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 (a, a, 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 verticall' direction and back again. The motion of the other set consists in a succession of undulations, which proceed in a uniform manner along the sides of the bar from one end to the other. It might be very easily mistaken for the circulation of globules of a fluid within a canal, more especially as the course of the undulations is different on the two sides of the bar, being directed on one side towards the edge of the gill, a.nd on the other towards the base. But besides that the undulations continue for some time in small pieces cut off from the gill, which is incon sistent with the progression of fluid in a canal, the cilia are easily distinguished when the un dulatory motion becomes languid. When it has entirely ceased, they remain in contact with each other, so as to present the appearance of a membrane, (d, d, fig. F.) Besides the two rows of cilia just described on each side of the bars, others are placed in a less regular manner on their external and internal borders. The in ternal (h, fig. C) are exceedingly small ; they extend upon the cross slips, (i,fig. C). Those on the external borders are very numerous and thick-set, and of considerable size, especially on the extremity of the bar at the edge of the gill (c, fig. C); their points are directed to wards the edge of the gill. It is probably by the agency of these last-mentioned cilia that the particles of food or other foreign matter are conveyed along the surface of the gill to its edge, and then onwards to the mouth, while the others may serve principally to force the water through the interstices of the bars into the space inclosed between the layers, and from thence out at the excretory orifice.
As in other instances, detached portions of the ciliated parts excite currents in the same direction as before their separation, or swim through the water in the opposite direction. It is very remarkable that when the parts are immersed in fresh water, the currents and mo tion of the cilia are almost instantaneously stopped.
The ciliary motion is equally apparent on the respiratory organs of the Oyster, River-mus sel, and other bivalve Mollusca which have been submitted to examination. Purkinje and Valentin pointed out its existence also in the alimentary canal of the River-mussel, which observation I have confirmed, and I have found the same to be true of the Sea-mussel. The impulsion appeared to me in both instances to be chiefly directed onwards, that is, towards the anus.
c. Tunicata (Ascidiee ).—In the paper pre viously referred to, I stated that I had not.been able to perceive the ciliary motion in the Ascidia, but added that the observation seemed inconclu sive, as the specimens examined had been some time out of the water. Since then I have seen the phenomena as distinctly in the Ascidim as in other Mollusca. The observations were made on a common species found adhering to rocks in the Frith of Forth at low water-mark, and as far as they go they agree with those lately made by Mr. Lister,* on a small aggregated species, the substance of which being nearly transparent enabled him to trace the currents more completely. For this reason it seems preferable to borrow his description.
The annexed figures (A and B) represent one of these Ascidim on its peduncle, with the opening of the mouth (g) and the funnel (f) in front. The outer covering is a tough coat (a), lined internally with a soft sub stance or mantle (b). A great part of the interior is occupied with the branchial sac (c), whose cavity terminates upwards at the oral opening, and is closed at the 'bottom.
tt is united to tne lope or to the mantle above and behind ; the juncture (e, e,) beginning,- in front of the oral ing, extends backwards on each side of it, and then downwards along the middle of the back (a', fig. A.) A vacant space (f;f;) is left between the sac and mantle at the sides and front, which ends in the opening of the funnel. The sac opens infeL riorly into the cesophagus (h), which leads to the stomach (i), the intestine passing forwards and opening by the vent (k) into the funnel. On its sides and front the branchial sac is forated by four rows of narrow vertical slits or spiracles (m, m), and through these the water, which flows constantly in at the mouth when its orifice is open, appears to be conveyed to the vacant space (f) between the sac and mantle, and it then escapes at the funnel. The sac seems extremely thin between the spiracles, but their edges are thickened, and they are lined with closely set cilia, which, by their motion, cause the current of water. When they are in full activity, the effect upon the eye is that of delicately toothed oval wheels, volving continually in a direction ascending on tile right and descending on the left of each oval, as viewed from without ; but the cilia them selves are very much closer than the apparent teeth, and the illusion seems to be caused by a fanning motion given to them in regular and quick succession, which will produce the ap pearance of waves, and each wave answers here to a tooth.
Whatever little substances alive or inanimate the current of water brings, if not ejected as unsuitable, lodge somewhere on the surface of the branchial sac, along which each particle travels horizontally with a steady slow course to the front of the cavity, where it reacheS a downward stream of similar materials (h') ; and they proceed together, receiving accessions from both sides, and enter at last, at the bottom, the cesophagus (h); this is a small flattened tube which carries them, without any effort of swallowing, towards the stomach.