on each side of it, diminishing in size as they approach its point. The currents on each of the gills are directed towards its apex, and also pass between the secondary laminx over their surface and along their edges : a, a, are the gills ; b one of the gills magnified, showing its laminT ; c the same viewed endwise. The arrows mark the direction of the currents.
e. Pectinibranchiata.— The common Buc cinum (fig. 304) may serve as an example of in one of thern, the D. cornuta, the ciliary motion was very strong on the club-shaped feelers ; perhaps it may be the same in all. I also examined the Tritonia and Eolis belong ing to this order, and found the ciliary motion in corresponding parts.
b. Cyclobranchiata.— In the Patella or Lirnpet (fig. 302, representing the under surface), the gills form a series of simple laminT (a, a) attached within the circular border of the mantle (b, b). The currents pass inwards from, the edge of the mantle to the gills, then over the surface and along the border of each branchial lamina, from its outer or lower to its inner or upper edge, as indicated in the figure by the arrows. In the Limpet the ciliary motion is also found on the inner surface of the alimentary canal.
In the Chiton or Oscabrion (fig. 303), the only other genus of this order, the gills are situated as in the Limpet, but are of a more complex structure. Each consists (at least in the species examined by me) of a triangular lamina, with a series of smaller lamin set this order. The gills, as accurately described by Cuvier, are attached to the roof of a bran chial cavity or recess formed between the man tle (a, a) and upper part of the body (b) in the last turn of the shell, and opening anteriorly by a broad slit. At the left end of the slit the edge of the mantle is prolonged in the form of a groove (c), which prolongation is called the syphon, and is lodged in a corresponding groove of the shell. On detaching the roof of the branchial cavity at the left side, and reflecting it (as represented in the figure), we find attached to it, first, the gills, consisting of a short double row (d) and a longer single row (e) of laminm, the latter being larger ; secondly, to the right of the gills, the so-named mucous lamin (for); thirdly, still more to the right, the rectum (g).
The water enters 'by the syphon, and issues at the right extremity of the branchial slit. The ciliary motion and currents take place on the gills, mucous laminw, and rectum, and ori the inner surface of the mantle, where it forms the roof of the branchial cavity. Their situ
ation and direction are indicated in the figures by the arrows. B is an enlarged view of a few laminw from the larger series, h the at tached border, i point, 271 left, and n right border. Currents pass between these laminw along the surface and border of each, as shewn in B; C is a magnified view of the laminm of the smaller set, on which the di rection of the currents is 'narked ; the direc tion on other parts will be understood by re ferring to figure A.
The ciliary motion is very manifest within the alimentary canal, in the gullet, stomach, and intestine; the direction of impulsion is from the mouth towards the anus.
The ciliary motion has been observed by myself and others in the Paludina vivipara, a fresh-water snail belonging to this order, in which also Purkinje and Valentin state that they observed it within the alimentary canal; and Gruithuisere has described the phenomenon as seen on the branchi2e of another fresh-water snail, which he names Valvata branchiata. He saw moving cilia, which caused an incessant agitation in the water; but he does not state whether the motion followed any constant direc tion, although we may infer that this was the case. He rightly attributed to these motions a respiratory function, but seems not to have observed that similar pheno mena existed in other Mollusca.
d. Pulmonifera. The ciliary motion is not confined to those Mollusca which breathe by gills, for it occurs also in the Lymnma and Planorbis, which, though they live in water, breathe air by a pul - . .
monary sac. in these instances the impulsion ot the water takes place on the surface of the teritacula, which is covered with cilia. If these parts are to be regarded as organs of sensation alone, the ciliary motion observed upon them, as well as that which occurs on the tentacula of bran chiferous species, must be considered as connected with the function of sensation; but the tentacula, which in the Lymna are broad vascular laminw, might be conceived also to perform the office of accessory organs of respiration, in which case the pulmoniferous Mollusca here mentioned would possess organs both of aerial and aquatic respiration. In the Lymnwa the motion has also been observed by Purkinje and Valentin within the alimentary canal.