Glimpses of Ocean Life 15

Glimpses of Ocean Life 15

These interesting investigations, which had been conducted thus far so

satisfactorily, were suddenly brought to a close by the death of my

protégé. This sad event occurred unexpectedly, not from overfeeding, as

some persons may suppose, but from natural causes.

 

Whether increase of food always produces a like effect to that

mentioned, is a point that I hope some of my brother naturalists will

be able to determine. That the moult was accelerated by such means

in my own specimen I have not the slightest doubt, for, on no other

grounds can I explain its unusually speedy occurrence.

 

I may here assure my readers that the above dates may be confidently

relied upon as correct, and also that each exuvium was produced by the

same crab--one specimen only being in the tank during the whole period.

 

Since the foregoing was written, I have again been fortunate enough to

have ocular demonstration of the phenomenon of exuviation, as occurring

in a _Cancer Pagurus_, about as large as a moderate-sized walnut.

 

While watching this crab, it flashed across my mind that it would be a

happy circumstance if by any means _I could arrest the process then

going on before my eyes, while it was yet only half completed_, in

order that others might also be enabled to witness the marvellous act

of exuviation.

 

But how to carry out this scheme was the rub. I knew that--

 

'If 'twere done, then 'twere well it were done quickly.'

 

One minute passed,--two minutes flew by;--the crab would speedily

complete his labours; still was I perplexed.

 

To plump it into fresh water would, I knew, be fatal to the animal,

but not in such a speedy manner as was desirable. Boiling water next

suggested itself, and doubtless would have answered the purpose

effectually, had a supply been near at hand at the time, but such was

not the case. I then thought of _spirits_. Ah! capital idea.... Before

the third minute had passed, I might be seen to rush frantically to the

sideboard, pour _something_ into a glass, then dart back to the tank,

dive down my trembling hand, bring up the poor unfortunate crab, and

drop it into a fatal pool of pure "Glenlivet."

 

The animal appeared to die quickly, and was next day transferred to a

vessel filled with Mythilated spirits. As it luckily turned out, the

whisky answered the intended purpose remarkably well.

 

The preparation in question is, as far as I can discover, _unique_; at

least I have neither read nor heard of another such existing in any

private or public museum in the kingdom.[6]

 

[6] A drawing of this crab will be found on Plate 3.

 

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.

 

[7] The fact of the exuvium of the Hermit-Crab being cast off in two

pieces, and at different periods, I have since confirmed 'many a time,

and oft.'

 

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.

 

 

 

 

CHAPTER VII.

 

Prawns and Shrimps.

 

 

'Men holden ye therefore prophanes

Ye eaten neither shrimps nor pranes,'

 

 

 

 

VII.

 

 

Although abundant at many parts of the Scottish coast, at Cockburnspath

(situated near the mouth of the Frith of Forth) only, has the writer

met with the very beautiful prawn, _Palæmon Squilla_. At this locality

specimens were very frequent in rock-pools situated near the shore, nor

were such difficult to capture. The small net being placed cautiously

over their head, the animals did not show signs of resistance, until

they found themselves, by a sudden jerk of the hand, drawn bodily out

of the water. Then, indeed, unless some degree of skill was used, the

captives would give a powerful spring, and escape, from the confinement

of the net, to the more congenial element from which they had been so

unceremoniously ejected.

 

Prawns (_Palæmonidæ_) exuviate very frequently, in some instances as

often as once or twice a month. No sooner is one coat thrown off and

the Palæmon recovered from the weakness which the process occasions,

than it commences, at first at intervals, and then almost incessantly

day and night, preparations for a renewal of the wonderful operation.

 

Every part of the body--eyes, antennæ, and especially the sub-abdominal

fins, to certain portions of which the ova are attached in groups,

and the lobes of the tail--are submitted to a severe rubbing and

brushing process. The appearance of the prawn at this period is

really most interesting, and, I may also add, amusing. Sometimes the

tail is compressed inwards, beneath the body for a few seconds, and

then suddenly elevated and forced out with donkey-like extravagance

of gesture, the animal the while standing upon its first pair of

forcep-like feet.

 

At the appointed time the shell opens at the back part of the head,

and the prawn becomes gradually freed from its old covering. The

marvellous process completed, like all its crustaceous brethren, the

creature becomes to a certain extent helpless, and if such convenience

be afforded, retires for protection beneath some shell or fragment

of rock, from whence it soon re-appears, and repeats its gymnastic

exercises, which cease, however, for a few days, as soon as the new

coat is sufficiently hardened.

 

The prawn is an extremely interesting occupant of an aquarium, from

the fact of its being constantly on the move, and also on account

of the pretty blue and orange markings of its many-jointed legs,

and the singular transparent appearance of its body. This latter

feature is made still more notable when the animal happens to have the

ova attached, as the latter are opaque, and of a deep brown colour

approaching to black. By the prawn the act of exuviation seems to be

considered an event of no slight importance, and, although occurring

so frequently, is fraught with danger. Specimens oftener die at the

moulting time than at any other. In fact, unless I am very much

mistaken, they are then subject to some peculiar disease, which is

apt to prove fatal. At all events, several of my little captives,

after having performed their gymnastic movements (before alluded

to) for several days, turned sickly, and died. The commencement of

their illness was always denoted by a small, white, opaque dot that

mysteriously appeared in the centre of the body. This object speedily

increased in size, until it eventually spread over the entire animal.

Then, no longer diaphanous, the flesh of the prawn seemed composed of a

solid substance not unlike lime or pounded chalk.