The rotifera are either naked or covered with a sheath, and many

inhabit a tube formed by themselves, attached by its lower end to some

water-plant, and open at the summit, from which the animal protrudes

when it would exercise its active instincts, and into which it retires

for repose from labour or for refuge from alarm. The majority, however,

have a furcated foot, which is often capable of contraction by a set of

telescopic sheathings or false joints, and by which they are enabled

to secure a hold of the minute stems of water-plants. This is their

ordinary position when keeping their wheels in action for a supply of

food or of water; but they have no difficulty in letting go their hold,

and either creeping along by alternate contractions and extensions or

swimming away in search of a new attachment. From the neck projects

a telescopic spur, supposed to be an organ of respiration, and just

below this are seen two minute red specks, supposed to be eyes. The

first rotifer was discovered by Leeuwenhoek, in 1702; now more than 180

species are known, and new discoveries are constantly adding to their

numbers. They are chiefly found in sweet water, but some are inhabitants

of the sea, as, for instance, the _Synchæta baltica_, remarkable for

its luminous powers. It measures about 1/125 of an inch in length, and

but 1/350 in width, so that it is invisible to the sharpest unassisted

sight: but when viewed through a microscope, it appears as a beautiful

and richly organised creature, clear as glass and perfectly colourless,

except that its stomach is usually distended with yellow food, and that

it carries a large red eye, which glitters like a ruby.

[Illustration: Philodina roseola.--(Highly magnified.)

_a._ Respiratory tube.

_b._ Alimentary canal.

_c._ Cellular mass.

_d._ Terminal intestinal pouch.

_e._ Anal orifice.

]

"Its motions too," says Mr. Gosse, "are all vivacious and elegant. It

shoots rapidly along or circles about in giddy dance, in company with its

fellows, sometimes near the surface, at others just over the bottom of

the vase in which it is kept. Occasionally the foot with the tiny toes

is drawn up into the body and then suddenly thrown down, and bent up

from side to side as a dog wags his tail. Sometimes the rotatory organs

are brought forward and then spasmodically spring back to their ordinary

position, when the little creature shoots forward with redoubled energy.

In all its actions it displays vigour and precision, intelligence and

will."

CHAP. XV.

MOLLUSCS.

The Molluscs in general.--The Cephalopods.--Dibranchiates

and Tetrabranchiates.--Arms and Tentacles.--Suckers.--Hooked

Acetabula of the Onychoteuthis.--Mandibles.--Ink Bag.--Numbers

of the Cephalopods.--Their Habits.--Their Enemies.--Their Use to

Man.--Their Eggs.--Enormous size of several species.--The fabulous

Kraken.--The Argonaut.--The Nautili.--The Cephalopods of the

Primitive Ocean.--The Gasteropods.--Their Subdivisions.--Gills of the

Nudibranchiates.--The Pleurobranchus plumula.--The Sea-Hare.--The

Chitons.--The Patellæ.--The Haliotis or Sea-Ear.--The Carinariæ.--The

Pectinibranchiates.--Variety and Beauty of their Shells.--Their

Mode of Locomotion.--Foot of the Tornatella and Cyclostoma.--The

Ianthinæ.--Sedentary Gasteropods.--The Magilus.--Proboscis of the

Whelk.--Tongue of the Limpet.--Stomach of the Bulla, the Scyllæa,

and the Sea-Hare.--Organs of Sense in the Gasteropods.--Their

Caution.--Their Enemies.--Their Defences.--Their Use to

Man.--Shell-Cameos.--The Pteropods.--Their Organisation and Mode

of Life.--The Butterflies of the Ocean.--The Lamellibranchiate

Acephala.--Their Organisation.--Siphons.--The Pholades.--Foot of

the Lamellibranchiates.--The Razor-Shells.--The Byssus of the

Pinnæ.--Defences of the Bivalves.--Their Enemies.--The common

Mussel.--Mussel Gardens.--The Oyster.--Oyster Parks.--Oyster Rearing

in the Lago di Fusaro.--Formation of new Oyster Banks.--Pearl-fishing

in Ceylon.--How are Pearls formed?--The Tridacna gigas.--The Teredo

navalis.--The Brachiopods.--The Terebratulæ.--The Polyzoa.--The

Sea-Mats.--The Escharæ.--The Lepraliæ.--Bird's Head Processes.--The

Tunicata.--The Sea-Squirts.--The Chelyosoma.--The Botrylli.--The

Pyrosomes.--The Salpæ.--Interesting Points in the Organisation of the

Tunicata.

Simple or compound, free or sessile, peopling the high seas or lining the

shores, the marine Molluscs, branching out into more than ten thousand

species, extend their reign as far as the waves of ocean roll. Though

distinguished from all other sea-animals by the common character of a

soft unarticulated body, possessing a complicated digestive apparatus,

and covered by a flexible skin or mantle, under or over which a

calcareous shell is generally formed by secretion, yet their habits are

as various as their forms. Some dart rapidly through the waters, others

creep slowly along, or are firmly bound to the rock; in some the senses

are as highly developed as in the fishes, in others they are confined to

the narrow perceptions of the polyp. Many are individually so small as

to escape the naked eye, others of a size so formidable as to entitle

them to rank among the giants of the sea; some are perfectly harmless and

unarmed, others fully equipped for active warfare. It is evident that

creatures so variously gifted, and consequently so widely dissimilar

in structure, cannot possibly be grouped together in one description,

and that each of the four orders, Cephalopoda, Gasteropoda, Pteropoda,

and Acephala (Lamellibranchiates, Brachiopods, Polyzoa, and Tunicata),

into which they have been subdivided, must be separately brought before

the reader, in order to give him a clear and faithful picture of their

organisation and mode of life.

The Cephalopods are the most perfect specimens of the molluscan type,

as the decapods are the first among the crustaceans. These remarkable

creatures consist of two distinct parts: the trunk or body, which,

in form of a sack, open to the front, encloses the branchiæ and

digestive organs, and the well-developed head, provided with a pair of

sharp-sighted eyes, and crowned with a number of fleshy processes, arms

or feet, which encircle and more or less conceal the mouth. It is to this

formation that the cephalopod owes its scientific name, for as the feet

grow from the circumference of the mouth, it literally creeps upon its

head.

All the cephalopods are marine animals, and breathe through branchiæ or

gills. These are concealed under the mantle, in a cave or hollow, which

alternately expands and contracts, and communicates by two openings with

the outer world. The one in form of a slit serves to receive the water;

the other, which is tubular, is used for its expulsion.

According to the different number of their gills, the cephalopods are

divided into two groups. The first, to which the poulp and common

cuttle-fish belong, and which comprises by far the majority of living

species, has only two sets of gills; while the second, which, in the

present epoch, is only represented by a few species of Nautilus, has

four, two on each side, according to the number of their arms or

feet--for these remarkable organs serve equally well for prehension

or locomotion. The first group is again subdivided into two orders,

Octopods and Decapods, the former having only eight sessile feet, while

the latter possess an additional pair of elongated tentacles, which serve

to seize a prey that may be beyond the reach of the ordinary feet, and

also to act as anchors to moor them in safety during the agitations of a

stormy sea.

[Illustration: Poulp (Octopus).]

Both the arms and tentacles are furnished with suckers disposed along

the whole extent of the inner surface of the former, but generally

confined to the widened extremities of the latter, where they are closely

aggregated on the inner aspect.

[Illustration: Calamary.]

In all the octopods the suckers are soft and unarmed. Every sucker

is composed of a circular adhesive disk, which has a thick fleshy

circumference and bundles of muscular fibres radiating towards the

circular orifice of an inner cavity.

This widens as it descends, and contains a cone of soft substance, rising

from the bottom of the cavity, like the piston of a syringe. When the

sucker is applied to a surface for the purpose of adhesion, the piston,

having previously been raised so as to fill the cavity, is retracted,

and a vacuum produced, which may be still further increased by the

retraction of the plicated central portion of the disk. So admirably are

these air-pumps constructed, and so tenacious is their grasp, that, when

they have once seized or fixed upon a prey, it cannot possibly disengage

itself from their murderous embrace.

In many of the decapods, who, generally seeking their prey in the deeper

waters, have to contend with the agile, slippery, and mucus-clad fishes,

more powerful organs of prehension have been superadded to the suckers.

Thus, in the Calamary the base of the piston is enclosed by a horny

hoop, the margin of which is developed into a series of sharp-pointed

curved teeth; and in the still more formidable Onychoteuthis each hoop

is produced into the form of a long, curved, and sharp-pointed claw

(_f_), which the predacious mollusc presses firmly into the flesh of its

struggling victim, and then withdraws by muscular contraction.

[Illustration: Section of an arm and suckers of a Poulp.

_e._ Soft and tumid margin of the disk.

_g._ Circular aperture.

]

Besides the hooked acetabula, a cluster of small simple unarmed suckers

may be observed at the base of the expanded part. These add greatly to

the animal's prehensile powers, for when they are applied to one another

(_e_), the tentacles are firmly locked together at that point, and the

united strength of both the elongated peduncles can be applied to drag

towards the mouth any resisting object which has been grappled by the

terminal hooks. There is no mechanical contrivance which surpasses the

admirable structure of this natural forceps.

The size of the arms and the arrangement of the suckers differ

considerably in the various species. In the octopods or poulps, which

generally lead a more sedentary creeping life, and, hidden in the

crevices of rocks, await the passing prey, the arms, in accordance

with their wants, are with rare exceptions longer, more muscular,

and stronger, than in the actively swimming decapods, where the two

elongated tentacles or peduncles are the chief organs of prehension. In

some species we find the arms distinct--in others they are united by a

membrane. Some have a double row of suckers on each arm, others four

rows, others again but one. So wonderful are the variations which nature,

that consummate artist, plays upon a single theme--so inexhaustible are

the modifications she introduces into the formation of numerous species,

all constructed upon the same fundamental plan, and all equally perfect

in their kind.

[Illustration: Arms and Tentacles of an Onychoteuthis.

_e._ Parts joined together by the mutual

apposition of the armed suckers.

_f._ Terminal expanded portions bearing

the hooks.

]

Thus well provided with the means for seizing and overcoming the

struggles of a living prey, the Cephalopods likewise possess adequate

weapons for completing its destruction; for their mouth is most

formidably armed with two horny or calcareous jaws, shaped like the

mandibles of a parrot, playing vertically on each other, and enclosing a

large fleshy tongue bristling with recurved horny spines. Hard, indeed,

must be the crab which can resist this terrible beak; and when the

cuttle-fish has once fixed on the back of a fish, though much larger

and stronger than himself, it is in vain for the tortured victim to fly

through the water: he carries his enemy with him till he sinks exhausted

under his murderous fangs.

Besides their arms, by help of which the Cephalopods either swim or

creep, the forcible expulsion of the water through the respiratory tube

or infundibulum serves them as a means of locomotion in a backward

direction. By those which have an elongated body and comparatively strong

muscles, this movement is performed with such violence that they shoot

like arrows through the water, or even like the flying-fish perform a

long curve through the air.

Thus Sir James Ross tells us, that once a number of cuttle-fish not only

fell upon the deck of his ship, which rose fifteen or sixteen feet above

the water, and where more than fifty were gathered, but even bolted

right over the entire breadth of the vessel, like a sportsman over a

five-barred gate. Finally, the fin-like expansion of their mantle renders

the nimble decapods good service in swimming. In the Sepias this finny

membrane runs along the sides of the body, while in the Calamary it forms

a kind of terminal paddle.

[Illustration: Sepia.

_b._ Finny membrane running along the sides of the body.

_c._ Arms with four rows of suckers.

_d._ Elongated retractile tentacles.

_e._ Eyes.

