The inherent chemical properties of this curious Zoophyte are very remarkable. When a Sponge has been immersed for fourteen or sixteen days in nitric acid (diluted with three parts of distilled water) it becomes nearly transparent, and when touched with ammonia, assumes a deep orange colour, inclining to a brownish red. But if much softened by the acid, the whole fabric immediately disappears, on being immersed in ammonia, and forms a deep orange-coloured solution. A Sponge, when boiled, gives out a considerable portion of animal jelly. The infusion of a small quantity of oak bark causes this to fall to the bottom of the vessel, as a sediment, and so entirely changes the nature of the Sponge, that, when dry, it crumbles between the fingers; and, when moist, it may be torn like wetted paper. In this state we should naturally conclude that it is entirely useless: but no; the operations of chemistry resemble a magic wand. Boil the same in water, with caustic potash, its latent qualities will be called forth; and, behold, a deposition of animal soap!
THE FRESH-WATER POLYPI AND THEIR MARINE ALLIES. (Hydroida.)
These are two species, which will fully illustrate the nature of the whole tribe. They are found in clear waters, and may generally be seen in small ditches and trenches of fields, especially in the months of April and May. They affix themselves to the under-parts of leaves, and to the stalks of such vegetables as happen to grow in the same water; and feed on the various species of small worms and other aquatic animals within their reach. When any of these pass near a Polyp, the latter suddenly catches it with its arms, and dragging it to its mouth, swallows it by degrees, much in the same manner as a snake gorges its prey. Two Polypi may occasionally be seen in the act of seizing the same worm at different ends, and dragging it in opposite directions with great force. It sometimes happens, that while one is swallowing the end it has seized, the other is employed in the same manner; and thus they continue swallowing, each his part, until their mouths meet. They then rest for some time in this situation, till the worm breaks between them, and each goes off with his share. But sometimes when the mouths of both are thus joined together a combat ensues, and the largest Polyp usually swallows his antagonist; the animal thus swallowed, however, seems to be a gainer by its misfortune, as after it has lain in the conqueror’s body for about an hour it issues unhurt, and often in possession of the prey that had been the original cause of contention. The remains of the animal, on which the Polyp feeds, are evacuated at the mouth, the only opening in the body. The species are multiplied by a kind of vegetation, one or two, or even more young ones, emerging gradually from the sides of the parent animal; and these young ones are frequently again prolific before they drop off; so that it is no uncommon thing to see two or three generations at once on the same Polyp. But the most astonishing fact respecting this animal is, that if a Polyp be cut in pieces, it is not destroyed, but is multiplied by dissection. It may be cut in every direction that fancy can suggest, and even into very minute divisions, and not only the parent stock will remain uninjured, but every section will become an animal. Even when turned inside out, it suffers no material injury; for, in that state it will soon begin to take food, and to perform all its other natural functions.
M. Trembley, of Geneva, ascertained that different portions of one Polyp could be engrafted on another. Two transverse sections brought into contact will quickly unite and form one animal, though each section should belong to a different species. The head of one species may be engrafted on the body of another. When one Polyp is introduced by the tail into another’s body, the two heads unite and form one individual. Pursuing these strange operations, M. Trembley gave scope to his fancy by repeatedly splitting the head and part of the body; he thus formed hydras more complicated than ever struck the imagination of the most romantic fabulist.
Though so difficult to destroy by division, all the Polyps, even those which form the corals, may be easily killed by depriving them of moisture, when they soon shrivel up, and the tissue of their skins is completely destroyed.
Of these Fresh-water Polypi, only a few kinds are known, but the sea nourishes a multitude of species which closely resemble the Hydras in their structure, from hence called Hydroid Polyps by Cuvier and many other naturalists. Most of these are compound creatures, of the kind shown in the above engraving, of which many species may be found on all our shores. A horny tube runs branching over the surface of a seaweed, or some other object, and from this, at intervals, rise slender stalks, often branched in the most elegant manner. Upon the delicate branches we find little horny cups, each of which is the habitation of a tiny Polyp, furnished with a mouth and stomach, and with a circlet of slender arms to enable it to capture its prey. Other species are enclosed only in a soft membrane, but all rise from creeping roots.