CHAPTER III.
FEBRUARY.
Visit to Hampstead—Small ponds—Water-fleas—Water-beetle—Snails—Polyps—Hydra viridis—The dipping-tube—A glass cell—The Hydra and its prey—Chydorus sphæricus and Canthocamptus, or friends and their escapes—Cothurnia—Polyp buds—Catching Polyps—Mode of viewing them—Structure of Polyps—Sarcode—Polyps stimulated by light—Are they conscious?—Tentacles and poison threads—Paramecium—Trachelius—Motions of Animalcules, whether automatic or directed by a will—Their restless character.
T has been a bitterly cold night, and as the sun shines on a clear keen morning, and glistens in the hoar-frost which covers the trees, it might seem an unpropitious time for visiting the ponds, in search of microscopic prey. We will, however, try our luck, and take a brisk trot to the top of Hampstead Heath, where the air is still keener, and the ice more thick. Arriving at the highest point, London appears on one side enveloped in its usual great coat of smoke, through which St. Paul's big dome, with a score or two of towers and steeples, can be dimly made out; while looking towards Harrow-on-the-Hill, or Barnet, we see the advantage of country air in the sharpness with which distant objects cut the blue sky. We leave the large ponds for another time, and hunt out the little hollows among the furze and fern. One looks promising from the bright green vegetation to be discovered under the sheet of ice, which is almost firm enough to bear human weight.
Breaking a convenient hole we hook up some of the water-plants, and place them in a wide-mouthed vial, which we fill with water, and cursorily examine with a pocket-lens. Some water-fleas briskly skipping about, and a beautiful little beetle, with an elegant dotted pattern on his brown back, and a glistening film of air covering his belly, show that we have not been unsuccessful, although we must wait till we get home to know the extent of our findings, among which, however, we can only discern the graceful spiral shell of a small water-snail, the Planorbis.
Arriving at home the bottle was left undisturbed for some hours in a warm light place, and then on being examined several specimens of that beautiful polyp, the Hydra viridis, were seen attached to the glass, and spreading their delicate tentacles in search of prey. One of the polyps is carefully removed by the dipping-tube, a small glass tube, open at both ends. The forefinger is placed upon the top, and when the other end is brought over the object the finger is raised for an instant, and as the water rushes in the little hydra comes too, and is placed in a glass cell, about half an inch wide, and one tenth of an inch deep. These cells are obtained from the opticians, and cemented with varnish or marine glue to an ordinary glass slide. After an object has been placed in one of them, a little water is taken up in the dipping-tube, and the cell filled until the fluid stands in a convex heap above its brim. We then select around glass cover, and press it gently on the walls of our cell. A few drops of superfluous water escape, and we have the cell quite full, and the cover held tight by force of the capillary attraction between the water and the glass.
Hydra viridis with developed young one, and bud beginning to sprout.
The polyp deposited in one of these water cages is then transferred to the stage of the microscope, and its proceedings watched. At first it looks like a shapeless mass of apple-green jelly. Soon, however, the tail end of the creature is fixed to the glass, the body elongates, and the tentacles (in this case eight) expand something after the manner of the leaves of a graceful palm.