I soon found that the gill was not the only part of the animal which excited motion in the water. Nearly the whole surface of the body produced the same effect. A general current commenced on the fore part of the head, pro ceeded along the back and belly and the two sides, to the tail, along which it continued to its extremity. It was not so strong as that on the gills, but agreed with it in other respects.
I continued for some time to observe the phenomenon in the larva of the Froo. in order ,D) to find out whether it underwent any alteration in the progress of the developement of that animal. It is known that after a time the ex ternal gills become covered by a fold of the skin, and inclosed in the same cavity with the internal gills, when they gradually shrink and at last disappear. On examining the animal while this change was taking place, and for some time after, it appeared that the external gills after their inclosure still retained their peculiar property, and continued to do so as long as any portion of them remained ; the current on the body remained the same; on the tail it acquired a twofold direction diverging from the middle part or continuation of the vertebral column, obliquely upwards and down wards towards the upper and lower edge. As the animal advanced in growth, the currents gradually disappeared over the greater part of the surface, continuing longest at the posterior part of the body; at length, when the pos terior extremities were so far advanced in growth that the thigh, leg, and toes could be discerned with a magnifying glass, which was the latest period of observation, the current existed only at the commencement of the tail, and on a small part of the body near the hind leg. The internal gills, though tried in various stages of development, did not exhibit the phenomenon.
I next sought for the same appearances in the larva of the Newt or Water Salamander, .which was first examined a few days after its exclusion from the egg when its oills are very simple. At this period the serface of the animal produces currents agreeing in almost every circumstance with those which take place in the larva of the frog at a correspond ing stage of its development. Particles of powder diffused in the water are carried along the surface of the body from before back wards; on the gills they are conveyed along each of the trunks from the root to the ex tremity. The gills also, when cut off, move
through the water with the cut extremity for wards, in a direction contrary to the currents. I have since found nearly the same phenomena in the gills at a much later period.
It was evident that the purpose of these currents was to effect a renewal of the water on the respiratory surfaces; respiration in these animals probably being performed not only by means of the gills, but also by the general sur face of the body.
It appeared that the power of impelling the water was wholly confined to the external sur face of the animal; a portion of the skin being raised and detached, floating bodies were moved along its external surface only. Parts cut off from the animal continued to excite currents for several hours after their separation, and the smallest portion produced that effect. In these cases the current always moved in the same direction relatively to the surface of the detached parts, as it had done previous to their separation.
At the time of making these observations I had not been able to detect Cilia in these larvw, although, from the analogy of the In vertebrata, I was led carefully to look for them. Since then I have succeeded in perceiving them with the aid of Wollaston's doublet of one-thirtyfifth of an inch focus, especially when a portion of the gill is compressed under a plate of' mica. They are to be distinguished chiefly by their waving motion, which is so charac teristic as to remove all doubt of their ex istence ; though here, as in other instances in which they are very minute it is not always possible to demonstrate ;heir existence by actual observation on every spot of the sur face.
Ova of the Batrachia.—In the course of the above-mentioned observations, I was led to enquire whether the phenomena in question appeared at a still earlier stage. With this view I examined the ova of the Newt, which for a considerable time may be procured in all degrees of advancement, and found that the ciliary motion presented itself in the embryo a considerable time before its exclusion from the egg. Since then I have observed the same with regard to the embryo of the Frog.