]

It might be supposed that the dibranchiate cephalopods, by their

swiftness, their arms, and their powerful jaws, were sufficiently

provided with means of attack or defence; but it must be remembered

that their body is soft and naked, and that, though well armed in

front, they may readily be attacked in the rear. To afford them the

additional protection they required, nature, ever ready to minister to

the real wants of her children, has furnished them with an internal bag

communicating with the respiratory tube, and secreting a large quantity

of an inky fluid, which they can squirt out with force in the face of

their foe, and which, mixing readily with the water, envelops them in

an opaque cloud, and thus screens them from pursuit. But this inky

fluid, thus useful to its owner, is often the cause of his destruction

by man, who applies it to his own purpose, for the Italian pigment,

called sepia, so invaluable to painters in water-colours, is prepared

from the inspissated contents of the ink bag of a cuttle-fish. Such

is the durability of this colour that even the inky fluid of fossil

species has been found to retain its chromatic property. We are told that

grains of wheat buried with Egyptian mummies three thousand years ago

have germinated; but it is surely still more astonishing that an animal

secretion, the origin of which is lost in the dark abyss of countless

ages, should remain so long unaltered.

The cephalopods are scattered in vast numbers over the whole ocean,

from the ice-bound shores of Boothia Felix to the open main; they seem,

however, to be most abundant in temperate latitudes. Some, like the

common poulp, constantly frequent the coasts, creeping among the rocks

and stones at the bottom; others, like the Cirroteuthis and Ommastrephes,

roam about the high seas at a vast distance from the land.

They are generally nocturnal or vespertine in their habits; they abound

towards evening and at night on the surface of the seas, but sink to a

greater depth, or retire into the crevices of the rocks, as soon as the

sun rises above the horizon. Some are of a recluse disposition, and lead

a solitary life in the anfractuosities of the littoral zone; others, of a

more social temper, wander in large troops along the shores, or over the

vast plains of ocean.

Possessing the organs of sense, and the means of locomotion in a high

degree of development, the cephalopods may naturally be expected to be

far more active and intelligent than the inferior orders of the molluscs.

On moonlight nights, among the islands of the Indian Archipelago,

Mr. Adams frequently observed the Sepiæ and Octopi in full predatory

activity, and had considerable difficulty and trouble in securing them,

so great was their restless vivacity, and so vigorous their endeavours to

escape. "They dart from side to side of the pools," says the naturalist

in his entertaining and instructive account of his journey to those

distant gems of the tropical sea, "or fix themselves so tenaciously to

the surface of the stones by means of their suckers that it requires

great force and strength to detach them. Even when removed and thrown

upon the sand, they progress rapidly, in a sidelong shuffling manner,

throwing about their long arms, ejecting their ink-like fluid in sudden

violent jets, and staring about with their big shining eyes (which at

night appear luminous, like a cat's) in a very grotesque and hideous

manner."

At the Cape de Verd islands, Mr. C. Darwin was also much amused by the

various arts to escape detection used by a cuttle-fish, which seemed

fully aware that he was watching it. Remaining for a time motionless, it

would then stealthily advance an inch or two, like a cat after a mouse,

and thus proceeded, till, having gained a deeper part, it darted away,

leaving a dusky train of ink, to hide the hole into which it had crawled.

All the cephalopods are extremely voracious; they destroy on shallow

banks the hopes of the fishermen, devour along the coasts and on the

high seas countless myriads of young fish and naked molluscs, and kill,

like the tiger, for the mere love of carnage. Thus they would become

dangerous to the equilibrium of the seas if nature, to counterbalance

their destructive habits, had not provided a great number of enemies for

the thinning of their ranks.

They form the almost exclusive food of the sperm-whales, and the

albatross and the petrels love to skim them from the surface of the

ocean. Tunnies and bonitos devour them in vast numbers, the cod consumes

whole shoals of squids, and man, as I have already mentioned, catches

many millions to serve him as a bait for this valuable fish.

At Teneriffe, in the Brazils, in Peru and Chili, in India and China,

various species of cephalopods are used as food. Along the eastern shores

of the Mediterranean, the common sepia constitutes now, as in ancient

times, a valuable part of the food of the poor. "One of the most striking

spectacles," says Edward Forbes, "is to see at night on the shores of the

Ægean the numerous torches glancing along the shores, and reflected by

the still and clear sea, borne by poor fishermen, paddling as silently

as possible over the rocky shallows in search of the cuttle-fish, which,

when seen lying beneath the water in wait for his prey, they dexterously

spear, ere the creature has time to dart with the rapidity of an arrow

from the weapon about to transfix his soft but firm body."

Animals exposed to the attacks of so many enemies must necessarily

multiply in an analogous ratio. Their numerous eggs are generally brought

forth in the spring. In the species inhabiting the high seas, they float

freely on the surface, carried along by the currents and winds, and form

large gelatinous bunches or cylindrical rolls, sometimes as large as a

man's leg.

The eggs of the littoral cephalopods appear in the form of dark-coloured,

roundish or spindle-shaped bodies, of the size and colour of grapes, and

hanging together in clusters. They are soft to the touch, with a tough

skin, resembling india-rubber; one end is attenuated into a sort of point

or nipple, and the other prolonged into a pedicle, which coils round

sea-weed or other floating objects, and serves to fix the berry-like bag

in its place. At an early stage these "sea-grapes," as they are called

by the fishermen, contain a white yolk enclosed in a clear albumen, and

nearer maturity the young cuttle-fish may be found within in various

stages of formation, until finally, hatched by the heat of the sun, it

emerges from the husk perfectly formed, and launches forth into the water.

[Illustration: Ova of the Cuttle-fish.]

Some species of cephalopods are only about the size of a finger, while

others attain an astonishing size. Banks and Solander, in Cook's first

voyage, found the dead carcass of a gigantic cuttle-fish floating between

Cape Horn and the Polynesian islands. It was surrounded by aquatic birds,

which were feeding on its remains. From the parts of this specimen, which

are still preserved in the Hunterian collection, and which have always

strongly excited the attention of naturalists, it must have measured at

least six feet from the end of the tail to the end of the tentacles.

Near Van Diemen's Land, Péron saw a sepia about as big as a tun

rolling about in the waters. Its enormous arms had the appearance of

frightful snakes. Each of these organs was at least seven feet long, and

measured seven or eight inches round the base. These well authenticated

proportions are truly formidable, and fully justify the dread and

abhorrence which the Polynesian divers entertain of those snake-armed

monsters of the deep; but not satisfied with reality, some writers have

magnified the size of the cephalopods to fabulous dimensions. Thus

Pernetti mentions a colossal cuttle-fish, which, climbing up the rigging,

overturned a three-masted ship; and Pliny notices a similar giant, with

arms thirty feet long and a corresponding girth. But all this is nothing

to the Norwegian kraken, a mass of a quarter of a mile in diameter, and a

back covered with a thicket of sea-weeds. When it comes to the surface,

which seems to be but rarely the case, it raises its arms mast-high into

the air, and, having enjoyed for a time the lovely daylight, sinks slowly

back again into abysmal darkness. Fishermen are said to have landed on

a kraken, and to have kindled a fire upon the supposed island for the

purpose of cooking their dinner. But even a kraken, thick-skinned as he

may be, does not like his back to be converted into a hearth, and thus

it happened that the treacherous ground gave way under the mistaken

mariners, and overwhelmed them in the waters. Strange that the oriental

tale of Sinbad the sailor should thus be re-echoed in the wild legends of

the north.

All the dibranchiate cephalopods are destitute of an outward shell, with

the sole exception of the Spirula, a small species chiefly found in the

South Sea, and of the far more renowned Argonaut, which poets, ancient

and modern, have celebrated as the model from which man took the first

idea of navigation. Its two sail-like arms expanding in the air, and the

six others rowing in the water, the keel of its elegant shell is pictured

as dividing the surface of the tranquil sea. But as soon as the wind

rises, or the least danger appears, the cautious argonaut takes in his

sails, draws back his oars, creeps into his shell, and sinks instantly

into a securer depth. Unfortunately there is not a word of truth in

this pleasing tale. Like the common octopus, the argonaut generally

creeps about at the bottom of the sea, or when he swims, he places his

sails close to his shell, stretches his oars right out before him, and

shoots backwards like most of his class by expelling the water from his

respiratory tube.

[Illustration: Argonaut.]

As he sits loosely in his shell, he was supposed by some naturalists

to be a parasite enjoying the house of the unknown murdered owner; but

this is perfectly erroneous, as the young in the egg already show the

rudiments of the future shell, and the full-grown animal repairs by

reproduction any injury that may have happened to it.

[Illustration: Pearly Nautilus.]

The tetrabranchiate cephalopods, or Nautili, are very differently

constructed from their dibranchiate relations. Here, instead of mighty

muscular arms, furnished with suckers or raptorial claws, we find a

number of small, sheathed, and retractile tentacles (_f_), surrounding

the mouth in successive series, and amounting to little short of a

hundred. The head is further provided with a large muscular disk (_g_),

which, besides acting as a defence to the opening of the shell, serves

also in all probability as an organ for creeping along the ground,

like the foot in the Gasteropods. The mandibles are strengthened by a

dense calcareous substance fit to break up the defensive armour of the

crustacean or shell-fish on which the animal feeds. There is no ink-bag,

no organ of hearing, and the eyes (_h_) are pedunculated, and of a more

simple structure. The handsome pearl-mother and spirally wound shell

is divided by transverse partitions (_a_), perforated in the centre,

into numerous chambers (_b_). The animal takes up its abode in the

foremost and largest (_b′_), but sends a communicating tube or siphon

(_c_) through all the holes of the partitions to the very extremity of

the spirally wound shell. Though the empty conch was frequently found

swimming on the waters of the Indian Ocean, or cast ashore on the

Moluccas or New Guinea, yet it was only in 1829 that the animal was known

with any certainty, one having been caught alive by Mr. George Bennett,

near the New Hebrides, which, preserved in spirits, is now in the museum

of the College of Surgeons. Since then three different species have been

found to abound in the waters of the above-named archipelago, of New

Caledonia, and of the Feejee and Solomon Islands, where they principally

sojourn among the coral reefs at depths of from three to six fathoms.

They usually remain at the bottom of the water, where they creep along

rather quickly, supporting themselves upon their tentacula, with their

head downwards and the shell raised above. After stormy weather, as it

becomes more calm, they may be seen in great numbers floating upon the

surface of the sea with the head protruded, and the tentacula resting

upon the water, the shell at the same time being undermost; they remain,

however, but a short time sailing in this manner, as they can easily

return to their situation at the bottom of the sea, by merely drawing in

their tentacles and upsetting the shell. They are caught in baskets by

the natives, who eat them roasted as a great delicacy.

What renders these animals peculiarly interesting is the circumstance

that they are the only living representatives of a class which once

filled in countless numbers the bosom of the primeval ocean, and whose

fossil remains (Orthoceratites, Ammonites) furnish the naturalist with

a series of historical documents, attesting the unmeasured age of our

planet. What are the ruins, thirty or forty centuries old, that speak of

the vanished glories of extinguished empires to these wonderful medals

of creation that lead our thoughts through the dim vista of unnumbered

centuries to the fathomless abyss of the past.

* * * * *

In point of development of organisation the Gasteropods or snails

rank immediately after the Cephalopods. They also have a head plainly

distinguishable from the rest of the body, and to which two brilliant

black eyes give an animated expression. But their nervous system is far

less developed, and while the lively cephalopod is able to swim about,

and rapidly to seize a distant prey, almost all the gasteropods creep

slowly along upon a flat disk or foot situated below the digestive

organs, a formation to which they owe their name of gasteropods or

stomach-footers.

The marine snails are divided into several groups according to the

different position and arrangement of their gills. In some species these

organs form naked or free-swimming tufts on the back (Nudibranchiata)

but generally they are variously disposed either in special cavities or

under the folds of the mantle. Thus in the Inferobranchiata they are

arranged under its inferior border on both sides of the body, or upon

one side only, while in the Tectibranchiata they are placed, as in the

Nudibranchiata, upon the dorsal aspect of the body, but are protected by

a fold of the skin. In the Cyclobranchiata they form a fringe round the

margin of the body, between the edge of the mantle and the foot, and in

the Scutibranchiata and Pectinibranchiata they are pectinated, or shaped

like the teeth of a comb, and placed in a large hollow chamber, which

opens externally at the side of the body or above the head.

