Glimpses of Ocean Life 14

Glimpses of Ocean Life 14

On one occasion I sat up all night, feeling confident, from symptoms

which a certain Cancer mænas exhibited, that he was speedily about to

exuviate. Alas! I was mistaken. On my endeavouring to expedite the

event by lifting up the carapace of the crab, I received a nip on my

finger so severe, that I shall never forget it.

 

But at length in the early portion of last year (1859), I, most

happily for my own peace of mind, did actually witness the entire

process of exuviation in a tolerably large specimen of the Common Shore

Crab. The animal in question, who was domiciled in a crystal vase,

or, in common language, a glass tumbler, rendered himself a favourite

from his constant habit of poking part of his head and his entire

claw (he had got but one), out of the water whenever he caught sight

of me. Who could resist such a powerful, though silent appeal to 'the

generous impulses of one's nature' as this? Certainly I could not,

and therefore, once a day at least, gave Master Cancer the half of a

newly-opened mussel, a tit-bit that was greatly relished. He would

sometimes get a grip of the valve, and allow himself and the Mytilus

to be entirely raised out of the water. Improving upon this, he would

then partly finish his meal while seated in my hand. On the morning

of the above mentioned eventful day, I gave the crab a portion of a

Pholas, but to my surprise, the heretofore high-class dainty remained

untouched. I was in ecstasies! for I felt morally certain that the

grand event, so long looked for, was soon to take place. Consequently,

I took out the crab, cleaned the windows of his dwelling in order

that I might the better see what was going on within, treated him to

some fresh water, as well as a new frond of sea-weed, and then again

introduced my pet to his old apartment.

 

Before doing this I had the animal closely examined, to see if any

signs of the approaching moult could be detected, but none were

visible, except that the glassy bags, if I may so call them, which for

some weeks had been gradually thrown out from the stumps of the three

mutilated limbs, appeared finer in texture than usual. Indeed, so

transparent had they become, that I could distinctly see the contour of

the new limb about to be reproduced, folded up within each capsule.

 

A few minutes after the crab had been placed in the tumbler, I gave a

peep to see how he was getting on. To my intense surprise, I observed

that his shell had just opened near the tail! My first feeling was

one of sorrow, thinking that in handling the specimen I had been too

rough, and had perhaps injured it. This apprehension was soon changed

to delight, as I became by degrees aware that exuviation had actually

commenced.

 

The operation did not extend beyond five minutes (although the time

appeared much longer to me), and was carried on by gentle, and at first

almost imperceptible degrees. The shell, or carapace, was slowly raised

over the back, and gave one the idea of the rear view of a lawyer's

white wig when tilted over his brow, thus exposing the natural black

hair on the occiput below; for, as the body of the animal came forth,

it was very dark in colour, while the old case assumed a whitish hue.

I need hardly say, the leg sheaths of the crab did _not_ split open,

and yet the corresponding limbs were drawn out with the greatest ease.

Moreover, they did not appear in view one by one, but in a cluster, as

it were, and packed close to the bent body of the crab.

 

During the entire process the animal appeared to use scarcely any

exertion whatever, certainly not half so much as any human being would

exhibit in throwing off the most trifling garment. In fact, the crab

seemed to swell painlessly, and gently roll or glide out in a kind

of ball. Until it had completely escaped from its old shell, I was

somewhat puzzled to guess what shape it would eventually assume. The

eyes and antennæ, so soon as they left their old sheaths, commenced,

together with the flabellæ, to work as usual, although as yet they were

still inside the exuvium. This circumstance was distinctly visible by

looking through the side of the half-cast shell.

 

It was a curious and extraordinary sight to see the eyes gradually lose

their brilliancy, and exhibit the filmy, lack-lustre-like appearance of

death, while the act of exuviation was being accomplished. I may add

that the tumbler which held my little captive stood upon a table near

a large window, and that the sloughing operation was watched through a

powerful hand lens.

 

On an after and well-remembered occasion, I saw a moderate-sized

Partane standing on the top of a bush of _Chondrus Crispus_ that grew

in my aquarium. The fronds were attached to a piece of sandstone,

placed uppermost upon a cluster of rock-work, situated, as before

mentioned, in the centre of the vessel, and rising slightly above the

level of the water. Thinking he was planning means of escape, I turned

away for a few moments to procure a simple instrument wherewith to

carry him to a less elevated position. On my return I saw him in the

act of backing out of his shell. It was a singular circumstance that

I should have just risen from the perusal of a talented author, who

informed me that 'the crab hitches one of its claws into some crack or

fissure, and from this point of resistance gives more power in emerging

and withdrawing itself from between the carapace and the tail.'

