The rays which pass through the several pyramids are prevented from mingling with each other by the isolating sheath of dark pigment; and no rays, except those which pass along the axis of each pyramid, can reach the opticnerve; all the rest being absorbed in the pigment of the sides. Hence it is evident, that as no two cornea? on the rounded surface of the compound eye can have the same axis, no two can transmit a ray of light from the very same point of any object looked at ; while, as each of the composite eyes is immovable, except as the whole head moves, the combined action of the whole 24,000 lenses insects: their ears and eyes. can present to the sensorium but the idea of a single, undistorted, unconfused object, probably on somewhat of the same principle by which the convergence of the rays of light entering our two eyes gives us but a single stereoscopic picture.

The soft blue colour of this Dragon-fly's eyes,-—as also the rich golden reflections seen on the eyes of other insects, as the Whame-flies, and many other Diptera,— is not produced by the pigment which I have alluded to, but is a prismatic reflection from the corner.

You would suppose that, having 24,000 eyes, the Dragon-fly was pretty well furnished with organs of vision, and surely would need no more ; but you would be mistaken. It has three other eyes of quite another character.

If you look at the commissure or line of junction of the two compound eyes on the summit of the head, you will see, just in front of the point where they separate, and their front outlines diverge, a minute crescent-shaped cushion of a pale-green colour, at each angle of which is a minute antenna. Close to the base of each antenna there is set in the black skin of the head that divides the green crescent from the compound eyes, a globose,, polished knob of crystal-like substance, much like the "bull's eyes " that are set in a ship's deck to enlighten the side-cabins. On the front side of the cushion there is a third similar glassy sphere, but much larger than the two lateral ones. What are these?

They are eyes, in no important respect differing from the individuals which comj)ose the compound masses, except that they are isolated. The shining glassy sphere is a cornea of hard transparent substance, behind which is situated a spherical lens, lodged in a kind of cup formed by an expansion of the optic nerve, and which is surrounded by a coloured pigment-layer.

You may study these simple eyes, or stem mata as they are called, in many other insects, though they are not so universally present as the compound eyes. On the forehead of the Honey-bee they are well seen, as three black shining globules, placed, as in the Dragon-fly, in a triangle.

It is reasonable to presume that some difference, important to the insect, exists between the perceptions obtained by means of the simple, and those obtained by means of the compound, eyes ; but what is the nature of the difference is certainly not known, and probably could not be imagined by us, who know only one kind of vision.

Chapter XI. Crabs and Shrimps

It is always interesting to trace the varied forms and •conditions under which any particular function is performed ; and particularly to mark, in creatures veryremote from us in the scale of being, the organs devoted to the senses which are so requisite to our own comfort. We have already seen some of these diversities in examples taken from the classes Mollusca and Insects; and will now examine some more, as they appear in the Crustacea.

If you look at the head of a Crab, a Lobster, or a Prawn, you will see that it is furnished, like that of Insects, with jointed antenna? ; but, whereas in insects there is never more than a single pair, in the creatures of which I am speaking there are two pairs. In the Prawn you may suppose, at first sight, that there are four pairs ; but that is because the internal antenna? terminate each in three many-jointed bristles, in structure and appearance exactly like the bristles of the outer pair, two of the three being nearly as long as the outer, while the third is short. In the Lobster, the internal are twobristled, both bristles rather short, while the external are very long. In the Flat-crabs each pair is simple, the inner minute, the outer long. In the great Eatable Crab each pair is very small, and they are dissimilar.

Now, taking the last-named animal as the representative of his class, let us examine one of his inner antennae first. It consists of a jointed stem and a terminating bristle ; the latter furnished with small hairs common to the general surface of the body, and with long, delicate, membranous filaments (setce, often improperly called cilia), which are larger and much more delicate in structure than the ordinary hairs.

The basal joint is greatly enlarged : if it be carefully removed from its connexion with the head, and broken open, it will be found to inclose in its cavity a still smaller chamber, with calcareous Avails of a much more delicate character than the outer walls. This internal cell is considered by Mr. Spence Bate to be a cochlea* from its analogy, both in structure and supposed use, to the organ so named in the internal ear of man and other vertebrate animals. It is situated, as has been said, in the cavity of the basal joint of the internal antenna, and is attached to the interior surface of its wall farthest from the median line of the Crab. It has a tendency to a spiral form, but does not pass beyond the limits of a single convolution.

If this interior cell does indeed represent the cochlea of more highly-constructed ears, to which it bears some resemblance both in form and structure, then it seems to identify, beyond dispute, these inner or upper antennae as the organs of hearing.