[Illustration: Tiara.]

[Illustration: Glaucus.]

[Illustration: Scyllæa.]

Nothing can be more elegant or various than the form and arrangement of

the gills in most of the nudibranchiate gasteropods. In the Glauci and

Scyllææ, we see at each side of the elongated body long arms branching

out into tufty filaments; in the Briarei a hundred furcated stems serve

for the aëration of the blood. On the back of the Eolides the gills are

arranged in rows; in the Dorides they form a wreath or garland round the

posterior intestinal aperture.

The beauty of these animals corresponds with their charming mythological

names, for every part of them which is not sparkling like the purest

crystal shines with the liveliest colours, red, yellow, or azure. Some

inhabit the coasts, where they creep along upon a well-developed foot,

others live in the deep waters, where they cling to the stems of floating

sea-weed with a narrow and furrowed foot, or swim upon their back,

using the borders of the mantle and of the branchiæ as oars. Though

chiefly living in the warmer latitudes, they are found in every sea,

and many interesting species inhabit the British waters: such as the

Sea-lemon (_Doris tuberculata_), which, when its horns and starry wreath

of branchiæ are concealed, bears a curious resemblance in size, form,

colour, and warty surface to the half of a citron divided longitudinally;

the exquisite _Eolis coronata_, whose crowded clusters of branchial

papillæ are radiant with crimson and cerulean tints; and the crested

Antiopa, whose transparent breathing organs are tipped with silvery white.

[Illustration: Eolis.]

Though they have no shell to cover them, the Nudibranchiata are not left

defenceless to the mercy of their enemies. The transparency of their

body is a cause of safety to many of them. Some conceal themselves under

stones or among the branches of the madrepores, and some on contracting

cast off a part of their mantle, which they leave in possession of their

hungry foe, while they themselves make their escape.

Among the British Inferobranchiata we find the rare golden or

orange-coloured _Pleurobranchus plumula_, thus named from its branchiæ

projecting like a plume from between the mantle and foot in crawling; and

among the Tectibranchiata the common sea-hare (_Aplysia punctata_), which

resembles a great naked snail; its back opening with two wide lobes,

which can be expanded or closed over the opening at the animal's will.

When open, they expose to view on the right side the finely fringed and

lobed branchiæ, seated in a deep hollow beneath a fold of the mantle.

The uncomely creature glides along over the stones upon its flat fleshy

foot and up the slender stems of sea-weeds by bringing the borders of the

same locomotive apparatus to meet around the stem, thus tightly grasping

it as if enclosed in a tube. While progressing, the fore part is poked

forward as a narrow neck furnished with two pair of tentacles, one pair

of which, standing erect and being formed of thin laminæ, bent round so

as to bring the edges nearly into contact, look like the ears of the

timid quadruped, from which the Aplysia has derived its common name. The

colour is a dark-brownish purple studded with rings and spots of white.

On being disturbed, the sea-hare pours out from beneath the mantle-lobes

a copious fluid of the richest purple hue, which however quickly fades,

and is of no value in the arts.

More than forty species of Aplysiæ are known, most of them inhabitants of

the warmer seas. The acrid humour exuded by the depilatory aplysia, or

_Aplysia depilans_, of the Mediterranean is still supposed by the Italian

fishermen to occasion the loss of the hair, and was used by the ancient

Romans in the composition of their venomous potions--though it is by no

means poisonous. Such are the prejudices resulting from the propensity of

man to associate evil qualities with an unprepossessing appearance.

[Illustration: Chiton squamosus.]

To the Cyclobranchiate order belong the Limpets and the Chitons. The

latter, which are the only multivalve shells among the Gasteropods, are

spread in more than two hundred species over every shore from Iceland to

the Indies, but they are particularly abundant on the coasts of Peru and

Chili. Some of the smaller species inhabit our coasts, where they may be

found adhering to stones near low water mark. They are coated with eight

transverse shelly plates, folding over each other at their edges like the

plates of ancient armour, and inserted into a tough marginal band, so as

to form a complete shield to the animal. Thus encased in coat of mail,

the chitons have the power of baffling the voracity of their enemies by

rolling themselves up into a ball like the wood-louse or the armadillo:

they are also able to cling with such tenacity to the rock that it is

difficult to detach them without tearing them to pieces. The Limpets, or

Patellæ, likewise attach their shield-like shell so firmly to a hard body

that it requires the introduction of a knife between the shell and the

stone to detach them. It has been calculated that the larger species are

thus able to produce a resistance equivalent to a weight of 150 pounds,

which, considering the sharp angle of the shell, is more than sufficient

to defy the strength of a man to raise them. They often congregate in

large numbers in one place, and an old writer compares them to nail-heads

struck into the rock. More than a hundred species are known; one of

which, the _Patella cochlear_ of the Cape, is almost invariably found

squatting upon the shell of another species of limpet. The finest and

largest varieties abound on the shores of the Oriental seas and the

coasts of the Mediterranean, but several of the smaller species are very

numerous in our littoral or sub-littoral zone, where they either feast

on the green sea-weeds that we find covering at ebb-tide the stones with

a thin emerald layer, or upon the coarser olive-coloured algæ. Thus

_Patella pellucida_ and _Patella lævis_, both remarkable for longitudinal

streaks of iridescent colours on an olive-shell, may generally be found

feeding either on the broad fronds or on the roots and stems of the

Laminariæ, or Oar-weeds. To their labours may indeed be partly attributed

the annual destruction of these gigantic algæ, for, eating into the lower

part of the stems, and destroying the branches of the roots, they so far

weaken the base that it is unable to support the weight of the frond, and

thus the plant is detached and driven on shore by the waves.

[Illustration: Limpet and Shell.]

The beautiful Sea-ear, or Haliotis, is the chief representative of the

scutibranchiate gasteropods. The flattened shell, perforated with small

holes on one side, is characterised by a very wide mouth or aperture, the

largest in any shell except the limpet. The outside is generally rough,

or covered with marine substances; the inside presents the same enamelled

appearance as mother-of-pearl, and exhibits the most beautiful colours.

The holes with which the shell is perforated serve to admit water to the

branchiæ, and are formed at regular intervals as it increases in size.

The foot is very large, having the margin fringed all round, and is able,

like that of the chiton or the limpet, to cling firmly to the rock. More

than seventy species of Haliotis are known, the greater part occurring in

the Pacific Ocean.

[Illustration: Haliotis.

_c._ Series of perforations.

_d._ Eye peduncles.

_e._ Tentacles.

_g._ Foot.

]

[Illustration: Carinaria.]

To the scutibranchiate gasteropods also belong the strangely formed

Carinariæ, which seem to be made up of disjointed parts. The gills (_g_)

project from under a thin vitreous shell (_f_), which projects from the

dorsal surface, and has a form not unlike that of the Argonaut or of a

Phrygian cap. The foot (_b_) is not formed for creeping, but constitutes

a muscular vertical paddle or fin, that serves them for swimming on the

back, and is furnished with a sucking disk (_c_), with which they are

enabled to attach themselves to floating objects.

The Pectinibranchiata comprise all the spiral univalve shells, and are by

far the most numerous of all the gasteropods, as their species are not

counted by hundreds, but by thousands. If their calcareous garment could

be drawn out, it would be found to consist of a tube gradually widening

from the apex to the base; but what an immense variety of form and

ornaments, what a prodigality of splendid tints, has not Nature spread

over this interminable host! The same fundamental idea appears to us in

thousands of modifications, one yet more elegant and capricious than the

other. Thus the passion of the shell collector is as conceivable as that

of the lover of choice flowers, and when we read that rich tulip-amateurs

have given thousands of florins for one single bulb, we cannot wonder

that many of the Volutes, Cones, Mitres, and Harps, are worth several

times their weight in gold; that more than a hundred pounds have been

paid for a Chinese wentle-trap, and that the _Cypræa aurora_, which the

Polynesian chiefs used to wear about the neck, is valued at thirty or

forty guineas.

[Illustration: Orange Cone-Shell.]

[Illustration: Mitre-Shells.]

[Illustration: Harp-shell.]

The mode in which these beautifully painted structures are formed is very

similar to what takes place among bivalve shells. They are secreted by

the glandular margin of the mantle or soft skin which clothes the upper

part of the body of the snail, and their form depends on the shape of the

body they are destined to cover, while the outline of the border is alike

regulated by that of the mantle. In the border of the mantle are placed

the glands through which colouring matter is added to the lime of which

the shell consists, and here also the whole of the outer coat of the

shell is formed by constant annual additions to the lip. The after-growth

of the shell proceeds, layer over layer, from the general surface of the

mantle, so that the calcareous robe constantly increases in thickness

with the age of the animal.

[Illustration: Chinese Wentle-trap.--(Scalaria pretiosa.)]

However different the form of a shell may be, its use is invariably

the same, affording the soft-bodied animal a shield or retreat against

external injuries. In this respect it is not uninteresting to remark that

those species which inhabit the littoral zone, and are most exposed to

the violence of the waves, have a stronger shell than those which live in

greater depths, and that the fresh-water molluscs have generally a much

more delicate and fragile coat than those which live in the ocean. The

greater the necessity of protection the better has Nature provided for

the want. Thus most of the gasteropods, besides possessing a stone-hard

dwelling, are also furnished at the extremity of the foot with an

operculum, or calcareous lid, which fits exactly upon the opening of

their house, and closes it like a fortress against the outer world. But

no animal exists that is safe against every attack, for the large birds

sometimes carry the ponderous sea-snails, whose entrance they cannot

force with their beaks, high up into the air, and let them fall upon the

rocks, where they are dashed to pieces.

The ordinary mode of locomotion of the testaceous sea-snails is by

creeping along on their foot: those that have a very heavy house to

carry, such as the Cassis or the Pteroceras, generally move along

very slowly, while others, such as the Olivæ, that are possessed of a

comparatively strong and broad foot, have rapid and lively movements,

and quickly raise themselves again when they have been overturned. The

Strombidæ and Rostellariæ place their powerful and elastic foot under

the shell in a bent position, when suddenly by a muscular effort they

straighten that organ and roll and leap over and over. The structure

of the foot of the _Tornatella fasciata_, an inhabitant of our coast,

is most remarkable: beaten incessantly by the waves, in the cavities

of rocks which it frequents, nearly on a level with the surface of the

sea, to the violence of which it is always exposed, it has need of

additional powers for retaining its hold; its foot is therefore divided

into two adhering portions, placed at each extremity, and separated by a

wide interval; when it crawls, it fixes the posterior disc and advances

the other, which it attaches firmly to the place of progression, and

this being effected, the hinder sucker is detached and drawn forwards,

locomotion being accomplished by the alternate adhesion of these two

prehensile discs. In Cyclostoma the foot is likewise furnished with two

longitudinal adhering lobes, which are advanced alternately. But the foot

of the marine snails is not merely an instrument of progression on a

solid surface, for in many species it is convertible at the will of the

animal into a boat, by means of which the creature can suspend itself in

an inverted position at the surface of the water, where by the aid of its

mantle and tentacles it can row itself from place to place.

[Illustration: Pteroceras scorpio.]

[Illustration: Oliva hispidula.]

[Illustration: Strombus pes pelicani.]

[Illustration: Ianthina communis.]

The Ianthinæ, or purple Sea-Snails, carry under their foot a vesicular

organ like a congeries of foam-bubbles, that prevents creeping, but

serves as a buoy to support them at the surface of the water.

[Illustration: Murex haustellum.]

