1867.] DR. J. S. BOWERBANK ON HYALONEMA MTRABILE. 23 



apjiear to touch each other, but are separately enveloped by keratode 

 in the manner that is so prevalent in the genus Dijsidea, 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 abun- 

 dantly ; they are dispersed over its surface without any appearance 

 of order. In some specimens they are nearly uniform in size, 

 seldom exceeding about a hne in height, while in others they vary 

 in that respect to a very considerable extent. In one specimen in 

 my 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 //. lusita- 

 nicum, described in the Society's 'Proceedings' for 1864, p. 26-J. 



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 mi- 

 nute corrugations. These structures, therefore, have every character 

 of contractile organs, supplying tlie place of muscles, so as to enable 

 the animal to open and close the oscular orifice at its pleasure. 

 ^Yithin 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 struc- 

 ture 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 fiat circular band around the 

 orifice, from the outer margin of which lines of thickened mem- 

 brane radiated towards the outer margin of the organ; and they 

 gradually expanded laterally, uniting and forming the extreme cir- 

 cumference of the perforated diaphragm, thus exhibiting a series of 

 contractile membranes for the opening and closing of the inner dia- 

 phragm 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. Tiie 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 dissepi- 

 mental 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 PI. IV., 



