Glimpses of Ocean Life 31

Glimpses of Ocean Life 31

Sea-Urchins are frequently taken in dredging. Several common species,

usually of a small size, are often found among the rocks situated

between tide marks.

 

Into the aquarium no specimens larger than from one to two inches in

diameter should be introduced, and even these require to be closely

watched, for if afflicted with a fatal illness, I know of no animal

whose remains sooner taint the water. Almost before life is extinct,

the Urchin throws out a light-coloured nauseous fluid, that speedily

poisons the surrounding water, and, of course, causes the destruction

of any inhabitants of the tank who may neither have the sense nor

opportunity to inhale copious draughts of fresh air. As a rule, if the

suckers are motionless, or if on touching the animal it is found not to

be adherent to any object, transfer it at once to your 'infirmary' for

further observation.

 

I have always found small specimens to be much more lively and

walkative, (if I may be allowed the __EXPRESSION__) than their more

corpulent brethren.

 

The inflexible, mail-like crust, or shell, as it is commonly called,

of the Echinus is perhaps one of the most marvellous objects on which

the eye can rest. Although at first sight it appears to be a solid

calcareous box, it is in reality composed of several hundred pentagonal

plates,[17] of various sizes, so closely dove-tailed together that

their marks of junction are scarcely perceptible. Upon a superficial

examination we are apt (most erroneously) to consider this wonderful

piece of work to be more elaborate than the wants of the animal demand.

The fact of the Lobster or Crab throwing off its entire shell at

certain seasons, to admit of the increased growth of the animal is a

truly marvellous phenomenon, still, it would more excite our wonder

were we to find that, instead of being cast away at all, the hard,

inelastic envelope which surrounds the bodies of crustaceans was made

to swell or expand proportionately with the soft parts of the animal!

Now, the mosaic-like shell of the Sea-Urchin, though built up, as

before stated, of several hundred pieces, is by a beautiful process

slowly and imperceptibly enlarged correspondingly with the growth of

the animal.

 

[17] In a specimen that I examined, and then carefully took to pieces,

there were exactly 1780 plates.

 

The gradual enlargement of the Echinus shell takes place in the

following manner:--

 

Over the entire surface of the globular shell, spines, and joints of

the living Urchin, there exists a delicate membrane that insinuates

itself between the pentagonal plates above mentioned, and continually

deposits around the edges a certain portion of calcareous matter

(carbonate of lime). The same process being also carried on by the

fleshy covering that surrounds the spines, &c., it must be evident that

so long as the vital power of the animal exists, each plate and spine,

still keeping to its original form, must be daily and hourly augmented

in size until the Sea-Egg has attained its full and mature dimensions.

 

As to how the spines retain their relative position in each plate, as

the latter gradually becomes enlarged, I cannot positively state; but

may be permitted to mention, that, judging from carefully prepared

sections of the plates when submitted to the microscope, each spine

appeared to my eye to be by some singular process urged along in a kind

of groove to its proper place.

 

The hedgehog-like spines that surround the globose body of the

Sea-Urchin are all moveable at the will of the animal,--each prickle

being connected by a ball-and-socket joint to a pearly tubercle, which

acts as the 'socket' on which the 'ball' of the spine revolves. If

the spine be removed, a comparatively smooth surface will be left, on

which are various sized tubercles systematically arranged. Situated at

regular intervals between the tubercles are ten broad bands, disposed

in pairs, and containing many hundreds of very minute perforations, or

ambulacral orifices, as they are generally termed by naturalists.

 

Through these apertures issue numerous sucker-like feet, closely

resembling those of the Star-fish, but endowed with far greater powers

of contraction and extension.

 

The number of suckers is very great. In an Urchin measuring exactly

three inches in diameter, by aid of a hand lens, I counted no less than

3300 pores in the ten avenues. Now, these pores are always situated in

pairs, and as each sucker occupies a pair of pores, it will give 1650

as the total amount of suckers.

 

There is no doubt that it is almost entirely by means of these curious

organs that the Sea-Urchin is enabled to move about from place to

place, although no less an authority than Professor Agassiz asserts

to the contrary. 'How, in fact,' says this author, 'could these small

tentacula, situated as they generally are in that part of the body

which is never brought into contact with the ground when the animal

moves, and overhung by calcareous solid spines--how, I ask, could

these flexible tubes be used as organs of motion? It is an undeniable

fact, and I have often observed it myself, that _it is with their

spines the Echini move themselves, seize their_ _prey, and bring it

to their mouths_ by turning the rays of their lower edge in different

directions. But the correction of an error respecting the functions

of the ambulacral tubes does not solve the problem relating to their

nature and use. This problem we are yet unable to solve, as we know

nothing more respecting them than that they are connected with the

aquiferous system.'

