B. Conchiferous Acephda.—The motion in question has been found in several bivalve Mollusca, both of salt and fresh water, and there can be little doubt that it exists in all.
The common Sea-mussel (fig. 305) will serve as an example of the class. It will be recollected that the gills of this animal (fig. A, c, c', d,) have the form of leaves, there being two on each side inclosed between the lobes of the mantle (a, a, a', a"). Between the gills are interposed what is called the foot (f) and the prominent part of the abdomen, which separates the two of the right side from those of the left. Each gill or leaf consists of two layers, which are made up of vessels set very close to one another (fig. D,) like the teeth of a comb, or like parallel bars, across the direction of the gill, and perpendicular to the great vascular trunks running along its base, with which they communicate. The two layers composing each gill are connected together at its edge, and by a few points of their contiguous surfaces. At the base only one layer is fixed, the other ter minating at this part by a thick unattached border (e, e), under which a probe may be passed into the interior space between the two layers. This is further explained by fig. B, which represents a section of the two gills of one side cut parallel to the bars. The layers (e c, f c,) are united at the edge of the gill (c), but separated at the base, the one being fixed at f; the other ending by a free margin, e. g, g, is the space between the layers ; it com municates with the excretory orifice (h, fig. A). Fig. C shews the upper part of the gill, (c, h, fig. B,) viewed similarly, but magnified eighteen diameters. Two bars, (e c, f c,) be longing to opposite layers, are seen ; they are shaped somewhat like the blade of a knife, ,with a thick round external border (e), and a thin internal edge (h) opposed to the corres ponding one of the other layer, with which it is connected at a few places by cross slips, i, fig. C, and k, k, fig. B, where they are longer, the space at this part being wider.
D is a small portion of one of the layers, (t, t, fig. A,) magnified eighteen diameters. The bars are connected laterally with the adja cent ones of the same layer at short intervals, by round projections on their sides, (a, a, a, a, in figs. D, C, and E,) in which last they are still more magnified. Each of these projec tions adheres but slightly to the corresponding one of the collateral bar, and its surface is covered with small filaments resembling the cilia in the other parts, only their motion is very slow. Besides the gills, the mussel has four triangular laminw (m, m, n, fig. A,) placed round the mouth, which probably serve for respiration ; they have been named labial ap pendages, tentacula, or accessory gills.
When a live mussel is placed in a vessel of salt water, it is soon observed to open slightly the two valves of its shell, and at the same time a commotion is evident in the water in its vicinity. This is occasioned by the water en
tering at the posterior or large end of the animal into the space between the lobes of the mantle in which the gills are lodged, and issuing near the same place by a separate orifice in a continued stream, as represented by the arrows, (g and h, fig. A), g being the entering and h the issuing stream. The existence of this con tinuous current is well known, but the agency by which the water is set in motion appears not to have been, at least generally, understood. It can readily be shewn that here, as in the in stances already described, the water receives its impulse from the ciliated surface of the gills and other parts over which it passes, and that it is carried along these surfaces in a determi nate direction. The whole surface of the gills and labial appendages or accessory gills, the inner surface of the cloak, and the surface of some other parts produce this effect, and the combined action of the cilia over this extensive surface gives rise to the main current which enters and issues from the animal.
On removing one of the valves, turning down the cloak, as represented at o, .and putting moistened charcoal powder on the surface of the gills, the finer part of the powder soon dis appears, having penetrated through the inter stices of the bars or vessels into the space between the two layers of the gill. On arriving there a part is often forced out again from under the border of the unattached layer at the base of the gill, but most of it is conveyed rapidly backwards between the two layers, and is carried out at the excretory orifice with the general current, its course being indicated by the dotted arrows in the figure. The coarser particles remain outside the gill, and are slowly carried to its edge, following the direction of the bars ; they then advance along the edge of the gill towards the forepart of the animal, as shewn by the entire arrows. It thus appears that the water first passes in between the lobes of the mantle to the external surface 0. the gills ; it is then forced into the space inclosed between their layers, from whence it is driven out at the excretory orifice, to which the inclosed spaces of all the gills lead. As this process continues to go on after the shell and lobe of the mantle of one side are removed, it is evident that the motion of the water must be mainly produced by the cilia of the gills, to be immediately described. By their agency the fluid is forced into the space within the gills, and this operation taking place over the whole extent of the gills, must, by its concentrated effect, give rise to a powerful issuing stream at the excretory orifice, of which the entering stream seems to be a necessary result.