Now with this conclusion agrees well the manner in which the living animal makes use of the organs in question. The Crab always carries them erectand elevated ; and is incessantly striking the water with them, with a very peculiar jerking action, now and then vibrating, and, as it has been called, " twiddling " them.

EAR OF CRAB PROM BEHIND. These antennae, be - therefore, on the . watch ap- . pear to always :

Cochlea, the Latin for Snail-shell, is the name given to one of the en vi ties of the ear, from its resemblance to the interior of a snailshell.

let the animal be at rest, let it be feeding, no matter, the inner antennae are ever elevated, and on constant guard.

The lengthened and delicate seta? with which they are furnished are, moreover, peculiarly adapted to receive and convey the most minute vibratory sensations from the medium in which they are suspended ; and, on the whole, it seems to be satisfactorily settled by Mr. Spence Bate (to whose excellent memoir* I am indebted for these explanatory details) that the inner antennae are real ears.

Having thus taken our Crab by the ears, we will endeavour next to tweak his nose. But stay, we must lind it first. We turn our horny gentleman up, and in his Hat ancient face we certainly discern little sign of a nasal organ. Our friend Mr. Bate must assist us again. He will tell us to look at the outer or lower antenna-. We will look, accordingly, magnifier in hand, while he makes it clear to us that these are a pair of noses.

Each of these organs is formed of a stem, consisting in general of five joints and a filament of many minute joints. In the Prawn and the Lobster all the five joints of the stem are distinct; but in the Crab the whole are, as it were, soldered together into a compact mass, so that the separate articulations are scarcely to be distinguished ; while in some species their position can be indicated only by the presence of the olfactory operculum or lid.

This important little organ varies in its construction in the different families of Crustacea. In the Crab it is a small movable appendage, situated at the point of junction between the second and third joints ; it is attached to a long calcareous lever-like tendon, at the extreme limit of which is placed a set of muscles, by which it is opened and closed ; to assist in which operation, at the angle of the operculum most distant from the central line of the animal are fixed two small hinges. When the operculum * u Annals of Xat. Hist." for July, 1S55.

is raised, the internal surface is found to be perforated! by a circular opening protected by a thin membrane.

In the Prawn, Shrimp, and Lobster, there is no operculum, but only the orifice covered by a membrane, which is placed at the extremity of a small protuberance, and is not capable of being withdrawn into the cavity of the antennae, as in the Crab.

In the latter animal, the little door, when it is raised, exposes the orifice in a direction pointing to the mouth ; and where there is no door, still the direction of the opening is the same, inwards and forwards, answering to the position of the nostrils in the higher animals. In each case it is so situated, that it is impossible for any food to be conveyed into the mouth without passing under this organ ; and there most conveniently the animal is enabled to judge of the suitability of any substance for food, by raising the little door and applying to the matter to be tested the sensitive membrane of the internal orifice.

Thus it is concluded that this lower or outer pair of antenna? are the proper organs of smell, as the upper and inner are of hearing.*

The eyes, though constructed on the same general principles as those of Insects, yet present some particulars worthy of your notice. In the Crabs and Lobsters they consist of numerous facets, behind each of which is a conical or prismatic lens, the round extremity of which is fitted into a transparent conical pit, corresponding to a vitreous body, while the conical extremity of these lenses is received into a kind of cup, formed by the filaments of the optic nerve. Each of these filaments, together with its cup, is surrounded by pigment matter, in a sheath-like manner. To see this structure would require anatomical skill ; but you may here examine with a low power portions of the cornea, or glassy exterior, of the eye of a Crab, and of a Lobster. In the * Op. cit.

former you see that the facets into which the cornea is divided are hexagonal, like those of most Insects, but in the latter they are square.

But Crustacea have a far greater faculty of circumspection than insects have; for besides the extensive convexity and numerous facets of their eyes, these organs are placed at the extremity of shelly foot-stalks, which are themselves movable on hinges, capable of beingprojected at pleasure, and of being moved in different directions, and of being packed snugly away, when not in active use, in certain grooves hollowed out expressly for them in the front margin of the shell.

We might find much more both instructive and amusing in examining microscopically the structure of the higher Crustacea ; but we will now dismiss them in order to discuss some of the lower forms, many of which are so minute that their whole bodies may be watched with ease performing all the functions of life, while confined under our eye, on the stage of the microscope. I refer to the tiny active little creatures known as Water-fleas, which are abundant in both fresh and salt water.

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