When the sea is quiet, these little creatures,

Like little wanton boys that swim on bladders,

appear in vast shoals on the surface, but as soon as the wind ruffles

the ocean, or an enemy approaches, they at once empty their air-cells,

contract their float, and sink to the bottom, pouring out at the same

time a darkened fluid like that of the Aplysia or the Murex, which no

doubt serves them as a defence against their foes, and, according to

Lesson, furnished the celebrated purple of the ancients. The Ianthinæ

inhabit the Mediterranean and the warmer regions of the Atlantic, but

especially towards the close of summer they are frequently drifted by the

Gulf Stream to the west coast of Ireland.

[Illustration: Magilus antiquus.]

[Illustration: Worm-Shell.]

While the vast majority of the gasteropods either creep or swim, some are

doomed to the sedentary life of the oyster, and remain for ever fixed

to the spot where they first attached themselves as small free-swimming

larvæ. Thus the _Magilus antiquus_, which in its young state presents

all the characters of a regular spiral univalve, establishes itself in

the excavations of madrepores, and as the coral increases around it, the

Magilus is obliged, in order to have its aperture on a level with the

surrounding surface, to construct a tube, lengthening with the growth of

the coral. As the tube goes on increasing, the animal abandons the spiral

for the tubular part of the shell, and in the operation it leaves behind

no partitions, but secretes a compact calcareous matter which reaches

to the very summit of the spiral part, so that in an old specimen the

posterior part of the shell presents a solid mass.

The Siliquariæ are generally found embedded in a similar manner in

sponges or other soft bodies, while the Vermetus, or Worm-Shell, usually

attaches itself, like the Serpulæ, to rocks, coral-reefs, or shells.

In these genera, which have been arranged by Cuvier in a separate order

(Tubulibranchiata), the foot is naturally reduced to the state of an

adhesive organ, its chief functions consisting in opening and closing the

lid.

The sea-snails are either predaceous or herbivorous; among the

pectinibranchiates, those with circular mouths to the shell are vegetable

feeders, while such as have an aperture ending in a canal are animal

feeders. Considerable modifications of internal structure indicate this

difference of food; and the external organs, particularly about the

mouth, exhibit a corresponding variety of form. In those which feed on

vegetables the mouth is generally a slit furnished with more or less

perfect lips, armed with a simple cutting apparatus, which is often

powerful enough to divide or dismember comparatively hard substances.

In most animal feeders the mouth presents the appearance of a proboscis

that can be protruded or shortened at the will of the animal, and

which, grasping the food, conveys it to a spine-armed tongue, by the

aid of which it is propelled into the gullet without mastication or any

preparatory change.

In the Whelk and its shell-boring allies, the alternate protrusion and

retraction of the proboscis, which is here of a much more complicated

structure, causes the sharp tongue to act as a rasp or auger, capable of

drilling holes into the hardest shells. It is this circumstance which

renders the whelk so formidable an enemy to mussel and oyster banks.

During the erection of Bell Rock lighthouse, an attempt was made to plant

a colony of mussels on the wave-beaten cliff, as they were likely to be

of great use to the workmen, and especially to the light keepers, the

future inhabitants of the rock; but the mussels were soon observed to

open and die in great numbers. "For some time," says Mr. Stevenson in his

interesting narrative, "this was ascribed to the effects of the violent

surge of the sea, but the Buccinum lapillus having greatly increased,

it was ascertained that it had proved a successful enemy to the mussel.

The buccinum was observed to perforate a small hole in the shell, and

thus to suck out the finer parts of the body of the mussel; the valves

of course opened, and the remainder of the shell-fish was washed away

by the sea. The perforated hole is generally upon the thinnest part of

the shell, and is perfectly circular, of a champhered form, being wider

towards the outward side, and so perfectly smooth and regular as to have

all the appearance of the most beautiful work of an expert artist. It

became a matter extremely desirable to preserve the mussel, and it seemed

practicable to extirpate the buccinum. But after we had picked up and

destroyed many barrels of them, their extirpation was at length given

up as a hopeless task. The mussels were consequently abandoned as their

prey; and, in the course of the third year's operations, so successful

had the ravages of the buccinum been that not a single member of the

imported mussel colony was to be found upon the rock." Thus the engineer,

whose skill and perseverance had gained so proud a triumph over the waves

of the stormy ocean, was defeated by an ignoble whelk.

[Illustration: Limpet's tongue.]

In the genera which have no proboscis, the tongue, acting as a prehensile

and rasping or abrading organ, is frequently of considerable length;

thus, in the Ear-shell, it is half as long as the body, and in the common

Limpet even three times longer than the entire animal. From the two

cartilaginous pieces (_b b_), placed on each side of its root, arise the

short and powerful muscles which wield the organ. The surface of this

curious piece of mechanism, a magnified view of which is given at B, is

armed with minute, though strong, teeth, placed in transverse rows, and

arranged in three series; each central group consists of four spines,

while those on the sides contain but two a-piece. It is only at its

anterior extremity (_d_), however, that the tongue, so armed, presents

that horny hardness needful for the performance of its functions, the

posterior part being comparatively soft; so that, probably in proportion

as the anterior part is worn away, the parts behind it gradually assume

the necessary firmness, and advance to supply its place. In the upper

part of the circumference of the mouth, we find a semicircular horny

plate, resembling an upper jaw, and the tongue, by triturating the food

against this, gradually reduces substances however hard. On opening the

limpet, the tongue is found doubled upon itself, and folded in a spiral

manner beneath the viscera.

Many of the Gasteropods which live on coarse and refractory materials are

provided with several digestive cavities, resembling in some degree the

stomachs of the ruminating quadrupeds; and frequently the triturating

power of these organs is still further increased by their being armed

with teeth variously disposed.

[Illustration: Bulla.]

In the Bulla, for instance, a genus belonging, like the sea-hares, to the

tectibranchiate order, the gizzard, or second stomach, contains three

plates of stony hardness attached to its walls, and so disposed that they

perform the part of a most efficacious grinding mill.

On opening the gizzard of the Scyllæa, it is found to be still more

formidably armed, for in its muscular walls there are embedded no less

than twelve horny plates (_e_), which are extremely hard and as sharp as

the blades of a knife.

[Illustration: Gizzard of Bulla.]

[Illustration: Gizzard of Syllæa.]

The Sea-hare, however, furnishes us with the most curious form of these

stomachal teeth, for here we see not only the gizzard (_b_) armed with

horny pyramidal plates, whose tuberculated apices, meeting in the centre

of the organ, must necessarily bruise by their action whatever passes

through that cavity, but the third stomach (_d_) is also studded with

sharp-pointed hooks (_c_), resembling canine teeth, and admirably adapted

to pierce and subdivide the tough leathery fronds of the olive sea-weeds

on which the animal feeds. Thus these deformed and disgusting molluscs

afford us one of the most interesting examples of the adaptation of

organs to their functions, which an enlightened research is continually

finding in creation.

[Illustration: Compound stomach of Sea-Hare.]

Though not so gifted as the cephalopods, many of the gasteropods possess

all the organs of sense. Like them, they have an apparatus specially

calculated to appreciate sonorous undulations, and consisting of a

membranous vesicle attached to an auditive nerve, and containing either

a single spherical otolithe or a larger number of similar smaller

calcareous bodies, which by their vibrations communicate the impression

of sound to the nerve. Their minute eyes are short-sighted, it is true,

and frequently either entirely wanting or, as in the Nudibranchiates,

scarcely able to distinguish light from darkness; but their inactive

habits require no wide field of vision, and thus they see as much of the

external world as is necessary for their humble sphere of existence. The

organs of sight are generally situated either on a prominence at the base

of the superior pair of tentacles or, as, for instance, in the Murex, at

the extremity of these organs (_a_, _b_), a position which enables the

animal to direct them readily to different objects.

[Illustration: Tentacles and eye of Murex.

_c._ Eye highly magnified.]

Many of the Gasteropods are evidently capable of perceiving odours; thus,

animal substances let down in a net to the bottom will attract thousands

of Nassæ in one night. We also may infer that they are not deficient

in taste from the presence of papillæ at the bottom of their mouth,

analogous to those found on the tongue of other animals; but, of all

their senses, that of touch is undoubtedly the most perfect. The whole

soft surface of the body is indeed of exquisite sensibility, but more

especially the vascular foot, and the tentacles, or horns, which vary

both in number and in shape in different genera. Yet, in spite of this

delicacy in the organisation of the skin, which makes it so sensible of

contact, it appears to have been beneficently ordered that animals so

helpless and exposed to injury from every quarter are but little sensible

to pain. Although they are deprived of all higher instincts, we find

among the Gasteropods a few examples of concealment under extraneous

objects, which remind us of the masks and artifices frequently employed

by the insects and crustaceans.

The Agglutinating Top (_Trochus agglutinans_) covers itself with small

stones and fragments of shells, and thus shielded from the view escapes

the voracity of many an enemy but little suspecting the savoury morsel

hidden under the mound of rubbish which he disdainfully passes by.

In animals which are only provided with passive means of defence, we may

naturally expect a considerable degree of caution, and in this respect

the gasteropods might give many useful lessons to man. How carefully they

protrude their tentacles as far as possible to sound every obstacle in

their way, before they creep onwards, and how rapidly they withdraw into

their shell at the least symptom of danger! What an example to so many of

us that leap before they look, and frequently break their necks in the

fall!

Yet, in spite of all their prudence and of the protection of their

stony dwellings, they serve as food to a host of powerful enemies. The

sea-stars, their most dangerous foes, not only swallow the young fry but

also seize with their long rays the full-grown gasteropods, and clasp

them in a murderous embrace.

They are preyed upon by fishes, crustaceans, and sea-birds, who pick them

up along the shores; but it will sometimes happen that a crow, while

endeavouring to detach a limpet for its food, is caught by the tip of its

bill, and held there until drowned by the advancing tide.

Man also consumes a vast number of sea-snails, for on every coast there

are some edible species; and it may be said that, with the exception of

very few that have a disagreeable taste, they are all of them used as

food by the savage. The miserable inhabitants of Tierra del Fuego chiefly

live upon a large limpet that abounds on the rocky shores of their

inhospitable land, and but for this resource would most likely long since

have been extirpated by hunger.

Many of the univalve shells are, moreover, highly prized as objects of

ornament or use both by savage and civilised nations. The South Sea

Islander makes use of a Triton as a war conch; the Patagonian drinks

out of the Magellanic volute, the Arab of the Red Sea employs a large

Buccinum as a water-jug, and the _Cypræa moneta_ is well known in

commerce as the current coin of the natives of many parts of Africa.

In Europe the iridescent Haliotis is frequently used for the inlaying

of tables or boxes, and various species of Helmet-shells and Strombi

(_Cassis rufa madagascariensis_, _Strombus gigas_), peculiar as being

formed of several differently coloured layers, placed side by side, are

in great request for the cutting of cameos, as they are soft enough

to be worked with ease, and hard enough to resist wear. More than two

hundred thousand of these shells are annually imported into France,

and the value of cameos produced in Paris alone amounts to more than a

hundred thousand pounds. A large number are also cut in the small town of

Oberstein on the Nahe (a river flowing into the Rhine at Bingen), which

has long been famous for the manufactory of agate ornaments and trinkets,

and has now added this new branch of industry to the more ancient sources

of its prosperity.

* * * * *

The Pteropods, or Wing-footers, move about by means of two fin-like

flaps, proceeding wing-like from the fore part of the body. They have no

disk to walk upon, nor arms for the seizure of prey, like the cephalopods

and gasteropods, but resemble them by the possession of a head distinct

from the rest of the body, which some, like the Hyaleas and Cleodora,

conceal in a thin transparent or translucent shell, in which they also

hide their head and wings at the approach of danger, and immediately sink

to the bottom; while others, like the blue and violet Clios, beautifully

variegated with light red spots, are perfectly naked. They generally

inhabit the high seas, and are but rarely drifted by storms or currents

into the neighbourhood of the land. They mostly swim about freely,

but sometimes also they are found clinging by their wings to floating

sea-weeds. They are small creatures, but propagate so fast that the _Clio

borealis_ and _Limacina arctica_ form the chief food of the colossal

whale.

