The size of the creature, whose name we may as well mention was _Pterodina patina_, rendered this practicable, but required some care. The longest diameter of the body, which was not quite round, was about 1--120", so that it was visible to the naked eye, and as a good many were swimming together, one could be captured without much difficulty, and transferred with a very small drop of water to the live-box. Then the cover had to be put on so as to squeeze the animal just enough to keep it still without doing it any damage, or completely stopping its motions. This was a troublesome task, and often a little overpressure prevented its success. Some observers always use in these cases an instrument called a _compressorium_, by which the amount of pressure is regulated by a lever or a fine screw; but whether the student possess one or not, he should learn to accomplish the same result by dexterously manipulating a well-made live-box. We will suppose the _Pterodina_ successfully caged, and a power of about one hundred and fifty linear brought to bear upon her, for our specimen is of the "female persuasion." This will suffice to demonstrate the disposition and relation of the several parts, after which one of from four hundred to five hundred linear may be used with great advantage, though in this case the illumination must be carefully adjusted, and its intensity and obliquity frequently changed, until the best effect is obtained. We find, on thus viewing the Pterodina, that it is a complex, highly organized creature, having its body protected by a _carapace_, like the shell of a tortoise, but as flexible as a sheet of white gelatine paper, which it resembles in appearance. Round the margin of this carapace are a number of little bosses or dots, which vary in different individuals. The cilia are not disposed, as at first appeared, in two separate and distinct disks, but are continuous, as in the annexed sketch. Down each side are two long muscular bands, distinctly _striated_, and when they contract, the ciliary apparatus is drawn in. As this contraction takes place, two apparently elastic bands, to which the ciliary lobes are attached, are bent downwards, till they look like the C springs behind a gentleman's carriage; and they regain their former position of slight curvature, when the cilia are again thrust out. [Illustration: Pterodina patina--tail-foot.]