The generation and development of reptilia and mollusca may be better studied by the use of jars for the specimens, than by their immersion in the aquarium. Tadpoles, the larva of newts, and the spawn of mollusks, may be preserved in the cabinet for purposes of study, much better than in the tank; each species being separate in a bright and portable vessel, every minute change can be observed, and a lens applied at any time, or the specimens removed for close inspection without difficulty. I find it a good practice to remove any spawn, which may be deposited on the large vessels, to the small jars on my shelves. There the little Lymnea and Planorbis are developed in hundreds, without molestation; and if increase of Paludina vivipara be required, a jar is at once converted into a breeding tank by throwing a couple into it, with a bunch of Callitriche, and any vegetable waste from the tanks. In the aquarium, the young mollusks are devoured almost as soon as they are born; and the pleasing spectacle of their increase, coming forth from the gelatinous mass in hundreds, like minute beads of gold, is lost without the aid of the cabinet in which to rear them. The young of most species of univalve mollusks are vagrant in their habits, and the jars in which spawn is hatched should be closely covered with perforated card or gauze, fitting closely by means of India rubber rings.
Since it is unnecessary in this work to give a classified history of the several creatures that may be kept in water-cabinets, I shall devote the remainder of the space at my command to notices of a few of the most attractive and best known species, and to a few hints on the general management of the cabinet.
LARVA.
The great class of insects comprises many remarkable and diverse forms, among the 560,000 species which Dr. Imhoff estimates to be now known to naturalists. Yet, various as they are, it is by no means impossible to define what are the distinctive features by which this class is separated from those which approach it in conformation and habits. A true insect has the body divided into three parts—the head, the thorax, and the abdomen. It has never more than six legs, and these are attached to the thorax. The segments of the body seldom exceed thirteen in number, one of which forms the head, three the thorax, and the remaining nine the abdomen. The possession of one or two pairs of wings gives them their prominent characteristic to the eye, but it is the successive metamorphoses that arrests universal attention, and calls forth the admiration and wonder of mankind. In the progress of an insect from the minute egg to its completed form, we see the most remarkable series of developments which animal life ever displays in all its endless procession of forms—the egg, the worm, the chrysalis, the fly—a strange unfolding, for the first time accurately observed by Swammerdam, who detected, under the wrinkled skin of the disgusting worm, the complete outline of the lovely butterfly.
This metempsychosis may be studied in its several strange details by the aid of the Water-Cabinet. The first condition of the newly-hatched egg is that known as the grub, or caterpillar—scientifically called the larva. The larva generally bears no earthly resemblance to the imago, or perfect insect, into which it is to be hereafter developed, but leads a life of sensual enjoyment—it eats, eats! it is gluttony concentrated in type and act. It changes its skin several times, slips one coat off and acquires a new; growing, and eating, and changing garments, till, like man himself, it seeks a temporary tomb, from which it is to soar to the skies like a soul liberated. This second form is popularly known as the chrysalis, or aurelia, scientifically called the pupa. In this form the insect remains in a state of complete or partial torpidity for a few days, weeks, or months, according to the particular species.
The day of its deliverance arrives, its bonds burst, and it comes forth "a thing of beauty" to sport in sunbeams, and, for but a brief season, lead a life of joy—
"Fluttering round the jasmine stems,
Like winged flowers or flying gems."
But beauty of the high poetic kind is not the inheritance of every member of the class insecta; and the water-cabinet presents us with many that have but analogical resemblances to the typical structure of the moth or the fly, though the naturalist finds beauty in a beetle, and points of profound interest in a maggot or grub.