[Illustration: Hyalea globulosa.]

While these two little pteropods, in spite of their minute proportions,

deserve to rank among the most important inhabitants of the northern

seas, the Mediterranean species belong mainly to the genera Hyalea,

Cleodora, and Criseis--forms wholly unknown to our own fauna except as

waifs. Vast shoals of these animals frequent the deeper parts of that

sea, leaving their remains strewed over its bed, between depths of from

one hundred to two hundred fathoms; they are short-lived creatures, and

have their seasons, being met with near the surface during spring and

winter, sparkling in the water like needles of glass.

"The pteropods are the winged insects of the sea," says M. Godwin-Austen,

"reminding us, in their free circling movements and crepuscular habits,

of the gnats and moths of the atmosphere; they shun the light, and if the

sun is bright, you may look in vain for them during the life-long day--as

days sometimes are at sea; a passing cloud, however, suffices to bring

some Cleodoræ to the surface. It is only as day declines that their true

time begins, and thence onwards the watches of the night may be kept by

observing the contents of the towing-net, as the hours of a summer day

may be by the floral dial. The Cleodoræ are the earliest risers; as the

sun sets, _Hyalæa gibbosa_ appears, darting about as if it had not a

moment to spare, and, indeed, its period is brief, lasting only for the

Mediterranean twilight. Then it is that _Hyalæa trispinosa_ and _Cleodora

subula_ come up; _Hyalæa tridentata_, though it does not venture out till

dusk, retires early, whilst some species, such as _Cleodora pyramidata_,

are to be met with only during the midnight hours and the darkest nights.

This tribe, like a higher one, has its few irregular spirits, who manage

to keep it up the whole night through. All, however, are back to their

homes below before dawn surprises them."

* * * * *

The lamellibranchiate Acephala, or headless molluscs with comb-like

gills, are distinguished from the preceding orders of molluscs by a

more simple organisation and the peculiar formation of their external

coverings. They are all contained within a bivalve shell, articulated

after the manner of a hinge, and to which some of their families are

attached by one strong muscle (Monomyaria), others by two (Dimyaria).

In this shell, which is secreted by two large flaps or folds of their

skin or mantle, they generally lie concealed like a book in its binding,

and bid defiance to many of their enemies. When danger menaces, the

sea-snail withdraws its head and closes the entrance of its hermitage

with a lid, but the bivalve shuts its folding-doors when it wishes to

avoid a disagreeable intruder. A strong elastic ligament connects the

two valves, and opens them wide as soon as the muscular contraction which

closed them ceases to act.

While the sea-snail creeps along upon a mighty foot, the bivalve is

frequently doomed to a sedentary life, and the former protrudes from its

shell a well-formed head, while the latter, like many a biped, has no

head at all. The lamellibranchiate Acephala have, however, been treated

by nature not quite so step-motherly as might be supposed from this

deficiency, for many of them have eyes, or at least ocular spots, which

enable them to distinguish light from darkness; and even auditory organs

have been discovered in many of them. Their circulation is performed by

a heart generally symmetrical, and their respiration by means of four

branchial leaflets equal in size, and symmetrically arranged on either

side of the body. The mouth is a simple orifice without any teeth,

bordered by membranous lips, and placed at one end of the body between

the two inner leaves of the branchiæ. The digestive apparatus consists of

a stomach or intestine of different lengths, a liver, and several other

accessory organs. A simple nervous system brings all the parts of the

body into harmonious action.

[Illustration: Bivalve deprived of shell, to show its various openings.]

In many lamellibranchiates the folds of the mantle are disjoined, as, for

instance, in the oyster, which, on opening its shell, at once admits the

water to its delicately fringed branchiæ; in others they are more or less

united, so as to form a closed sack with several openings, an anterior

one (_h_) for the passage of the foot, and two posterior ones (_g_, _f_)

for the ingress and egress of the water which the animal requires for

respiration. These posterior openings are often prolonged into shorter

or longer tubes or siphons, sometimes separate, and sometimes grown

together so as to form a single elongated fleshy mass. The use of these

prolongations becomes at once apparent when we consider that they are

chiefly developed in those species which burrow in sand, mud, wood, or

stone, and which therefore require to be specially guarded against the

danger of suffocation. The interior of these siphonal canals is lined

with innumerable vibratory cilia, by the action of which the water is

drawn towards the branchial orifice and conveyed in a current through the

canal over the surface of the gills; then, having been deprived of its

oxygen, it is expelled by a similar mechanism through the other tube;

and it is by the force of this anal current that the passage is kept

free from the deposit of mud or other substances, which would otherwise

soon choke it up. The cleaning action of the anal current is assisted by

the faculty the burrowing molluscs possess of elongating and contracting

their siphons, and the degree to which this may be accomplished depends

on the depth of the cavity which the species is accustomed to make. Yet

since many particles of matter float even in clear water, which from

their form or other qualities might be injurious to the delicate tissue

of the viscera to be traversed, how is the entrance of these to be

guarded against in an indiscriminating current? A beautiful contrivance

is provided for this necessity. The margin of the branchial siphon,

and sometimes, though more rarely, of the anal one, is set round with

a number of short tentacular processes, endowed with an exquisite

sensibility and expanding like feathery leaves. In _Pholas dactylus_

this apparatus, which is here confined to the oral tube, is of peculiar

beauty, forming a network of exquisite tracery, through the interstices

or meshes of which the water freely percolates, while they exclude

all except the most minute floating atoms of extraneous matter. Thus

admirably has the health and comfort of the lowly shell-fish been provided

for that spend their whole life buried in sepulchres of stone or sand.

[Illustration: Donax.

_a_, _b_. Siphons.]

The fragile shell of the pholades seems to have prompted them to seek

a better protection in the hard rock; a similar necessity may have

induced the ship-worm to drill a dwelling in wood. Its shells, which

are only a few lines broad, are very small compared with the size of

the vermiform body, and are therefore completely inadequate for its

defence. For better security it bores deep passages in submerged timber,

which it lines with a calcareous secretion, closing the opening with two

small lids. Unfortunately, while thus taking care of itself, it causes

considerable damage to the works of man. It is principally to guard

against the attacks of this worm that ships are sheathed with copper,

and the beams of submarine constructions closely studded with nails.

During the last century, the Teredo caused such devastations in the dykes

which guard a great part of Holland against the encroachments of an

overwhelming ocean that the Dutch began to tremble for their safety; and

thus a miserable worm struck terror in the hearts of a nation which had

laughed to scorn the tyranny of Philip II., and bid defiance to the

legions of Louis XIV.

[Illustration: Pholas striata.]

[Illustration: Ship-worm.--(Teredo navalis.)]

But while blaming the teredo for its damages, justice bids us not pass

over in silence the services which it renders to man. If it here and

there destroys useful constructions, on the other hand, it removes the

wrecks that would otherwise obstruct the entrance of rivers and harbours;

and we may ask whether these services do not outweigh the harm it causes.

The pholades also belong to the noxious animals; they perforate the

walls and calcareous jetties which man opposes to the fury of the sea,

or raises for the creation of artificial harbours and landing places,

destroy their foundations, and gradually cause their destruction.

[Illustration: Petunculus.

_a._ Foot.]

The foot of the lamellibranchiates presents a great variety of form,

and is found in various degrees of development, gradually passing into

a rudimentary state, until finally it is completely wanting in the

oyster family. In most of those which live at large it is strong and

muscular, serving either as an excellent spade for speedy concealment

in the sand when an enemy approaches, or to dig a furrow into which

the animal forces itself partially, and then advances slowly by making

slight see-saw or balancing motions, or even to jump along with tolerable

rapidity. Thus, the common Cockle protrudes its foot to its utmost

length, bending it and fixing it strongly against the surface on which it

stands; then by a sudden muscular spring it throws itself into the air,

and, by repeating the process again and again, hops along at a pace one

would hardly expect to meet with in a shell-bound mollusc.

Even some of those which have but a very rudimentary foot, incapable of

subserving locomotion, are able to move from place to place by the sudden

opening or shutting of their valves. In this manner the scallop, which

inhabits deep places, where it lies on a rocky or shelly bottom, swims or

flies through the water with great rapidity, and the file or rasp mussel,

a closely related genus, principally occurring in the Indian Ocean,

glides so swiftly through the water that the French naturalists Quoy and

Gaimard were hardly able to overtake it.

[Illustration: Cockle.

_a._ Foot.]

In the stone or wood-boring bivalves the functions of the foot with

regard to locomotion are much more limited than in the Cockle, or

Tellina, as they merely consist in moving the animal up and down in the

cavity where it has fixed its residence. In the Razor-Shells, which will

sometimes burrow to the depth of two feet, and very rarely quit their

holes, the cylindrical foot, no longer fit for horizontal locomotion,

serves the animal for rising or sinking in the sand, for when about to

bore, it attenuates it into a point, and afterwards contracts it into a

rounded form so as to fix it by its enlargement when it desires to rise.

In places where the razor-shells abound, they are sought after as bait

for fish, and taken in spite of their mole-like facility of concealment,

for when the tide is low, their retreat is easily recognised by the

little jet of water they eject when alarmed by the motion of the

fishermen above. Having thus detected their burrow, the wily enemy who

is well aware that, though inhabiting the salt water, the Solen does not

like too much of a good thing, merely throws some salt into the hole,

which, sadly irritating the nerves of the poor creature, generally brings

it to the surface. He must, however, be very quick in grasping it firmly,

for should he fail, the animal speedily sinks again into the sand and

will remain there, being either insensible to the additional irritation

or its instinct of self-preservation teaching it to remain beneath.

The pholades, which have very delicate milk-white valves, burrow holes in

limestone or sandstone rocks, though occasionally they content themselves

with houses of clay or turf. How creatures invested with shells as thin

as paper and as brittle as glass are able to work their way through hard

stone has long been a puzzle to naturalists, some of whom asserted that

they attained their object by means of an acid solvent, others that

they bored like an auger by revolving; but recent investigations have

discovered that their short and truncated foot is the chief instrument

they use in their mining operations, being provided at its base with a

rough layer of sharp crystals of flint, which, when worn off, are soon

replaced by others, and act as excellent files.

[Illustration: Solen, or Razor-Shell.

_a._ Foot.]

In several of the sedentary genera the rudimentary foot, though incapable

of locomotion, makes itself useful by spinning a bundle of silken

threads, called _byssus_, or beard, which serve to anchor the animal to

any solid submarine object as firmly as a ship in harbour. Generally

the connection is permanent, but some species, among others the edible

mussel, are able to detach the filaments from the glandular pedicle

situated at the inferior base of the foot which originally secreted them,

and then to seek another point of attachment.

If the byssus be examined under a powerful lens, before any of the

filaments are torn, it is easy to perceive that these are fixed to

submarine bodies by means of a small disc-like expansion of their

extremities of various extent, according to the genus and species.

Certain genera are celebrated for the abundance and fineness of their

byssus; that of the Pinnæ, or Wing-Shells, among others, which are very

common in some parts of the Mediterranean, and attain a considerable

size, is so long and firm that in Naples it is sometimes manufactured

into gloves and other articles of dress, though more as an object of

curiosity than for use.

Thus we find in the same class of animals the same organ most variously

modified in form and structure; now serving as a foot, now as a spade,

or as a rasp, or as a spinning machine, and, throughout all these

modifications, admirably adapted in every case to the mode of life of its

possessor.

[Illustration: Pinna.

_c._ Pedicle from which the filaments are detached.

_d._ Inferior base of the foot.

]

The whole construction, and generally the extremely restricted

locomotion, of the bivalves tells us at once that they are unable

to attack their prey, but must be satisfied with the food which the

sea-currents bring to the door of their shells, or within the vortex

of their branchial siphons. But they have as little reason to complain

as the equally slow or sessile polyps, bryozoa, and ascidians, for the

waters of the ocean harbour such incalculable multitudes of microscopic

animals and plants that their moderate appetite never remains long

unsatisfied. The same streams which aërate their blood also convey to

their mouth all the food which they require.

