But these conclusions were quite set aside by the brilliant discovery of Thompson, that Zoea and Megalopa were the same animal in different stages of existence ; and that, moreover, both were but the early states of well-known and familiar forms of larger Crustacea ;— which therefore undergo a metamorphosis, as complete as that by which the caterpillar changes to a chrysalis, and the chrysalis to a butterfly, and in every essential point parallel to it.

In the Cove of Cork this naturalist met with a considerable number of Zoeas, which he kept in captivity. Some of these passed into the Megalopa form, which in turn changed to the most abundant of all our larger Crustacea, the common Shore-crab (Carcinus mcenas). " Thus, in its progress from the egg to its final development, the Crab was proved to pass through two temporary conditions, which had previously been regarded as types, not of genera only, but of different families ; and both strikingly dissimilar from the group to which, in its perfect state, it belongs."

I have not myself examined the transformations of this species ; but, as they have been well worked out, and as the animal is so abundant everywhere on the coast that you may easily verify what has been observed, I will cite you the elaborate account of Mr. R. Q. Couch, of Penzance, who has investigated the subject with great skill, zeal, and success.

Having procured some specimens of the Shore-crab laden with eggs just ready for shedding, he goes on to say, " These were transferred to captivity, placed in separate basins, and supplied with sea-water ; and in about sixteen hours I had the gratification of finding large numbers of the creatures alluded to above, swimming about with all the activity of young life. There could be but little doubt that these creatures were the young of the captive Crabs. In order, however, to

ZOEA OF SHOKE-CKAB.

secure accuracy of result, one of the Crabs was removed to another vessel, and supplied with filtered water, that all insects might be removed ; but, in about an hour, the same creatures were observed swimming about as before. To render the matter, if possible, still more certain, some of the ova were opened and the embryos extracted ; but shortly afterwards I had the pleasure of witnessing, beneath the microscope, the natural bursting and escape of one precisely similar in form to those found so abundantly in the water. Thus, theD, there is no doubt that these grotesque-looking creatures are the young of the Cavchms memos; but how different they are from

the adult needs hardly be pointed out any further than by referring to the figure. When they first escape they rarely exceed half a line in length. The body is ovoid, the dorsal shield large and inflated ; on its upper edge and about the middle is a long spine, curved posteriorly, and rather longer than the diameter of the bodv, though it varies in length in different specimens; it is hollow, and the blood may be seen circulating through it. The upper portion of the body is sap-green, and the lower semi-transparent. The eyes are large, sessile, and situated in front, and the circumference of the pupil is marked with radiating lines. The lower margin f the shield is waved, and at its posterior and lateral margin is a pair of natatory feet. The tail is extended, longer than the diameter of the shield ; and is composed of five equal annulations, besides the terminal one ; its extremity is forked, and the external angles are long, slender, pointed, and attached to the last annulation by joints. Between the external angles and on each side of the median line are three lesser spines, also attached to the last ring by joints. Between the eyes, and from near the edge of the shield, hangs a long, stout, and somewhat compressed appendage, which, as the animal moves, is reflexed posteriorly between the claws. Under each eye is another appendage, shorter and slightly more compressed. The claws are in three pairs ; each is composed of three joints, and terminates in four long, slender, hair-like appendages. These claws are generally bent on the body, but stand in relief from it. If the animal be viewed in front, the lower margin of the dorsal shield will be found to be waved into three semi-circular festoons, the two external of which are occupied by the eyes, and between which the middle one intervenes ; the general direction of the claws will be seen to be at right angles to the body. As the young lies inclosed within the membranes of the egg, the claws are folded on each other, and the tail is flexed on them so far as the margin of the shield, and, if long enough, is reflected over the front of the shield between the eyes. The dorsal spine is bent backwards, and lies in contact with the dorsal shield ; for the young, when it escapes from the egg, is cmite soft, but it rapidly hardens and solidifies by the deposition of calcareous matter in what may be called its skin. The progress of this solidification may be very beautifully observed by watching the circulation in the

SECOND STAGE OF SHORE-CBAB.

(Meijulopo.) -dorsal spine. When the creature has just effected its liberation from the egg, the blood-globules may be seen -ascending to the apex; but, as the consolidation advances, the circulation becomes more and more limited in its extent, and is finally confined to the base. These minute creatures, in this early state of their existence, are natatory, and wonderfully active. They are continually swimming from one part of the vessel to the other ; and, when observed free in their native pools, are, if possible,

even more active than when in confinement. Their swimming is produced by continued flexions and extensions of the tail, and by repeated beating motions of their claws ; this, together with their grotesque-looking forms, gives them a most extraordinary appearance when under examination. As the shell becomes more solid they get less active, and retire to the sand at the bottom of the vessel, to cast their shells and acquire a new form. They are exceedingly delicate, and require great care and attention to carry them through the first stage ; for unless the water be supplied very frequently, and in great abundance, thev soon die.

