In the inside of the body the membranous stomach (g) occupies the middle part, and from it a pair of lobed cceca (h,h,) (and i, i, cut short) pass into each ray. Within the rays also we find inferiorly the rovvs of vesicles (k, k) which form part of the feet (/, /), and the ovaries. All the rays communicate through the middle part, and the whole inside is lined by a transparent membrane (n, n), which, like a sort of peritoneum, covers the stomach and cceca, attaches each of the cceca by a me sentery (o, o) to the roof of the ray, lines the fibrous patietes of the body, and is probably reflected over the vesicles of the feet and the ovaries. Each mesentery encloses a space (o, fig. B) between its sides, vvhich opens inth the general cavity at the root of the cceca. The lining membrane passes into the perforated pits (e), by which the tubes (c) communicate with the cavity, and sends prolongations through the perforations -intO the tubes lining them to their points. The space (s, s, fig. B) lined by this membrane contains sea-water, which is generally described as entering and issuing by the respiratory tubes.* I find the ciliary motion in four situations, namely, 1. on the external surface; 2. within the cavity of' the body, or in the space (s) between its parietes and the viscera ; 3. within the stomach and cceca ; 4. within the' feet. In all these situations moving cilia are visible with the microscope on the respective surfaces; they are every where comparatively small, in some parts excessively so. Though I have not traced them over the entire extent of each sur face, I have no doubt they exist at every point where currents are produced.
1. On the external surface. The ciliary mo tion as indicated by the application of pow dered charcoal, occurs over nearly its entire extent, but with different degrees of intensity. The strongest currents pass along the outer surface of the tubes from the base to the point, as at c' ; they are also pretty strong on the claw-like processes (b' ) and intermediate skin; on the feet they are evident but less vigorous.
2. Within the body the currents take place on the lining membrane and its reflections. A longitudinal current runs along the roof, and another along the floor of each ray, forwards or towards its point : (see the arrows infig. A.) These advancing currents are .confined to the median line and its immediate vicinity; two retiring currents (r, r,) run backwards (one on each side) at the place where the sides join the floor of the ray. Two longitudinal currents also exist on each of the cceca, an advancing one (h' ) on the inferior surface, and a retiring one superiorly (h, h, fig. A) in the space (o,fig. B) inclosed within the mesentery, w! ich, as already mentioned, opens into the general cavity. The longitudinal currents, except those within the mesentery, are, if for the sake of explanation we may so express it, connected by others which run vertically and transversely on the cceca and on the roof and sides of the cavity, (see the arrows in fig. B ;) on the vesicles of the
feet the course of these cross currents is varied by.the curved surraces. As the lining membrane of the cavity extends into the respiratory tubes, so currents exist within these likewise, as at t, fig. C. This is proved by injecting turbid fluid into the ray, when particles are seen moving within the tubes; and if a few of the tubes with a portion of the skin be cut off and placed under the microscope, the fluid which will still be retained by some of them may be seen to be in motion, the floating particles moving from the base to the point and back acrain as in the arms of the Actinim.
3., The Motion is very distinct on the inner surface of the stomach and cceca; the currents within the cceca follow the same direction as on their external surface, that is, an advancing current runs inferiorly from the root to the point and a returning one superiorly ;. and at the sides currents run upwards, following the ridges or folds of the internal membrane which result from the lobulated structure of the mca.
4. The ciliary motion exists distinctly within the feet, though the cilia are very small; these became visible on viewing the edge of a folded portion with Wollaston's doublet of one-thirty fifth of an inch focus.
The currents described, as far as I have been able to perceive, preserve always the same determinate direction. Even when portions of the ciliated surface are detached, the motion on them continues, and its direction is the same as before their separation.
As to the use of these motions, it is most probably connected chiefly with respiration ; and if such be the case, it would show that in this animal a great extent and variety of parts are concerned in that function. The ciliary motion on the inner surface of the stomach and cceca is probably subservient also to the process of digestion. It is conceivable that by means of this provision the dissolved or digested food might be introduced into the cceca, and spread over their internal surface, there to be duly mixed with secreted fluids ' and subjected to the process of absorption ; the returning cur rent serving to bring back the residue or to. convey secreted fluids into the stomach'. Or, considered as subservient to respiration the cili ary motion, in diffusing the digested fo' od over the internal surface of the cceca, may at the same time expose it to the respiratory influence of the water on their outside.
These phenomena in the Asterias seem not to have been previously noticed. Tiedemann,'' it is true, had observed an eddying motion of the water in the vicinity of the respiratory tubes while the animal was slowly distending or emptying itself, but he conceived it to be nothing more than the commotion necessarily produced by the passage of the water through the tubes. There can be little doubt that the phenomenon he saw was caused by the ciliary motion on the external surface, though he was not aware of this.