Deprived of more active weapons, most bivalves rely upon their shells

as their best means of defence, and to answer this purpose, their stony

covering must naturally increase in solidity the more its owner is

exposed to injury. The pholades, lithodomes, and teredines, which scoop

out their dwellings in stone or wood, and thus enjoy the protection of

a retrenched camp, can do with a thin and brittle or even with a mere

rudimentary shell. The solens, which at the least alarm bury themselves

deeper and deeper in the sand, likewise require no closely-fitting

valves; but the oysters or mussels, which have no external fortress to

retire to, and are unable to move from the spot, would be badly off

indeed if they could not entirely conceal themselves within their thick

shells, and keep them closed by strong muscular contraction.

Bernardin de St. Pierre, in his "Studies of Nature," points out another

admirable provision for the safety of molluscs. Thus, those which crawl

and travel, and can consequently choose their own asylums, are in general

those of the richest colours. Such, among the Gasteropods, are the

gaudily-tinted Nerites, and the polished marbled Cowries, the Olives,

richly ornamented with three or four colours, and the Harps, which have

tints as rich as the most beautiful tulips; while among the bivalves

the vivacious Pectens, coloured scarlet and orange, and a host of other

travelling shells, are impressed with the most lively colours. But those

which do not swim, as the Oysters, which are adherent always to the

same rocks, or those which are perpetually at anchor, as the Pinnas and

Mussels, or those which repose on the bosom of Madrepores, such as the

Arcs, or those which are entirely buried in the calcareous rocks, as the

Lithodomi, or those which immovably, by reason of their weight, pave the

surface of the reefs, as the Tridacna, are of the colour of the bottoms

or floors which they respectively inhabit, in order, no doubt, that they

shall be less perceived by their enemies.

But even so the best guarded of the bivalves fall a prey to innumerable

enemies, and when we see the strand covered for miles and miles with

their débris, we may rest assured that but few of the quondam inmates

of these fragmentary shells have died a natural death. Annelides

and Sea-snails, crustaceans and star-fishes, strand birds and even

quadrupeds, all fatten upon their delicate flesh, and man devours

incalculable numbers.

[Illustration: Edible Cockle.]

In vain the Pholas buries itself in stone, or the cockle in the sand;

their security was at an end as soon as man had found out that they

were grateful to the palate. The former was reckoned a delicacy by the

ancients, and the latter is preferred by some to the oyster itself. So

much is certain, that, during the years of famine caused by the potato

disease, it preserved the lives of many of the poor Shetlanders and

Orcadians.

The Razor-Shells, particularly when roasted, and the Clam-Mussels, which

are not only a favourite repast of the Greenlander but also of the white

bear and arctic fox, are equally reckoned among the most delicate of

bivalves.

The common Mussel (_Mytilus edulis_), which is found in the littoral

zone on almost every rocky shore, is eaten in vast numbers by the coast

inhabitants, and carried in enormous masses into the interior of the

country; it furnishes an equally cheap and agreeable food, but is not

easy of digestion, and sometimes produces symptoms of poisoning, which

have been ascribed to the eggs of asterias, on which it feeds during the

summer. In the northern countries it is also in great request as a bait

for cod, ling, rays, and other large fishes that are caught by the line.

In the Frith of Forth alone from thirty to forty millions of mussels are

used for this purpose, and in many places they are enclosed in _gardens_,

the ground of which is covered with large stones, to which they attach

themselves by their byssus or beard.

[Illustration: Edible Mussel.]

It is a curious fact that the rearing of mussels should have been

introduced into France as far back as the year 1235, by an Irishman

of the name of Walton. This man, who had been shipwrecked in the Bay

de l'Aiguillon, and gained a precarious living by catching sea-birds,

observed that the mussels, which had attached themselves to the poles

on which he spread his nets over the shallow waters, were far superior

to those that naturally grow in the mud, and immediately made use

of his discovery by founding the first "_bouchot_," or mussel-park,

consisting of stakes and rudely interwoven branches. His example soon

found imitators, and, strange to say, the method of construction adopted

by Walton, six centuries ago, has been maintained unaltered to the

present day. It may give some idea of the immense resources that might

be obtained from so many utterly neglected lagunes when we hear that

the fishermen of l'Aiguillon, although they sell three hundredweight of

mussels for the very low sum of five francs, or four shillings, annually

export or send them into the interior to the amount of a million or

twelve hundred thousand francs.

The praise which Pliny bestowed on the oyster, calling it the palm or

glory of the table, is still re-echoed by thousands of enthusiastic

admirers. We know that this king of the molluscs congregates in enormous

banks, often extending for miles and miles, particularly on rocky ground,

though it is also found on a sandy or even on a muddy bottom. Along the

shallow alluvial shores of many tropical lands, great quantities of

oysters are often found attached to the lower branches of the mangroves,

where they are so situated as to be covered when the flood sets in, and

to remain suspended in the air when it retires, swinging about as the

wind agitates their movable support. The oyster inhabits all the European

seas from the shores of the Mediterranean to the Westenfiord in Norway,

where it finds its northern boundary, lat. 68° N., but the British

waters may be considered as its headquarters, for nowhere is it found

in greater abundance and of a richer flavour. After the ancient Romans

had once tasted the oysters of Kent--the renowned _Rutupians_--they

preferred them by far to those of the Lucrine lake, of Brindisi, and of

Abydos, and Macrobius tells us that the Roman epicures in the fourth

century never failed to have them at table. The "Pandores" of Edinburgh,

and the "Carlingfords" of Dublin, are likewise celebrated for their

delicious flavour; and if we turn to the Continent, we find the Bay

of Biscay, and the coasts of Brittany and Normandy, of Holland and of

Schleswig-Holstein, renowned for the excellence of their oysters.

Three sorts of oysters are distinguished in the trade. The first

comprises those which are dredged from the deeper banks. These are the

largest-sized, but also the least valued. The second consists of those

that are gathered on a more elevated situation. Being accustomed to

the daily vicissitudes of ebb and flood, they retain their water much

longer, and can therefore be transported to much greater distances than

the former. Those are preferred that grow on a clear bottom near the

estuaries of rivers. The third and most valued sort of oysters are those

that are cleaned and fattened in artificial _parks_ or stews.

This branch of industry was already known to the Romans, and Pliny

tells us that Sergius Orata, a knight, was the first who established an

artificial basin for the cultivation of oysters, and realised large sums

of money by this ingenious invention. At present Harwich, Colchester,

Whitstable, and many other sea-ports along our coast are famed for their

oyster-stews, as are, in France and Belgium, Marennes, Havre, Dieppe,

Tréport, and Ostend, where real British natives are cleaned and fattened

for continental consumption.

The renowned oyster-parks of Ostend, the oldest of which celebrated its

hundredth anniversary in 1860, are extensive walled basins, communicating

by sluices with the open sea, so that the water can be let in and out

with every returning tide. As microscopic algæ and animalculæ are

produced in much greater numbers in these tranquil reservoirs than in

the boisterous sea, the oysters find here much more abundant food, and

being detached one from the other, they can also open and close their

shells with greater facility, so that nothing hinders their growth. Thus

fostered and improved by constant attention, they are greatly superior

in flavour to the rough children of nature that are sent without any

further preparation to market and condemned to the knife soon after

having been dragged forth from their submarine abode. The highly prized

_green_ oysters owe their colour to the number of ulvæ, enteromorphæ, and

microscopic infusoriæ, that are abundantly generated in the parks, and

communicate their verdant tinge to the animal that swallows them.

In spite of their high price, which unfortunately debars the poorer

classes from their enjoyment, the consumption of oysters is immense; so

that in a commercial point of view they are by far the most important of

all the mollusc tribes. Of the quantities eaten in London alone, it is

impossible to give even an approximate guess, as no reliable statistics

can be arrived at. Exclusive of those bred in Essex and Kent, in the

rivers Crouch, Blackwater, and Colne, and in the channel of the Swale and

the Medway, vast numbers are brought from Jersey, Poole, and other places

along the coast. The Channel Islands alone, which export about 100,000

bushels a year, send a great part of their oysters to the metropolitan

market.

The luxurious tables of Paris likewise consume unnumbered millions, and

when we consider that, thanks to the railroad, even the most distant

inland towns of the Continent may now be supplied with Ostend oysters, we

cannot wonder that their price has risen enormously with the constantly

increasing demand.

This great augmentation of value has naturally directed attention to

the creation of new oyster-banks, and to the better management of those

already existing, and fortunately the manner in which the mollusc

propagates renders its culture in appropriate localities a by no means

difficult task.

The oyster spawns from June to September. Instead of immediately

abandoning its eggs to their fate, as is the case with so many

sea-animals, it keeps them for a time in the folds of its mantle, between

the branchial lamellæ, and it is only after having thus acquired a more

perfect development that the microscopic larvæ, furnished with a swimming

apparatus and eyes, emerge from the shell, and are then driven about

by the floods and currents, until they find some solid body to which

they attach themselves for life. In this manner the oyster produces in

one single summer a couple of millions of young, which, however, mostly

perish during the first wandering stage of their existence. Thus we see

what rich rewards may be gained by protecting and fixing the oyster-larvæ

at an early date; and that this can be done in many places without any

great outlay of capital is proved to us by successful examples both in

ancient and modern times.

Between the Lucrine Lake, the ruins of Cumæ, where of yore the Sibyl

uttered her ambiguous oracles, and the promontory of Misenum, lies a

small salt-water lake, about a league in circumference, generally from

three to six feet deep, and reposing on a volcanic, black, and muddy

bottom. This is the old Acheron of Virgil, the present Fusaro. Over

its whole extent are spread from space to space great heaps of stones,

that have been originally stocked with oysters brought from Tarentum.

Round each of these artificial mounds stakes are driven into the ground,

tolerably near each other, and projecting from the water, so as to be

pulled up easily. Other stakes stand in long rows several feet apart,

and are united by ropes, from which bundles of brushwood hang down into

the water. All these arrangements are intended to fix the _oyster-dust_,

that annually escapes from the parental shells, and to afford it a vast

number of points to which it may attach itself. After two or three years

the microscopic larvæ have grown into edible oysters. Then, at the proper

season, the stakes and brushwood bundles are taken out of the water, and

after the ripe berries of the marine vineyard have been plucked, they

are again immersed into the lake, until a new generation brings a new

harvest. Thus the indolent Neapolitans have for ages given an example

which has but recently been imitated by the men of the North. In 1858

a mason named Beef (a name which, if not misspelt, would seem to point

out an English origin) inaugurated the modern era of oyster cultivation,

at the island of Ré, near La Rochelle, by laying down a few bushels of

growing oysters among a quantity of large stones on the fore shore. His

success encouraged his neighbours to follow his example, so that now

already upwards of 4,000 beds or _claires_ extend along the coast.

Between March and May 1859 a quantity of oysters taken from different

parts of the sea were distributed in ten longitudinal beds in the Bay of

St. Brieux, on the coast of Brittany. The bottom was previously covered

with old oyster-shells and boughs of trees arranged like fascines, which

afford a capital holding-ground for the spat. In 1860 three of the

fascines were taken up indiscriminately from one of the banks, and found

to contain about 20,000 oysters each, of from one inch to two inches in

diameter. The total expense for forming the above bank was 221 francs,

and reckoning the number of oysters on each of the 300 fascines laid down

on it at only 10,000, these sold at the low price of 20 francs a thousand

would produce the sum of 60,000 francs, thus yielding a larger profit

than any other known branch of industry.

Encouraged by these successful examples, an English company has obtained

a grant by Act of Parliament of a piece of fore shore lying between the

Whitstable and Faversham Oyster Companies' beds, and thus admirably

situated for receiving a large quantity of floating spawn from these

establishments. There can be no doubt that oyster cultivation will

spread further and further, and that ultimately all the worthless bays

and lagunes along our coasts will be converted into rich oyster-fields,

yielding a good profit to their owners and enjoyment to millions of

consumers.

