without this muscular base and resemble the prostomial tentacles in appearance. One more point to be noted in this view of the dorsal surface is the anus, which lies just beyond the end of the dorsal respiratory channel, separated from it by the transverse ridge already referred to. The two ventral pygidial cirri represent tbe ventral cirri of the pygidium, and are directed backwards, as are also the dorsal cirri of three pairs of parapodia immediately anterior to the pygidium. Thus four pairs of these tactile organs may be seen curling up round the edge of the elytra at the extreme posterior end. It seems rather remarkable that there should be such a good supply of " feelers " at this end, and indicates that it is the habit of the animal to explore crannies and holes among stones &c, from which it must often make its exit backwards. The Head.-The head consists of prostomium and peristomium (the first segment), the whole being retractile to a slight degree under the next segment, which bears the first pair of elytra (Plate LXI. fig. 4). The prostomium is irregularly oval, convex above, well defined posteriorly. Its base is, in the preserved specimens, overhung by the first "dorsal tubercle," which belongs to the peristomial segment. There are two pairs of eyes-a posterior larger, and an anterior smaller eye on each side,-which are so |