" The second form of transmutation is equally as remarkable as the first, and quite as distinct from the adult animal. In the species now under consideration this second transformation is marked by the disappearance of the dorsal spine ; the shield becomes flatter and more depressed, the anterior portion more horizontal and pointed, the three festoons having disappeared. The eyes, from being sessile, are now elevated on footstalks, the infra-orbital appendages become apparently converted into antenna?. The claws undergo an entire revolution : the first pair become stouter than the others, and are armed with a pair of nippers," the others being simple ; " but the posterior pair are branched near the base, and one of the branches ends in a bushy tuft. The tail is greatly diminished in its relative size and proportions, and is sometimes partially bent under the body, but is more commonly extended. This form is as natatory as the first. They are frequently found congregating around floating sea-weed, the buoys and strings of the crab-pot marks, and other floating substances, both near the shore and in deep water. Their general form somewhat resembles a Galathea." *

Thus, under Mr. Couch's eye, the Zoea had changed to * " Rep. Cornw. Polyt. Sec," 1843.

a Megalopa ; and this latter became, after a short time, a Crab, in which were all the characters that belong to the order to which the parent belongs ; but not those of the genus nor even of the family. Its form bore a close resemblance to that of the Sargasso Crabs (Gra-psidcr) ; for the shield, instead of being large and arched in front, and narrowed behind, was nearly square, while the front was (taking in the eyes) almost straight, the lateral angles much advanced.

THIRD STAGE OF SHOBE-CRAB.

(Grapsoid.)

This Crab, however, was still very minute : and many sloughings were before it. In the course of these it was destined gradually to attain, not only the dimensions of its parents, but also their form. This, however, would be matter of development rather than metamorphosis : the lateral outlines of the shield would more and more approach each other behind, while the series of points that now belonged to these lateral outlines would become thrown into the front margin, which would by degrees assume an arched form, as you may see in the figure of the adult Crab.

Though I cannot at this moment show you specimens of the Carcinas in its earlier stages, yet I have here a good number of the Zoeas of one of those intermediate form s which are the connecting links between the Crabs and Lobsters : I mean Galathea. The adult animal is

of a broad squat form, something like what you might suppose a Lobster to be, if it had been flattened between two stones, without being actually destroyed. We have two or three species, one of which is adorned with brilliant scarlet and azure paintings ; but I cannot tell to which of them all this infant form belongs.

ADULT SHORE-CKAB.

You perceive that there is a general similarity between these transparent little creatures and the Zoea described 1 »y Mr. Couch ; but there are great differences in detail. The glassy shield or carapace shoots out in front in a stiff', inflexible, very fragile spine. This is perfectly straight, and nearly thrice the length of the whole shield. It is beset, on various lines on its surface, with short slender spmules jointed to shoulder-like angles, and not serratures. Its interior is perforated by a canal which dilates and narrows irregularly. The carapace posteriorly is semi-oval, projecting a transparent convex vault over the part where the abdomen is attached to it, as is seen when the latter

Lends down. Its extremity gradually tapers into two straight, sub-parallel, stiff spines, about as long as the carapace itself, each terminating in a hooked point.

The abdomen ends in a spinous plate, which is very elegantly lozenge-shaped and beset with spines. Each of the two latero-posterior edges of the lozenge is cut into six rectangular teeth, and each tooth bears on its hinder face a long spine, articulated to it, and most delicately plumose all along its sides. The hindmost pair of spines are short, and are set close together, side by side. Besides these jointed spines, each lateral angle of the lozenge-shaped tail-plate projects into a spine-like tooth.

Though the individuals before us are all in the same state as to the stage of their development, there is some difference in size, indicating, doubtless, a correspondingdiversity in age. "We will isolate a few of the largest and put them into a glass trough for microscopical examination.

The largest, during the few minutes which I have occupied in the process of dipping them out, has undergone a metamorphosis. You observed that, after skipping about the trough for a few minutes, it sank quietly to the bottom, where it lay on its back; the next thing that you see is a much more crab-like animal, more opaque, redder, much larger, but lying on its back in the very spot where a moment before you had seen a Zoea: while close by it lies the transparent filmy skin which has been cast off.

Page 29 of 66