A shell nearly related to the oyster produces the costly pearls of the

East that have ever been as highly esteemed as the diamond itself.

The most renowned pearl-fisheries are carried on at Bahrein, in the

Persian Gulf, and in the Bay of Condatchy, in the island of Ceylon, on

banks situated a few miles from the coast. Before the beginning of the

fishery, the government causes the banks to be explored, and then lets

them to the highest bidder, very wisely allowing only a part of them

to be fished every year. The fishing begins in February, and ceases by

the beginning of April. The boats employed for this purpose assemble in

the bay, set off at night at the firing of a signal-gun, and reach the

banks after sunrise, where fishing goes on till noon, when the sea-breeze

which arises about that time warns them to return to the bay. As soon as

they appear within sight, another gun is fired, to inform the anxious

owners of their return. Each boat carries twenty men and a chief; ten

of them row and hoist up the divers, who are let down by fives,--and

thus alternately diving and resting keep their strength to the end of

their day's work. The diver, when he is about to plunge, compresses his

nostrils tightly with a small piece of horn, which keeps the water out,

and stuffs his ears with bees'-wax for the same purpose. He then seizes

with the toes of his right foot a rope to which a stone is attached, to

accelerate the descent, while the other foot grasps a bag of network.

With his right hand he lays hold of another rope, and in this manner

rapidly reaches the bottom. He then hangs the net round his neck, and

with much dexterity and all possible despatch collects as many oysters

as he can while he is able to remain under water, which is usually about

two minutes. He then resumes his former position, makes a signal to

those above by pulling the rope in his right hand, and is immediately

by this means hauled up into the boat, leaving the stone to be pulled

up afterwards by the rope attached to it. Accustomed from infancy to

their work, these divers do not fear descending repeatedly to depths of

fifty or sixty feet. They plunge more than fifty times in a morning,

and collect each time about a hundred shells. Sometimes, however, the

exertion is so great that, upon being brought into the boat, they

discharge blood from their mouth, ears, and nostrils.

[Illustration: Ceylon Pearl-Oyster.]

While the fishing goes on, a number of conjurors and priests are

assembled on the coast, busily employed in protecting the divers by their

incantations against the voracity of the sharks. These are the great

terror of the divers, but they have such confidence in the skill or

power of their conjurors that they neglect every other means of defence.

The divers are paid in money, or receive a part of the oyster-shells in

payment. Often, indeed, they try to add to their gains by swallowing

here or there a pearl, but the sly merchant knows how to find the stolen

property. The oysters, when safely landed, are piled up on mats, in

places fenced round for the purpose. As soon as the animals are dead, the

pearls can easily be sought for and extracted from the gaping shells.

After the harvest has been gathered, the largest, thickest, and finest

shells, which furnish mother-of-pearl, are sorted, and the remaining heap

is left to pollute the air. Some poor Indians, however, often remain for

weeks on the spot, stirring the putrid mass in the hopes of gleaning some

forgotten pearls from the heap of rottenness. The pearls are drilled and

stringed in Ceylon, a work which is performed with admirable dexterity

and quickness. For cleaning, rounding, and polishing them, a powder of

ground pearls is made use of.

The Pacific also furnishes these costly ornaments to wealth and beauty,

but the pearls of California and Tahiti are less prized than those of the

Indian Ocean.

Pearl-like excrescences likewise form on the inner surface of our oysters

and mussels, and originate in the same manner as the true pearls.

The formation of the pearl, however, is not yet quite satisfactorily

accounted for. Some naturalists believe that the animal accumulates

the pearl-like substance to give the shell a greater thickness and

solidity in the places where it has been perforated by some annelide or

gasteropod; and according to Mr. Philippi, an intestinal worm stimulates

the exudation of the pearl-like mass, which, on hardening, encloses and

renders it harmless.

Brilliancy, size, and perfect regularity of form are the essential

qualities of a beautiful pearl. Their union in a single specimen is rare,

but it is of course still more difficult to find a number of pearls of

equal size and beauty for a costly necklace or a princely tiara.

Nature has given the bivalves the same beauty of colouring and wonderful

variety of elegant or capricious forms as to the sea-snails; so that

they are equally esteemed in the cabinets of wealthy amateurs. Among the

most costly are reckoned the Spondyli, which are found in the tropical

seas, where they grow attached to rocks. They are distinguished by

the brilliancy of their colours, but particularly by the long thorny

excrescences with which their shells are covered. A Parisian professor

once pawned all his silver spoons and forks to make up the sum of six

thousand francs which was asked for a _Royal Spondylus_; but on returning

home was so _warmly_ received by his lady that, overwhelmed by the

hurricane, he flung himself on a chair, when the terrific cracking of the

box containing his treasure reminded him too late that he had concealed

it in his skirt-pocket. Fortunately but two of the thorns had been broken

off, and the damage was susceptible of being repaired; his despair,

however, was so great that his wife had not the heart to continue her

reproaches, and in her turn began to soothe the unfortunate collector.

[Illustration: Tridacna gigas.]

The gigantic Tridacna, which is now to be found in the shop of every

dealer in shells, was formerly an object of such rarity and value that

the Republic of Venice once made a present of one of them to Francis I.,

who gave it to the Church of St. Sulpice in Paris, where it is still made

use of as a basin for holy water. The tridacna attains a diameter of

five feet, and a weight of five hundred pounds, the flesh alone weighing

thirty. The muscular power is said to be so great as to be able to cut

through a thick rope on closing the shell. It is found in the dead rocks

on the coral reefs, where there are no growing lithophytes except small

tufts. Generally only an inch or two in breadth of the ponderous shell

is exposed to view, for the tridacna, like the pholas, has the power of

sinking itself in the rock, by removing the lime about it. Without some

means like this of security, its habitation would inevitably be destroyed

by the roaring breakers. A tuft of byssus, however strong, would be a

very imperfect security against the force of the sea for shells weighing

from one to five hundred pounds. It is found in the Indian Ocean and the

Pacific as far as the coral zone extends. The animal of the tridacna,

and of the nearly related Hippopus, distinguishes itself by the beauty

of its colours. The mantle of the _Tridacna safranea_, for instance,

has a dark blue edge with emerald-green spots, gradually passing into a

light violet. When a large number of these beautiful creatures expand the

velvet brilliancy of their costly robes in the transparent waters, no

flower-bed on earth can equal them in splendour.

[Illustration: Hippopus maculatus.]

* * * * *

Like the Lamellibranchiate Acephala, the Brachiopods are covered with

a bivalve shell, but their internal organisation is very different.

Instead of being disposed in separate gills, their respiratory system is

combined with the ciliated mantle on which the vascular ramifications are

distributed, but their most striking feature is the possession of spiral

fringed arms or buccal appendages which serve to open the shell and

occupy the greater part of its cavity. These curious organs are in some

Brachiopods quite free, in others attached to a complicated cartilaginous

or calcareous skeleton. None of the existing molluscs of this class are

capable of changing place, but are either fixed to extraneous substances

by the agglutination of one of their valves or by a muscular peduncle

passing through a perforation of their shells. There are no more than

forty-nine living species, chiefly belonging to the genera Terebratula

and Crania, and generally found at great depths in the Southern Ocean;

but the fossil remains of 1,370 species prove their importance in the

primitive seas, where they rivalled the lamellibranchiates in numbers and

variety. Though now so rare or so local in the British seas that ordinary

collectors are not likely to meet with any, they abound in many of our

oldest rocks. "A visit to the quarries at Dudley," says E. Forbes, "or

an Irish lime-kiln, or an oolitic section on the Dorsetshire coast, or

a green sand ravine in the Isle of Wight, will afford more information

about the Brachiopods than an examination of the finest collection of the

living species. In each of the above excursions a different set of forms

would be collected, for many of the palæozoic genera have altogether

disappeared when we rise among the secondary rocks, and in the latter

we find forms which closely remind us of existing species, but which,

though very near, are yet unquestionably distinct. In formations of all

epochs, a few generic types are common, and the Lingulæ of the earliest

sedimentary formations, presenting traces of organic life, strikingly

remind us of the species of that curious group living in exotic seas at

the present day."

* * * * *

[Illustration: Leaf-like Sea-Mat.]

At the lower extremity of the great series of molluscous animals we find

the Polyzoa (Bryozoa, or Sea-Mosses) and Tunicata. The former, which

comprise the Sea-Mats (Flustræ, Escharæ), the Sea-Scurfs (Lepraliæ), the

Retepores, the Cellulariæ, and several other families, were formerly

reckoned among the polyps, whom they greatly resemble in appearance

and mode of life, but far surpass by the complexity of their internal

organisation. The Sea-Mats are among the commonest objects which the tide

casts out upon our shores, for you will hardly ever walk upon the strand

without finding their blanched skeletons among the relics of the retiring

flood. Their flat leaf-like forms might easily cause them to be mistaken

for dried sea-weeds, but a pocket-lens suffices to show that they are

built up of innumerable little oblong cells, placed back to back like

those of a honey-comb, and each crowned by four stout spines, which give

their surface a peculiarly harsh feel when the finger is passed over it

from the apex to the base. "The individual cells," says Mr. Gosse, "are

shaped like a child's cradle, and if you will please to suppose some

twenty thousand cradles stuck side to side in one plane, and then turned

over, and twenty thousand more stuck on to these bottom to bottom, you

will have an idea of the framework of a flustra. And do not think the

number outrageous, for it is but an ordinary average. I count in an area

of half an inch square sixty longitudinal rows, each of which contains

about twenty-eight cells in that space; this gives 6,720 cells per square

inch on each surface. Now a moderate-sized polyzoary contains an area

of three square inches, i. e. six on both surfaces, which will give the

high number of 40,320 cells on such a specimen. Many, however, are much

larger."

Before the stormy tide detached them from the bottom of the sea, and

left them to perish on the shore, each of the cells contained a living

creature whose mouth was surrounded by a coronet of filiform and ciliated

tentacles, destined to produce a vortex in the water, and thus to provide

the tiny owner with its food. The body was bent on itself somewhat like

the letter V, the one branch (_a_) being the mouth and throat, the other

(_b_) the rectum, opening by an anus, and the middle part (_c_) the

stomach. Each of these tiny members of the flustra colony possessed a

considerable number of muscles; each was furnished with a movable lip or

lid to block up the entrance of his cell when he courted retirement; each

had his individual nerves, and consequently his individual sensations,

though feeling and moving simultaneously with his fellow citizens by the

agency of a system of nerves common to the whole republic, and sending

forth a delicate filament to the inmate of each cell.

[Illustration: Flustra in its cell. (Highly magnified.)]

Such are the wonders which but for the microscope would for ever have

remained unknown to man.

The Escharæ greatly resemble the Flustræ, for here also the cells

are disposed side by side upon the same plane, so as to form a broad

leaf-like polyzoary, which, however, is not of a horny or coriaceous

texture, as in the latter genus, but completely calcified, so as to

present something of the massiveness of the stony corals. The annexed

wood-cuts, showing us _Eschara cervicornis_, first A, in its natural

size; then B, a few cells magnified twenty diameters, and ultimately C,

a single individual so highly magnified as to reveal some of the details

of its otherwise invisible structure, give us a good idea of the truly

remarkable organisation of the Polyzoa.

In the Escharæ and Flustræ the cellular extension of the common stock

or polyzoary is unbroken, and opening on both surfaces, while in the

Retepores we find the cells opening only on one side, and the leaf-like

expansion pierced like network.

[Illustration: Eschara cervicornis. (Natural size.)]

[Illustration: Portion of a branch of the polypary of Eschara cervicornis,

magnified twenty diameters, to show the form and arrangement of cells.]

