I first observed the ciliary motions in the Aphrodita aculeata in 1830, at the same time with the late Mr. Cheek, who gave notice of the fact in the journal of which he was conductor ;t but most of the observations on which the preceding account is founded, were made more recently. There is no mention of the existence of the phenomenon in the Aphro dita to be found in systematic works on com parative anatomy, nor in any of the special memoirs on that animal which I have had an opportunity of consulting.
The ciliary motion exists in several other animals belonging to the class Annelida. It is remarkably distinct, and easily observed, on the branchia2 or gills of the Serpula. These organs consist of two bunches of pinnated or feather-like processes,_which the animal pushes forth from tbe calcareous tube in which it lives, and spreads out in a radiating form. The edges of the branchim, both of the stems and of the leaflets, are fringed with cilia, which exhibit their vibrating and undulating motions, and cause a constant current of water over the surface of the gills, serving here, no doubt, as in analogous instances, at least chiefly for respiration.
In a paper already referred to,* I mentioned having observed the phenomena in question in the Amphitrite. The animal meant was a com mon marine tubicolar worm (fig. 300), which appeared to be the same with that figured by Ellis (Corall. plate 36), and described by Cuvier as the Amphitrite a ruche, with which figure it agrees, except that it bears two rows of simple filaments on the back, which, for reasons that -vvill appear, I was led to regard as gills. But if these are really gills, the animal must, it seems, be arranged with the Dorsibranchiata, probably as a Sabella. The currents in this worm proceed forwards along the back, be tween the rows of gills (as marked in fig. B), and along the gills themselves (see C), whose _ points are directed forwards. The conical fila ment of which each gill consists is marked on one side by ridges (see C, D), crossing it obliquely like segments of a spiral; and on these ridges as well as on the point of the gill the most conspicuous cilia are placed. The cilia. are comparatively large and curved, their points being turned towards the summit of the gill, which figure they retain when their motion is stopped. The gills contain large bloodvessels, which when distended give them a bright red colour.
The ciliary motion occurs also on what seem to be the branchiee of another tubicolar worm, the name of which is unknown to me; the organs in question are placed at the anterior extremity of the animal, concealed by a pro fusion of long serpentine tentacula.
Lastly, Mr. Cheek* observed the ciliary motion in the Sandworm (Arenicola piscato rum). It was seen on the inner surface of the internal vesicles, which Sir Everard Home de scribes as livers. Nothina similar exists on the tufts of filaments which fo°rm the gills.t 8. Mollusca.— The ciliary motion prevails very extensively in this division of the animal kingdom. It seems to exist generally in the Gasteropodous and Acephalous Mollusca. There is some uncertainty as to its existence in the Cephalopoda ; I have repeatedly sought for it in that class, but without success.
It occurs on the surface of the respiratory organs, and often on other surfaces over which the water has to pass in the act of respiration. It also exists within the alimentary canal, at least this has been ascertained in several spe cies of Gasteropoda and Acephala, and may be presumed of the rest. Moreover in some of the Gasteropoda, it is very manifest on the horns or feelers, which suggests the possibility of its aiding in these instances in the exercise' of the sense of touch or smelling. In all cases the impulsion maintains a determinate direction, which continues the same in parts detached from the animal. In salt-water spe cies, the action of the cilia and impulsion of the fluid, are instantly stopped by putting the parts into fresh water.
The ciliary motion also occurs in the embryo of the Mollusca within the egg, which pheno menon will be considered in the next section.
A. Gasteropodous Mollusca.—Of this class the phenomena have been observed by myself and others in the orders of Nudibranchiata, Cyclobranchiata, Pectinibranchiata, and the aquatic Pulmonifera, in one or more species of each.
a. Nudibranchiata.—In this order, in which the gills are entirely exposed, the currents can be very easily observed. .The Doris, a species of which is represented in the adjoining figure (301), may serve as an example. The arbo rescent gills (a, a) are ranged in a circle round the anus, and their stems and branches are covered with cilia. Currents pass over their surface, the general direction being towards the points; small portions detached still ex cite currents in the same direction, and, if free, move through the water in the opposite one. I have examined three species of Doris, and LIA.