7. Annelida.—In proceeding to describe the ciliary motion in animals of this class, in several of which it occurs, it seems advisable to begin with the Aphrodita, as the phenomena in this animal present a remarkable analogy with those we have been considering in the Echinodermata.
A great part of the body of the Aphrodita aculeata, or Sea-mouse, (of which fig. 299, A, represents a cross section,) is occupied by the abdominal cavity, ( a, a, a.) Along the superior wall of this cavity a row of cells ( b ) is placed on each side, which below open into the abdo men, but above, or exteriorly, project on the dorsal,surface as oblong transverse eminences. Each alternate cell on the back bears a broad , membranous scale (c, c), and each of the in termediate ones a small indented process. On the back a covering of felt-like substance (d ) is stretched from side to side like a roof over the cells and scales, inclosing them in a space (e) to which the water has free access. Re turning to the abdomen, we find the nearly straight alimentary canal, its anterior third (f, B) forming the stomach, the remaining part or intestine (g, fig. A and B) being fur nished on each side with a number of long cceca (h ), whose branched extremities (i, i) are in part lodged in the before-mentioned cells. The abdomen is lined with a de licate peritoneal membrane, which also lines the cells, and is reflected over the viscera.
In the living Aphrodita the water freely enters and issues from the space (e) beneath the felty membrane, passing over the external surface of the cells and their appendages. The flow of the water in this passage is produced, as I have repeatedly observed, by the elevation and depression of the scales, and on no part of the surface over which the fluid passes is the ciliary motion to be observed. But the water also enters the cavity of the abdomen, though it is doubtful by what orifices this takes place, for my endeavours to find those de scribed by Treviranus in the alternate in tervals of the feet have never been successful. In whatever way it may happen, however, there can be no doubt of the fact that the water enters the abdomen, and consequently fills the dorsal cells and surrounds the intestine and its cceca, which last organs, according to Sir Everard Home and Treviranus, exercise a respiratory function, an opinion which derives additional' probability in considering the phe nomena of the ciliary motion to be here de scribed. The ciliary motion exists in two
situations, 1st, on the external surface of the intestine and cceca and the internal surface of the cells, which surfaces are in contact with the contained water ; 2dly, within the intes tine and cceca, or on their internal surface. The motion as usual persists for some time in de tached parts, and the direction of the currents is constant. On the intestine the currents pass from the inferior surface round the sides to the upper part (as marked by the arrows). On the cceca the direction is outwards or towards the cells, and the motion is very distinct at their extremities. The direction on the inner surface of the cells was not completely made out, but it seemed to be chiefly downwards. Nor was the direction of the impulsion satis factorily ascertained on the internal surface of the intestine and cceca, though of the ex istence of the phenomenon in that situation there could be no doubt.
From what has been stated, it appears then, first, that in the Aphrodita the water finds access to the outside of the cells, over which it is conveyed by the elevation and depression of the dorsal scales, and to the inside of the cells, over which, as well as over the external surface of the intestine and its ccecal appen dages, it is moved by the action of cilia. In both situations the motion of the fluid is -pro bably subservient to the respiratory function, and if it really be so, we must reckon the scales, the cells, the alimentary canal, and its appendages, as constituting the respiratory ap paratus. Secondly, that the ciliary motion exists also on the internal surface of the in testine and cceca, where it is likely connected both with respiration and digestion. In all this we cannot overlook the analogy which subsists between the Aphrodita and Asterias. In both the water is conveyed, though by a different mechanism, over the external surface of the body; in both it enters the cavity con taining the viscera; in both it is moved along the parietes of the cavity and surface of the viscera in a determinate direction by the agency of cilia ; and, lastly, in both the ciliary motion occurs on the internal surface of the digestive organs.