OCR Text |
Show 1867.] DR. J. S. BOWERBANK ON HYALONEMA MIRABILE. 23 appear to touch each other, but are separately enveloped by keratode in the manner that is so prevalent in the genus Dysidea, Johnston. The inner layer has few such adventitious matters imbedded in it: but in place of such material there are numerous cylindro-cruciform and other siliceous spicula dispersed throughout its whole length. From this thick coriaceous dermis the oscula are projected abundantly ; they are dispersed over its surface without any appearance of order. In some specimens they are nearly uniform in size, seldom exceeding about a line in height, while in others they vary iu that respect to a very considerable extent. In one specimen in m y possession a few only are as short as a line, while others vary from 6 lines in height to scarcely an elevation of the apex of the organ above the dermis of the cloaca. The apical terminations of these organs also vary considerably ; they are more frequently slightly oval than circular, and in many instances they are quite as much oval as those figured by M . Barboza du Bocage from his //. lusitanicum, described in the Society's 'Proceedings' for 1864, p. 264. I cut off the corrugated apical portion of several of these oscular bodies and mounted them in Canada balsam : the outer surface in most of them was so thickly studded with closely adhering grains of sand that no part of the dermal surface could be distinctly seen; but in some the central orifice was partly open, and the radiating structure was more than usually distinct. In these specimens it was apparent that the radiating ridges within the outer surface do not extend from the circumference to the centre, but only to the outer margin of a central circular membrane with concentric lines of minute corrugations. These structures, therefore, have every character of contractile organs, supplying the place of muscles, so as to enable the animal to open and close the oscular orifice at its pleasure. Within the outer portion of the apex of the osculum, at about the distance of one-third or one-fourth of its diameter, there is situated a second membranous diaphragm, of much less complicated structure than the outer one. This also was not entirely closed ; the inner margin of this membrane also exhibited a series of numerous concentric corrugations, forming a flat circular band around the orifice, from the outer margin of which lines of thickened membrane radiated towards the outer margin of the organ; and they gradually expanded laterally, uniting and forming the extreme circumference of the perforated diaphragm, thus exhibiting a series of contractile membranes for the opening and closing of the inner diaphragm in a similar manner to that of the outer one. The radial lines of the inner diaphragm do not correspond with those of the outer one, and they are not so numerous. The apical and the inner diaphragms are connected by a circular series of dissepimental membranes, the planes of which are at right angles to the upper and lower diaphragms ; so that the internal aspect of this complicated valvular structure bears no very distant resemblance to the dissepimental structures of many seed-vessels of plants, supposing sections at right angles to their axes to have been made. Sections of this valvular structure in its natural condition are represented in Pl. IV., |