 

Certainly no statement could more inadequately describe what I had

witnessed in both of my crabs. Not only was the whole operation

performed with perfect ease, but I am much inclined to believe with a

degree of pleasure. For a while one of my crabs stood in juxtaposition

to the shadow of its former self, and rubbed his antennæ and wee

peeping eyes as if awakening from a sleep. He had been lately, there

was no doubt, living in an oppressed state, and might probably have

surveyed things around him somewhat darkly, but now all was bright and

clear again. On turning, the first object that caught his awakened

eye was his cast-off vestment, which he seemed to scan as dubiously

as a grown man would an exhumed pair of boyish corduroys, and mutter

musingly, while stroking his chin, 'Well, come what will, it can never

be my _case_ again.'

 

On taking it in my hand, the Partane felt quite soft and velvetty to

the touch, and exhibited no signs of alarm.

 

Since then I have repeatedly had shells of crabs cast _in smooth glass

globes, containing nothing else but clear salt water_. This fact, in

my opinion, completely subverts the statements of certain writers, who

assert that these animals require extraneous assistance when about to

exuviate.

 

Some writers have questioned the truth of the generally-received

opinion that the new parts of the crab are derived from the old: that,

for instance, a claw is regenerated within a claw, a limb within a

limb, eyes within the eyes, and that on exuviation each is withdrawn

from the pre-existing organ as from a sheath. But my operations tend

fully to confirm the popular and existing belief.

 

There is yet one curious point connected with this subject which

requires explanation, as it is not generally understood. I allude to

the apparent disproportionate smallness of the 'glassy bag,' situated

at the stump, as compared with the size of the regenerated limb, which

is supposed to be folded up within the bag previous to exuviation.

On looking at the newly-formed member, we can scarcely believe it

possible that the transparent case could by any possibility have held

it. The mystery vanishes if the new limb or claw be examined; for,

although in shape it is perfect, even to the most minute particular, it

remains for a certain period comparatively useless to the animal, from

the fact of its being utterly devoid of flesh.

 

The new limb, therefore, can be considered merely as an expanded case,

which, by a wonderful law of nature, becomes slowly filled up and

completed. Immediately after exuviation has taken place, and a claw is

introduced in the place of some mutilated stump, if any one will pull

off the new member, he can readily confirm the truth of what I have

stated, and, moreover, be able to test into how very small bulk the new

limb may be rolled.

 

As the reader may remember, Goldsmith states that the crab casts its

shell 'regularly once a-year, at the beginning of May.' Professor

Owen fixes the date in the month of August. Professor Bell states,

that 'there is no doubt exuviation takes place _annually_ with great

regularity, until the growth is completed, which, in many species, is

not before the animal is many years old.' Another professor, treating

on the same subject, thus writes, 'We are told that all this coat

of mail is _annually_ thrown off in a single piece by the contained

animal,--the great proficient in Chinese puzzles may well be posed at

this greater puzzle.' In fact, all writers whose works I have had

opportunity of examining repeat the statement. Mr. Ball, who writes

from personal observation, apparently confirms beyond a doubt, the

annual moult of Crustacea. This gentleman, we learn, kept a Cray-fish

alive for two years in a vase, and found that _during each year its

exuvium was shed but once_.

 

It may readily be believed, with such a formidable array of contrary

evidence, that I offer my own observations with modesty. But at the

same time, I feel justified in confidently stating that the moult of

the crab, (in its comparatively youthful state, at all events), takes

place not only once, but many times during each year of its existence.

My specimens may, perhaps, be considered exceptions to the general

rule, but the facts I relate cannot by any possibility admit of doubt.

The cast-off shells lie before me as I write.

 

Here is a set of three belonging to the same animal, exhibiting with

marvellous exactness the gradual development of a broken claw. In the

first the member appears very diminutive, in the second it is nearly

twice its size, while in the third it has advanced to its natural form

and bulk. To my regret, I cannot state the exact period that elapsed

between each successive moult, but I am confident that the trio were

cast in the course of a very few months.

 

I may here take the liberty of informing the uninitiated, that the

appearance of the above objects is extremely pleasing; for, as the

exuvium becomes dry, its colour changes to a bright scarlet, somewhat

resembling that which the crab assumes when placed for a time in

boiling water.

 

The next series of specimens, five in number, possess even still

greater interest than the first examples. They were produced by a

youthful _C. mænas_, at the following consecutive intervals:--

 

The first moult took place on 11th April 1858; the second on the 22d

of May following; the third on July the 3d; the fourth on the 30th of

August; and the fifth on the 26th of September in the same year. So

that between the first and second period of exuviation there was an

interval of forty-one days, between the second and third forty-two days

elapsed, between the third and fourth fifty-eight days, but, singular

to state, between the fourth and fifth moult _only twenty-seven days

intervened_.

 

My first impression was, that as the creature grew older, its shell

would be renewed less frequently, and the dates of the sloughings

seemed to support this idea--until the fourth moult. It had occurred

to me that perhaps the operation might be accelerated by the amount

of diet which the crab consumed. In order to test this, I fed the

animal carefully every day, as though he were a prize beast about to be

exhibited at some Christmas show. Nothing loath, he ate of everything

that was placed before him with a gusto that would have done credit to

an alderman. The result was, that the shell was renewed in less than

half the time that elapsed between the preceding moults.