There is a peculiarity in the form of the cilia in some animals, of which the Beroe and other Ciliograde NIedusm afford a gootl example. In these, in place of cilia of the usual form and arrangement, there are rows of broad flattened organs, each of which is made up of several simple filaments joined together by a common base, according to Eschscholz, or according to Dr. Grant by a connecting membrane in their whole length. The entire oraan is raised or depressed at once, so that thefilaments are all moved simultaneously, like the eye-lashes. The compound cilia in some of the Rotatoria, de scribed by Ehrenberg, are probably of the same nature.
5. Of the wearance of the cilia in motion. —On examining these organs with a lens of 23 inch focus, when their motion is not very rapid, the manner in which the individual cilia move may be distinguished with tolerable cer tainty. iVlost commonly they have a fanning or lashing motion, that is, the cilium is bent in one direction and returns again to its original state. The flexion takes place chiefly at the base or root, but not wholly there, for the rest of the organ is obviously bent and altered in' figure ; nay, the more elastic cilia, when their motion abates in intensity, appear some times to bend only near the point, the base and adjoining part remaining motionless.
When a number of cilia are affected in suc cession with this motion, the appearance of a progressive wave is produced, and as in such a case they are again and again moved in the same way at very short intervals, successive waves proceed along them in the same direc tion, which might be compared to those pro duced by the wind in a corn-field. Such at least seems to be tile true explanation of the Undulatory motion which so often occurs, although it must be confessed that the motion of the cilia individually cannot be distinctly seen when the undulation is most perfect. The Undulations succeed one another along a range bf cilia with great regularity, and except in the Rotifera, and perhaps some other Infusoria, they seem always to maintain the same direc tion in the same parts.
• Purkinje and Valentin describe the motion of the individual cilia as being more frequently rotatory, or, as they term it, infundibuliforrn ; and Ehrenberg states this to be the common Mode in the Infusoria; the cilium describing a circle with its point, while the base is the centre of' motion. From my own observation, how
ever, I would. be inclined to infer that this motion is by no means the most common.
6. Duration rif the ciliary motion ofter death and in separated parts.—The continuance of the ciliary motion for some time after death, and the perfect re?-ularity with which it goes on in parts separateifrom the rest of the body, are facts which have been already repeatedly stated, and sufficiently prove that the motion is quite independent of the will of the animal, and also that it is not immediately' influenced by the circulation of the blood, even in the respiratory prgans.
The time _which it continues after death differs in differe'nt species of animals, and also, but in a much smaller degree, in different parts of the same aniinal. Its duration is influenced also by the temperature of the air, and by the nature of the fluid in contact with the surface. In Mammaliaand Birds the period varies from half an hour to four hours, being longer in summer than in winter ; but it is still further prolonged when the parts are covered with blood. In the gills of Batrachian larvw I have seen the motion continue six hours ; but of all vertebrated animals it is most enduring in the Tortoise, in which animal Purkinje and Valen tin affirm they observed it fifteen days after death, when putrefaction was far advanced ; the irritability of the muscles remained in the same animal for seven days. Among the in vertebrata .the River-mussel affords an instance of the great pertinacity of the motion, which ceases only when putrefaction has advanced so far as actually to destroy and dissolve the tissues.
7. Effects of external agents on the ciliary motion.— Steinbuch, Purkinje, and Valentin allege that on touching the parts, or giving them a gentle shock by merely striking against the object plate of the microscope, the motion is rendered. brisker when it has become languid, or is even renewed in parts where it has ceased. They, however, attribute more importance to this fact than it seems to deserve ; for it may be doubted whether the concussion in renewing the vivacity of the cilia does not act merely by removing obstacles which impede their play.