In cabinets of natural history, the species commonly called Neptune's

ruffles will rarely be found wanting. It is a native of the

Mediterranean, but individuals of a smaller size are also found in the

British seas.

[Illustration: An individual of Eschara cervicornis, highly magnified.

_a._ Tentacula

_b._ First digestive cavity.

_d._ Stomach.

_f._ Anus.

]

[Illustration: Retepora cellulosa. (Neptune's Ruffle.)]

The Lepraliæ, or Sea-Scurfs, form thin calcareous crusts of a

white-yellow or reddish colour on rocks, shells, and sea-weeds. To the

naked eye they appear as rude unsightly eruptions, so as to justify

their name derived from the hideous leprosy of the East, but, when

magnified, their cells, generally disposed in regular concentric rows,

exhibit a surprising diversity and elegance of structure. Forty species

are found in the North Sea alone; hence we may judge how great the number

of still unknown forms must be that spread their microscopic traceries

over the algæ and shells of every zone.

It would lead me too far were I minutely to describe the Cellulariæ with

their cells disposed in alternating rows on narrow bifurcated branches;

the Tubulipores, with their mouths at the termination of tubular cells

without any movable appendage or lip; the Bowerbankias and Lagunculas,

with their creeping stems and separate cells; suffice it to say that a

wonderful exuberance of fancy displays itself in the structure of the

numerous varieties of the Polyzoa.

[Illustration: A. Portion of a Cellularia, magnified.

B. A Bird's Head Process, more highly magnified, and seen in the act of

grasping another.]

But a closer inspection reveals still greater miracles to the marine

microscopist, for most genera, and chiefly the Cellulariæ, possess

very remarkable appendages, or processes, presenting the most striking

resemblance to the head of a bird. Each of these processes, or

"aviculariæ," as they have been named, has two "mandibles," of which one

is fixed like the upper jaw of a bird, the other movable like its lower

jaw; the latter is opened and closed by two sets of muscles, which are

seen in the interior of the head, and between them is a peculiar body,

furnished with a pencil of bristles, which is probably a tactile organ,

being brought forwards when the mouth is open, so that the bristles

project beyond it, and being drawn back when the mandible closes. During

the life of the polyzoon, these tiny "vulture-heads," which are either

sessile or pedunculated, keep up a continual motion, and it is most

amusing to see them see-sawing and snapping and opening their jaws, and

then sometimes in their incessant activity even closing upon the beaks of

their neighbours.

It is still very doubtful what is their precise function in the economy

of the animal; whether it is to retain within reach of the ciliary

current bodies that may serve as food, or whether it is like the

pedicellariæ of the sea-urchins to remove extraneous particles that may

be in contact with the surface of the polyzoary. The latter would seem to

be the function of the "vibracula," which are likewise pretty generally

distributed among the polyzoa. Each of these long bristle-shaped organs,

springing at its base out of a sort of cup, that contains muscles by

which it is kept in almost constant motion, sweeps slowly and carefully

over the surface of the polyzoary, and removes what might be injurious

to the delicate inhabitants of the cells, when their tentacles are

protruded. So carefully have these lowly molluscs been provided for!

The polyzoa can neither hear nor see, at least as far as we are able to

ascertain, but the delicacy of their sense of touch is very great. "When

left undisturbed in a glass of fresh sea-water," says Dr. Johnston,[R]

"they push their tentacula beyond the mouth of the cell by straightening

the body, and then expanding them in the form of a funnel or bell,

they will often remain quiet and apparently immovable for a long time,

presenting a very pretty and most interesting object to an observer of

the 'minims of nature.' If, however, the water is agitated, they withdraw

on the instant, probably by aid of the posterior ligament or muscle; the

hinder part of the body is pushed aside up the cell, the whole is sunk

deeper, and by this means the tentacula, gathered into a close column,

are brought within the cell, the aperture of which is shut by the same

series of actions. The polyzoa of the same polyzoary often protrude their

thousand heads at the same time, or in quick but irregular succession,

and retire simultaneously, or nearly so, but at other times I have often

witnessed a few only to venture on the display of their glories, the

rest remaining concealed, and if, when many are expanded, one is singled

out and touched with a sharp instrument, it alone feels the injury, and

retires, without any others being conscious of the danger, or of the

hurt inflicted on their mate. The polyzoa propagate by gemmation and by

ova or eggs, which, germinating on the inner surface, escape at a later

period into the visceral cavity, and are finally discharged into the wide

sea, so to fulfil their mission in creation, and people the shores of

every clime with myriads of busy workers in horn and in lime, which, with

subtle chemistry, they draw from a fluid quarry and build up in textures

of admirable beauty and heaven-ordered designs."

[Footnote R: "History of the British Zoophytes," 2nd edit. vol. i. p.

259.]

Each polyzoon begins with a single ovum. The original or seminal cell

of a flustra or lepralia has no sooner fixed itself upon some stone,

shell, or alga, than new buds begin to shoot forth, which in their turn

produce others from their unattached margins, so as rapidly to augment

the number of cells to a very large amount. Thus a common specimen of

_Flustra carbasea_ presents more than 18,000 individual polyzoa, and as

each of these has about twenty-two tentacula, which are again furnished

with about a hundred ciliæ a piece, the entire polyzoary presents no less

than 396,000 tentacula and 39,600,000 ciliæ. The Rev. David Landsborough

calculated that a specimen of _Flustra membranacea_ five feet in length

by eight inches in breadth had been the work and the habitation of above

two millions of inmates, so that this single colony on a submarine island

was about equal in number to the population of Scotland. As the tentacula

are numerous in this species, four thousand millions of ciliæ must have

provided for its wants, about four times the number of the inhabitants of

this globe!

[Illustration: Clavellina producta. Group of two adult and several young

individuals, magnified about five times.

_c._ Branchial orifice.

_e._ Branchiæ.

_i._ Anal orifice.

_l._ Stomach.

_o._ Heart.

_u_, _u′_, _u″_. Reproductive

buds, springing from the abdomen of the adults.

]

[Illustration: Ascidia mammillata.

_a._ Branchial orifice, open.

_b._ Anal orifice, closed.

]

The Tunicata are so called because their soft parts are not enclosed in

a calcified shell such as invests the majority of their class, but in a

more or less coriaceous envelope or tunic which is either bag-shaped, and

provided with two apertures, or tube-shaped, and open at the ends. They

present a strong resemblance to the Polyzoa, not merely in their general

plan of conformation, but also in their tendency to produce composite

structures by gemmation; they may, however, be at once distinguished from

them by the absence of the ciliated tentacula which form so conspicuous

a feature in the external aspect of a flustra or a retepore. Their

branchiæ, which have generally the form of ridges (_e_), occupy a large

sac, forming, as it were, the antechamber of the alimentary canal, which

is barely distinguishable into gullet, stomach, and intestine, and always

convoluted or folded once on itself. The Tunicata are exclusively marine,

and widely spread from the arctic to the tropical seas. All of them are

free during the earlier parts of their existence; some remain permanently

so (Pyrosomidæ, Salpæ), but the generality (Ascidiæ, Botrylli) become

fixed to shells and other marine bodies; some exist as distinct

individuals (Ascidiæ, Cynthia), whilst various degrees of combination are

effected by others (Botryllus, Clavellina, Pyrosoma), and some are simple

in one generation and combined in the next (Salpæ).

Thus the whole family is divisible into two groups, the _simple_ and the

_aggregate_; both branching out into numerous genera, of which my limits

only allow me to mention some of the most remarkable. The simple Ascidiæ,

or Sea-Squirts, are very common on our shores. "Rarely," says Forbes,

"is the dredge drawn up from any sea-bed at all prolific in submarine

creatures without containing few or many of their irregularly shaped

leathery bodies, fixed to sea-weed, rock, or shell, by one extremity, or

by one side, free at the other, and presenting two more or less prominent

orifices, from which, on the slightest pressure, the sea-water is ejected

with great force. On the sea-shore, when the tide is out, we find similar

bodies attached to the under surface of rough stones. They are variously,

often splendidly, coloured, but otherwise are unattractive or even

repulsive in aspect. Some are of a large size, several inches in length.

As may easily be imagined, they lead a very inactive life, except in the

young state, when by means of a long tail they rapidly swim about, until

finally settling in some convenient spot, they gradually assume the form

and adopt the quiet life of the parent from which they sprang."

[Illustration: Chelyosoma Macleayanum.

_a._ Branchial orifice.

_b._ Anal orifice.

_c._ Coriaceous envelope of the sides.

_d._ Stone to which the animal is fixed.

]

To the simple Tunicata belong also the Chelyosomata, whose coriaceous

envelope, consisting of eight somewhat horny angular plates, reminds

one of the carapace of the turtle. Their small and prominent orifices,

perforating the plated surface, are each surrounded by six triangular

valvules.

Some species of simple Ascidians on the coasts of the Channel and the

Mediterranean are valued as articles of food. At Cette sea-squirts

are taken regularly to market, and _Cynthia microcosmus_, although so

repulsive externally, furnishes a very delicate morsel.

[Illustration: Botryllus violaceus. Two of the stems magnified.

_a._ Common test.

_b._ Some of the branchial orifices.

_c._ The common anal orifice of one of the systems.

]

[Illustration: Diazona violacea (magnified).]

While in the Clavellinidæ the animals are connected by creeping tubular

prolongations of the common tunic through which the blood circulates,

the Botrylli form translucent jelly-like masses of various hues of

orange, yellow, purple, blue, grey, and green; sometimes nearly uniform

in tint, sometimes beautifully variegated, and very frequently pencilled

as if with stars of gorgeous device; now encrusting the surface of the

rock, now descending from it in icicle-like projections. They are also

frequently attached to the broad-leaved fuci, investing the stalks, or

clothing with a glairy coat the expanse of the fronds. "In examining

their bodies," says the distinguished naturalist previously quoted,

"we find that it is not a single animal which is before us, but a

commonwealth of beings bound together by common and vital ties. Each star

is a family, each group of stars a community. Individuals are linked

together in systems, systems combined into masses. Few bodies among the

forms of animal life exhibit such exquisite figures as those which we see

displayed in the combinations of these compound Ascidians."

In the genus Diazona, which has its chief seat in the Mediterranean, the

animals, which are very prominent and arranged in concentric circles,

form a single system expanded into a disc like that of a flower or of an

Actinia. The anal orifices, it will be seen, are situated close to the

branchial apertures at the free end of the single animals, while in the

Botrylli they open into a central excretory cavity.

[Illustration: A single individual of Pyrosoma giganteum, cut out of the

common test and magnified.

_a._ Branchial or external orifice.

_b._ Anal or internal orifice.

_d._ Stomach.

_e._ Liver.

_f._ Branchiæ.

]

In the Pyrosomes we find large colonies of small individuals aggregated

in the form of a cylinder open at one end. Their mouths or anterior

extremities are situated on the exterior of this hollow body, which they

bristle with large and longish tubercles (_a_), whilst the opposite or

anal orifices (_b_) open into the cavity of the cylinder, whose smooth

wall they perforate with numerous small holes. By a simultaneous action

the central cavity is either narrowed or enlarged, and by this means the

strange social republic glides slowly through the waters.

The Pyrosomes inhabit the Mediterranean and the warmer parts of the

ocean. In the former at times their abundance is a source of great

annoyance to the fishermen, sometimes even completely clogging their

nets, and on the high seas they are not seldom met with in almost

incredible profusion. Their delicate and transparent forms, their

elegant tints, and their unrivalled phosphorescence render them objects

of admiration to the voyager, and entitle them to rank amongst the most

resplendent living gems of the ocean.

[Illustration: Salpa maxima.

_a._ Upper lip or posterior orifice.

_b._ Anterior orifice.

_c._ Prolongations of the test by which the

animal is adherent to its neighbours.

]

[Illustration: Salpæ, isolated and associated.

A. _Salpa runcinata_, solitary.

B. _Salpa runcinata_, associated.

C. _Salpa zonaria_, aggregated.

]