says was sent by the Deity to punish the growing pride of the Hollanders—I have not been able to ascertain. Writers in general speak of it as "very great;" and Dr. Job Baster mentions the Teredo as an animal "which has done so many millions damage to these countries." In our own country it has done, and continues to do, extensive mischief. The soundest and hardest oak cannot resist these noxious creatures; but in the course of four or five years, they will so drill it as to render its removal necessary, as has repeatedly happened in the dockyard of Plymouth. To preserve the timbers used there, and exposed to them, the plan now adopted is to cover the parts under water with short broad-headed nails, which, in salt water, soon invests the whole with a strong coating of rust impenetrable by their augers. The plan appears to have proved effectual, for, in the harbours of Plymouth and Falmouth, where the Teredo was once abundant, it is now rare or not to be found; but in other parts it has still a residence, and within these few years it has materially injured or destroyed many of the piles used in the construction of the pier at Port Patrick, on the coast of Ayrshire; and the Limnoria terebrans, a crustaceous insect, co-operating with it, the result of their united efforts can hardly fail to be the utter and speedy destruction of all the timber in the pier."[13]
Another kind of injury is dependent on the fact that certain species, which are generally eatable and even wholesome, become at certain times highly poisonous. Some foreign species are liable to this fatality, particularly oysters, both in the East and West Indies. But we need not go to distant countries for cases in point. The Mussels of our own rocks, though generally sold and eaten by many persons without fear, are well known to be fickle in their qualities, and many cases are on record in which their use has proved fatal. One of these, well authenticated, and investigated by scientific medical men, occurred at Leith in June 1827. Many of the poor of this town were poisoned by eating mussels which had been collected in the docks.
The cause of this occasional liability to become poisonous seems involved in almost total obscurity. Dr. Johnston, who discusses at some length the many loose and vague conjectures that have been hazarded on the subject, has shown, that not one of them is tenable, unless it be that in some cases the poisonous principle proceeds from some particular food which, not fatal to the Mollusks, yet generates a diseased condition of the body, deadly to other creatures. The Leith mussels, he adds, were living in a dock, where we may presume they were nurtured and fattened amid putrescent matters; and Dr. Coldstream, than whom no one is better qualified to decide the point, gave it as his opinion that the liver was larger, darker, and more brittle than in the wholesome fish, and satisfied Dr, Christison that there was a difference of the kind. It must be confessed, however, that these observations leave the question pretty nearly where it was before.
Some peculiar secretions of the Mollusca remain to be noticed. And first the black liquor or ink of the Cuttles and Squids, which has already been mentioned as useful to man, but which is doubtless much more useful to the animals themselves. These animals, when in danger, are known to pour forth from a funnel-like orifice a liquor of a blackish-brown colour in considerable abundance; this fluid, readily diffusing itself and mingling with the surrounding water, produces such a cloud of obscurity as frequently enables the crafty animal to escape, enveloped in the mist of its own making—as the deities in Homer are represented as concealing their favourite heroes.
Somewhat analogous to this is a secretion of a rich purple hue produced and poured forth under excitement, by those large and naked mollusks, the Aplysiæ. I have found one of these animals, on being put into a vessel of clean sea-water, change the whole to a brilliant purple in a very few minutes; and on the water being renewed even again and again, produce the same result. This was a West Indian species, but there is one found occasionally upon our own coasts which has the same property.
APLYSIA.
This liquor must not be confounded with that which constitutes the purple dye of Murex, Purpura, &c. already mentioned, for it is so volatile as to be unsuitable for the purposes of dyeing. According to Cuvier, the secretion in drying assumes the beautiful deep hue of the sweet Scabious, and remains unaltered by long exposure to the air. Nitric acid, in small quantity, heightened the tint, but a larger dose changed it to a dirty orange colour, while potash turned it to a dingy vinous grey.
A very common shell in ponds and ditches, (Planorbis corneus,) coiled up like a ram's horn, is said to have the same property; a purple fluid is poured out from beneath the mantle, but it is so fugitive that no application can prevent its speedily turning to a dull rusty colour.
Colonel Montagu mentions one of our marine shell-fish (Scalaria clathrus) as secreting a purple juice. "It may be collected either from the recent or dried animal, by opening the part behind the head; and as much can be procured from five individuals as is sufficient, when mixed with a few drops of spring-water, to cover half a sheet of paper. Neither volatile nor fixed alkali materially affects it; mineral acids turn it to a bluish green, or sea-green; sulphuric acid renders it a shade more inclining to blue; vegetable acids probably do not affect it, since cream of tartar did not in the least alter it. These colours, laid on paper, were very bright, and appeared for some months unchanged by the action of the air or the sun; but, being exposed for a whole summer to the solar rays, in a south window, they almost vanished. The application of alkali to the acidulated colour always restored it to its primitive shade, and it was as readily changed again by mineral acid."[15]
SCALARIA.
I have already mentioned some thread-spinners among the Mollusca; there are others which have the power of forming threads of silky substance much stronger and more durable than those of our pond snails. The Common Mussel (Mytilus edulis) is one of these marine silk-worms; and we have a good many others. The bundle of threads, familiar to many of my readers as the beard of the shell-fish, is the substance in question, termed by naturalists byssus, a Greek word originally signifying silk; and the use to which it is applied by the animal itself is that of a cable to moor itself to the solid and immovable rock, that it may not be washed away by the violence of the waves. The mode in which the threads are formed, and the organ by which they are secreted, are thus described by Professor Rymer Jones:—
"The foot in the Mussel is of small dimensions, being useless as an instrument of progression. By its inferior aspect it gives attachment to the horny threads of the byssus, which are individually about half an inch in length, or as long as the foot itself, by which, in fact, they are formed, in a manner quite peculiar to certain families of Conchifera; no other animals presenting a secreting apparatus at all analogous, either in structure or office, to that with which these creatures are provided. The manner in which the manufacture of the byssus is accomplished is as follows: A deep groove runs along the under surface of the foot, at the bottom of which thin horny filaments are formed by an exudation of a peculiar substance, that soon hardens and assumes the requisite tenacity and firmness. While still soft,
