622 



CILIA. 



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 (Jig. 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,fc,) are united at the edge of the gill (c), 

 but separated at the base, the one being fixed 

 at J\ 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, 

 ( f > h, fig. B,) viewed similarly, but magnified 

 eighteen diameters. Two bars, (e c, f f,) 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 (A) opposed to the corres- 

 ponding one of the other layer, with which it 

 is connected at a few places by cross slips, 

 i, i, Jig. C, and k, k, fig. B, where they are 

 longer, the space at this part being wider. 

 Fig. I) 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 laminae (m, m, n, Jig. 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 A, fig. A), g being the entering and A 

 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 o. 

 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. 



The cilia are found on the gills, the acces- 

 sory gills, the inside of the mantle, and the 

 foot. Only those on the gills require particular 

 notice. Most of them are arranged along the 

 sides of the vessels or bars (, , fig. F), com- 

 posing the gills, in two sets, one nearer the 

 surface consisting of longer and more opaque 

 cilia, (b, b,) the other close to the first, but a 

 little deeper, and consisting of somewhat shorter 

 and nearly transparent cilia, (c, c.) Both sets 

 are in constant motion, but of this it is difficult 

 to convey a correct idea by description. The 

 more opaque cilia, or those of the exterior 

 range, appear and disappear by turns, as if they 

 were continually changing from a horizontal to 

 a vertical* direction and back again. The 



* By vertical is here meant a direction perpendi- 

 cular to the plane of the gills, which direction is 

 vertical when the gills are spread out under the 

 microscope. 



