Tadpoles of the frog (which, of course, are only obtainable in their season) are good subjects for showing the circulation of the blood. They are best suited for the microscope when about one inch long. The tadpoles of the newt and toad also are equally suitable. They may be placed in a very shallow glass trough with a little water, and a narrow band of linen bound lightly round in some part not required for examination, to keep them from moving; or they may be laid upon a glass plate with a drop or two of water, and a thin glass covering lightly bound upon it. Dr. Carpenter, however, places them first in cold water, gradually adding warm until the whole becomes about 100°, when the tadpole becomes rigid, whilst the circulation is still maintained. I have not, however, found this necessary, the thin glass accomplishing all that is desired. The tail is generally the most transparent, and shows the circulation best; but in some of the newt larvæ the blood may be traced down to the very extremities if they are not _too old_. Mr. Whitney places the tadpole upon its back, by which means the heart and other internal arrangements may be seen. Amongst fishes also may be found subjects for the same purpose, but they seldom furnish as good examples as those before mentioned, because the blood-vessels are not nearly so abundant, as in the foot of the frog, &c. The stickleback is, however, procurable almost in any place during the summer months, and may be laid in a shallow trough, loosely bound down as the tadpole. The tail may be covered with a piece of thin glass to prevent him curling it to the object-glass. The power needed with this will be about the same as with the other subjects--viz., half to quarter inch object-glass. It is not absolutely necessary to go to reptiles or fishes for this curious sight, as some other animals serve very well. In the wings of the common bat may be found a good subject. These must be stretched out on something resembling the frog-plate before described, when those parts near to the bones will show the _largest_ vessels very clearly. The ear of a young mouse is an illustration of the same phenomenon, but it is very difficult to fix it in a good position, as these animals are so very timid and restless. Amongst insects also the same law may be observed, by placing them in the “cage,” or “live-box,” so as to keep them still, but not to injure them by too much pressure. In certain larvæ this is particularly well shown, as that of the day-fly and plumed gnat; but in some of these the blood is almost colourless. In the wings also of many insects this circulation is well seen, as in those of the common housefly; but as these parts become dry in a few days, the subject should not be more than twenty-four hours old.