Dragon-fly larvæ: A ordinary aspect;
B with the huge nipper-like jaw extended.
Fig. 14.
A Limnæca Stagnalis, or water snail.
B Planorbis.
Fig. 15.
Paludina Vivipara.
Continuing our search, we light upon the fat, sluggish, ungraceful larva of the graceful and brilliant Dragon-fly, the falcon of insects ([Fig. 13]). He is useful for dissection, so pop him in. Among the dead leaves you perceive several small leeches, and flat oval Planariæ, white and brown; and here also is a jelly-like mass, of pale yellow colour, which we know to be a mass of eggs deposited by some shell-fish; and as there are few objects of greater interest than an egg in course of development, we pop the mass in. Here ([Fig. 14]) are two molluscs, Limnæus and Planorbis, one of which is probably the parent of those eggs. And here is one which lays no eggs, but brings forth its young alive: it is the Paludina vivipara ([Fig. 15]), of which we learned some interesting details last month. Scattered over the surface of the net and dead leaves, are little dabs of dirty-looking jelly—some of them, instead of the dirty hue, are almost blood-red. Experience makes me aware that these dirty dabs are certainly Polypes—the Hydra fusca of systematists. I can’t tell how it is I know them, nor how you may know them again. The power of recognition must be acquired by familiarity: and it is because men can’t begin with familiarity, and can’t recognize these Polypes without it, that so few persons really ever see them. But the familiarity may be acquired by a very simple method. Make it a rule to pop every unknown object into your wide-mouthed phial. In the water it will probably at once reveal its nature: if it be a Polype, it will expand its tentacles; if not, you can identify it at leisure on reaching home, by the aid of pictures and descriptions. See, as I drop one of these into the water, it at once assumes the well-known shape of the Polype. And now we will see what these blood-red dabs may be; in spite of their unusual colour, I cannot help suspecting them to be Polypes also. Give me the camel-hair brush. Gently the dab is removed, and transferred to the phial. Shade of Trembley! it is a Polype![23] Is it possible that this discovery leaves you imperturbable, even when I assure you it is of a species hitherto undescribed in text-books? Now, don’t be provokingly indifferent! rouse yourself to a little enthusiasm, and prove that you have something of the naturalist in you by delighting in the detection of a new species. “You didn’t know that it was new?” That explains your calmness. There must be a basis of knowledge before wonder can be felt—wonder being, as Bacon says, “broken knowledge.” Learn, then, that hitherto only three species of fresh-water Polypes have been described: Hydra viridis, Hydra fusca, and Hydra grisea. We have now a fourth to swell the list; we will christen it Hydra rubra, and be as modest in our glory as we can. If any one puts it to us, whether we seriously attach importance to such trivialities as specific distinctions resting solely upon colour, or size, we can look profound, you know, and repudiate the charge. But this is a public and official attitude. In private, we can despise the distinctions established by others, but keep a corner of favouritism for our own.[24]
I remember once showing a bottle containing Polypes to a philosopher, who beheld them with great calmness. They appeared to him as insignificant as so many stems of duckweed; and lest you should be equally indifferent, I will at once inform you that these creatures will interest you as much as any that can be found in ponds, if you take the trouble of studying them. They can be cut into many pieces, and each piece will grow into a perfect Polype; they may be pricked, or irritated, and the irritated spot will bud a young Polype, as a plant buds; they may be turned inside out, and their skin will become a stomach, their stomach a skin. They have acute sensibility to light (towards which they always move), and to the slightest touch; yet not a trace of a nervous tissue is to be found in them. They have powers of motion, and locomotion, yet their muscles are simply a network of large contractile cells. If the water in which they are kept be not very pure, they will be found infested with parasites; and quite recently I have noticed an animal, or vegetal, parasite—I know not which—forming an elegant sort of fringe to the tentacles: clusters of skittle-shaped bodies, too entirely transparent for any structure whatever to be made out, in active agitation, like leaves fluttering on a twig. Some day or other we may have occasion to treat of the Polypes in detail, and to narrate the amusing story of their discovery; but what has already been said will serve to sharpen your attention and awaken some curiosity in them.