THE ZOEA OF THE COMMON CRAB.
The Crabs are a very numerous tribe, and contain many singular species, as far as regards their form, which assumes an endless variety of curious shapes, sometimes elegantly decorated with the most brilliant colours; this more particularly applies to those which are found in the seas of hot climates.
The singular little animal represented in the preceding page, was placed by naturalists among the Crustacea, and considered a perfect animal of a distinct species; it was named [Zoea]. Mr. Thomson, the experienced naturalist we have already noticed, was the first to discover the real nature of this little creature. We cannot do better than to introduce the subject by the following observations of the author of this discovery, showing the reason so little is known respecting these inhabitants of the deep.
“The sea (which is the habitation of the greater part of the Crustacea) to the casual observer offers nothing but an immense body of water, here and there presenting a solitary whale, or a vagrant troop of some of the smaller cetaceous animals; the appearance of a fish of almost any other kind in the track of a vessel over a vast expanse of the open ocean, is regarded, even by the mariner, as a kind of phenomenon, and creates an interest not to be appreciated by those who have not engaged in distant voyages. The fathomless parts of the ocean certainly do not offer the same profusion of inhabitants with the shores of islands and continents, or those parts where the bottom is within reach of the sounding-line, or where the surface is interspersed with fields of Sargosa[6]. On due examination, however, we shall not fail to find it everywhere peopled by a considerable variety of animals, either of small size, or possessed of such a degree of translucency as to render them invisible, or scarcely perceptible, even when on or near to its surface; that it should possess its share of the organized beings which we see spread over every other part of the surface of our globe, is a conclusion we might arrive at indirectly, from the consideration of oceanic fishes and birds being observed in those parts of the ocean most distant from the land, and the provident care of the Deity, which we invariably witness throughout the domain of nature, to furnish food for all, even the meanest of his creatures; the more minute and invisible inmates of the sea, then, must constitute the food of oceanic fishes and birds.
“Few of these marine animals, except some of the larger and most conspicuous, have as yet been observed, so that the investigation of them holds out a promise of a rich harvest to the naturalist, and a vast field of exploration, replete with novelty and interest; to accomplish this, however, he must use the greatest diligence, seizing every opportunity, when the way of a ship does not exceed three or four miles per hour, to throw out astern a small towing-net of gauze, bunting, or other tolerably close material, occasionally drawing it up, and turning it inside out into a glass vessel of sea-water, to ascertain what captures have been made. When a ship goes at a greater rate, and in stormy weather, a net of this kind may be appended to the spout of one of the sea-water pumps, and examined three or four times a day, or oftener, according to circumstances.”
Although naturalists were decided in calling the [Zoea] a crustaceous animal, they were still far from agreeing as to the place in the system it ought to occupy, for the different species were so unlike each other: but it will be no longer a matter of surprise, when it is known, that this singular creature is not a perfect animal, but merely the larva, or imperfect state of the Common Crab. This fact is perfectly new, and interesting in a double point of view, not only proving their real nature, but also that the Crustacea are not, as described in most systems, animals undergoing no metamorphosis, and on that account to be separated from the insects, but that they do undergo a metamorphosis, and that of a most wonderful nature.
It was in the Spring of 1822 that Mr. Thomson first met with Zoeæ, in the harbour of Cove, and that in considerable abundance; the year following, at the same season, one of considerable size occurred; this was considered a fit object for experiment, and was carefully supplied with fresh sea-water, from May 14th to June 15th, when it died in the act of changing its skin. That portion of its new form which it had been able to disengage was sufficient to show that it bore a great resemblance to the division of the Crustacea, in which the crabs and lobsters are placed. “This proof,” says the author, “might be considered incomplete, if I had not had the good fortune to succeed in hatching the ova of the Common Crab, during the month of June, which presented exactly the appearance of the Zoea taurus.”
The Common Crab, Cancer major, is so well-known, that any description of its appearance would be useless.