It shows at a glance the increase that instantaneously takes place in the size of the crab after the act of exuviation is performed, the portion exuded being on a scale considerably larger than the old covering, which, however, is capacious enough to hold that half of the animal that had not effected its deliverance at the moment when the novel arrestment was so unceremoniously served.
The fourth and fifth pair of legs are free, while the eyes and antennæ are also drawn out of their sheaths. (This is not very evident now, but such is really the fact, I having distinctly seen those organs in motion when the animal was in the living state.) The chelæ, or large claws, being still undetached, serve to bind the crab to its old integument, and thus enable the act of exuviation, or one phase of it at least, to be distinctly apparent.
I know of no work on Natural History that speaks of the Hermit-Crabs (Anomoura) casting their shells, and on this account I have given some attention to them. These animals being so common, I kept by me at least a dozen specimens for the purpose of observing some of them, if possible, in the act of exuviation. The result of my labours has not been so satisfactory as I could wish, from my not having been able to collect any 'sets' of exuviæ. I cannot, therefore, speak with certainty as to the frequency of this phenomena. By this time my readers will know that the tail of the Hermit-Crab is very tender and fleshy, being covered merely with an extremely delicate membraneous skin, while the carapace, claws, and antennæ of the animal are protected by a hard crust, similar to the Lobster, Cray-fish, &c.
From this peculiar formation of the crab, I was not at all surprised to find, on several occasions, the upper part of its body alone cast off, and therefore came to the very natural conclusion, that as the tail was soft, it would grow and increase in proportion to the other parts of the animal, without ever needing the skin to be changed.
Each morning and evening during the time my experiments were being conducted, I examined all the tanks attentively, to see whether an exuvium had been cast. If visible, the object was picked out and gummed in a box, and a date placed above it for future reference. After having performed an operation of this kind one afternoon in October 1858, I saw a Hermit-Crab (who had cast his shell on the previous day) hurriedly leave his testaceous dwelling, then scrape away at his tail, and after a moment's interval, leap into his old seat again. On inspection, I found to my surprise that he had actually slipped off the skin of his tail![7] much in the same fashion as we would draw off a well-fitting glove. Here was a strange and unexpected discovery.
On submitting the exuvium to the microscope, we find that the covering of the false feet, and the cilia attached to the same, all remained fixed in their natural position to the tail-piece. Although in several cases I have had no difficulty in discovering the rejected cuticle of the tail, at other times it has eluded my search. The cause of this I cannot explain. It may be that the animal, adopting the habits of the toad, swallows a portion of its exuviæ as soon as cast. On two occasions I found the slough of the body and claws of a crab, and waited patiently for several days, without success, expecting to get the tail portion. Growing impatient, it occurred to me that it would be a curious experiment to try and draw off the exuvium with my fingers. This was easy to talk about, but difficult to perform.
In the first place, the crab would not, if he could help it, allow himself to be handled even in the most gentle manner. To overcome this difficulty the shell had to be broken. This was done; but, alas! the shock nearly killed the poor little Hermit. After some trouble, I carefully unwound his body from the whirls of the Top Shell, and proceeded to perform the intended operation. Reader, have you ever seen a child take a rose-bud in his hands, and force open its half-pouting blossom, in the belief that by so doing he was assisting nature? If so, you must have watched the puzzled expression of the boy's countenance when he beheld the leaves fall one by one at his feet, and the bud itself exhibit evident signs of approaching decay.
In just such a position did I stand with regard to the poor Hermit-Crab, for, in spite of all my care in manipulation, the skin of the animal was so tender and delicate that the first gentle pinch caused a puncture which proved fatal; and as to drawing off the covering, the thing I now believe to be impossible, even under the most favourable circumstances.
The upper portion of the Soldier-Crab, I may mention, is cast off in one piece, while the animal is seated in its turbinated dwelling. The act is performed with the most perfect ease. Unlike the Brachyura, the Anomoura do not exhibit signs of such rapid growth immediately after exuviation, but increase in size very gradually indeed.