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Show 1893.] MR. A. E. SHIPLEY ON THE GENUS SIPUNCULUS. 327 those of the Common Earthworm, and their presence gives the Sipunculus a remarkable resemblance to that animal. The rings correspond with elevations of the cutis extending over several bundles of the circular muscles which lie just under the cutis. On looking at the skin with a hand-lens it is at once seen that each ring is composed of a number of rectangular oblong areas (Plate X X V I . fig. 4) side by side. Each of these oblong areas corresponds in width with one of the bundles of longitudinal muscles which lie within the circular muscles. The mouth of Sipunculus indicus is situated in the centre of the anterior end of the body; it is slightly elongated transversely (Plate X X V I . fig. 4). Around the mouth lies a circular ring of tentacles, and between the mouth and the ring of tentacles a number (7 or 8) of ridges radiate ; probably these correspond with certain vascular spaces which supply the tentacles. Viewed from in front the mouth and tentacles are very much like those of Stephanostoma (Phascolosoma) hanseni as figured by Danielssen and Koren (4) in their monograph on the northern Gephyrea. Owing to the fact that Sipunculids usually die with their introvert inverted, the arrangement of the parts of their bead has been difficult to make out, and with the exception of the figure given by H . B. Ward (3) it does not seem to m e that this part of the body has been adequately depicted. The genus Sipunculus differs from many other unarmed Gephyrea in having a ciliated web or membrane round the mouth, which web has not been broken up into discrete and independent tentacles. The arrangement of the web is very various even amongst members of the same species. The simplest form is that presented by Sipunculus tessellatus (Plate X X V I . fig.7), in which the web cannot be said to be broken up into tentacles at all, although its free border is very irregular ; it forms a complete ring around the mouth and is not incurved at any point. The inner surface of the web bears numerous ridges and intervening depressions which are lined by ciliated cells. In the specimen depicted in Plate X X V I . fig. 7, some of these ridges were especially marked and continued some way into the mouth. In Sipunculus indicus the membrane has been much more broken up into tentacles, which appear more or less aggregated into tufts, and there seems to be a certain relation between the tufts and the conspicuous radial ridges which run toward the mouth. In two of the four specimens which I received from Zanzibar the head was extended : one of them is depicted in Plate X X V I . fig. 4, and it will be noticed in this one that the tentacles are rather thicker on one side, the dorsal, than on the other; in the second specimen with extended head the tentacles were even more concentrated in this region, and showed a tendency to be incurved, so that the outline of their base took the form of a double horse-shoe. In Sipunculus nudus the membrane has not broken up into tentacles, but remains as a web with a ciliated internal surface and |