 

Many other writers, among whom is Professor Forbes (from whose work

on Star-fishes I have transferred the foregoing extract), assert, in

opposition to the great Swiss naturalist, that the Echinidæ move by

the joint action of their suckers and spines. 'The argument,' says the

great British naturalist, 'against the suckers being organs of motion,

founded on their position above as well as below, would equally apply

to the spines, to which organs Professor Agassiz has attributed all

progressive powers in these animals.'

 

The fact is now so well established, that it is scarcely necessary

for the writer to state, that from personal observation he can fully

confirm the evidence of Professor Forbes relative to the functions

of the suckers of the Sea-Urchins. But although that talented author

entertained no doubt as to the organs in question being powerful

locomotive agents, he evidently seems to have felt himself unable to

suggest any purpose they could possibly serve when situated on the back

or upper part of the animal.

 

My own experience incontestibly proves that the suckers in question

are used for precisely the same purpose as those situated in any

other part of the body. I am enabled to state, from having repeatedly

witnessed the phenomenon, that _the Echinus can walk about with equal

facility while lying on its back as in its more natural position_. The

advantage of this power to the animal under certain circumstances will

be apparent upon a little reflection.

 

With regard to the spines, I fancy their purpose is almost solely to

assist the Urchin to burrow in the sand, and to protect it from the

attacks of its enemies. It may be, however, that at particular times

they serve as aids to locomotion, but that their assistance can be, and

is, often dispensed with entirely by the animal, I can most positively

assert.

 

My experiments were always conducted in glass vases, up the smooth,

polished sides of which my specimens frequently advanced. Upon reaching

the surface of the water, I have seen an Urchin roll completely round

and move along on its back, then after a time change its position, and

travel round the circumference of the vessel _while attached by its

side_, the body of the animal being sometimes inverted.

 

At such times as these it must be quite evident that the spines would

be totally useless, and that by the suckers alone did the animal

perform its interesting movements.

 

According to a certain writer, there are some foreign species of the

Echini remarkable for possessing spines, which act both as offensive

and defensive weapons. 'On one occasion' (this writer says) 'when

searching for a fish in the crevice of a coral rock, I felt a severe

pain in my hand, and upon withdrawing it, found my fingers covered

with slender spines, evidently those of the Echinus, of a grey colour,

elegantly banded with black.

 

'They projected from my fingers like well-planted arrows from a target,

and their points being barbed could not be removed, but remained

for some weeks imbedded as black specks in the skin. Its concealed

situation did not permit me to examine this particular Echinus. In some

experiments I approached the spines with so much caution, that had they

been the most finely pointed needles in a fixed state no injury could

have been received from them, yet their points were always stuck into

my hand rapidly and severely.'

 

In addition to those above described, the Sea-Urchin is provided with

other organs, in shape somewhat resembling minute pincers, supported on

fleshy stems, which always keep up an incessant motion when the animal

is in a healthy condition. They are scattered in great numbers over

the surface of the body, among the spines, and around the mouth of the

Urchin.

 

The use of these singular objects--by naturalists termed

Pedicellariæ--is totally unknown. Some writers think they are an

integral part of the Echinus, others describe them as distinct and

parasitic animals. There is good reason to believe that the former will

eventually be proved to be the correct explanation of the matter.

 

Its masticatory apparatus is not the least wonderful portion of the

Sea-Urchin. The teeth, five in number, which may frequently be seen

protruding from the mouth, are of extreme hardness, and of seemingly

disproportionate length. They are not fixed in sockets as ours are,

or they would be speedily worn away by their action on the shelled

mollusca upon which the animal feeds, but fresh substance is added

to each tooth as fast as it is worn away by use, as in the case of

many gnawing animals. 'In order to allow of such an arrangement, as

well as to provide for the movements of the teeth, jaws are provided,

which are situated in the interior of the shell, and these jaws, from

their great complexity and unique structure, form perhaps the most

admirable masticating instrument met with in the animal kingdom. The

entire apparatus removed from the shell consists of the following

parts. There are five long teeth, each of which is enclosed in a

triangular bony piece, that for the sake of brevity we will call jaws.

The five jaws are united together by various muscles, so as to form a

pentagonal pyramid, having its apex in contact with the oval orifice

of the shell, while its base is connected with several bony levers by

means of numerous muscles provided for the movements of the whole.

When the five jaws are fixed together in their natural position, they

form a five-sided conical mass, aptly enough compared by Aristotle

to a lantern, and not unfrequently described by modern writers under

the name of "the lantern of Aristotle." The whole of this complicated

machinery is suspended by muscles from a frame-work fixed in the

interior of the shell, and may often be picked up upon the beach, or

still better exposed _in situ_ in a dead Echinus, by those who would

examine closely this wonderful piece of mechanism.