SEA-SNAILS AND SEA-SLUGS.
Mollusks that have their shell all in one piece are technically known
as the Gasteropoda, or belly-footed creatures; but for our purpose the
term sea-snail will serve admirably, for it is a popular term that
will not cause misunderstandings, as many popular general terms do.
The sea-snails, as living creatures, are more amenable to study by
the shore-naturalist, than is the case with the bivalves; and every
rock, whether it be thickly clothed with weeds, or bare and exposed
to the full fury of the waves, will provide us with specimens. It is
true, that all visitors to the sea-shore are well acquainted with the
most plentiful of these--the periwinkle, the purple, and the limpet.
But though they are familiar with the forms and names of such common
objects, there may be among my readers some to whom the principal facts
in the economy and structure of these species may be new or interesting.
[Illustration:
LIMPETS. PURPLES.
]
I fear, that in popular estimation, there is but one kind of Limpet.
As a matter of greater exactitude, I may say that eight or nine
species may be found on our shores; and we may find some points of
interest even in the too common species (_Patella vulgata_). Only
those perhaps who have been badly in want of bait for a little fishing
have troubled to see what is beneath the conical shell; but the shell
itself is worthy of a little attention. What could be better adapted
for the animal’s mode of life than this? The Limpet is not a deep-water
mollusk, but lives between tide-marks, where it receives the full force
of the waves as they beat and hammer the rocks in stormy weather.
But the Limpet has a broad foot, which exudes a thick glue, whereby
it sticks tightly to the rock. Then his muscles are powerful, and by
their aid he pulls his conical roof well down till its edges fit into
the little pit he has sunk in the rock-surface, and thus ensconced he
can defy the hardest gale that may chance to blow and the heaviest
water-hammers that the sea uses against the land. The Limpet is
typical of the Briton, alike in his tenacity of purpose and his love
of privacy. But with all his exclusiveness John Bull likes to open his
doors and windows wide to let in the air, and we shall find the Limpet
resembling him in this detail; for if you seek him when the tide is
out, you may surprise him with his roof so lifted up that the edges are
a quarter of an inch away from the rock. Then is the time to take him
unawares, and force his foot from its firm hold. Having secured him, we
are at liberty to inspect the owner of this strange house, but we can
best do this by placing him in our clear glass bottle, and letting him
crawl up the side.
That which is known as the mollusk’s “foot” has no relationship with
the feet of vertebrate animals, the name being suggested by the similar
use to which dissimilar organs are put. We have already explained that
the term gasteropod signifies “belly-foot,” and if we were to cut
through the “foot” of the Limpet, we should find that it is indeed its
belly, for it contains the principal portion of its viscera. We are not
going into the anatomy of the mollusca, just now, but will confine our
attention to its exterior. It has now begun to climb up the glass, and
we can see that the foot is spread out so that it occupies the greater
portion of the area covered by the shell. At the fore part it has a
distinct head, with a pair of tentacles, ditto eyes, and a very evident
mouth, for the Limpet’s principal occupation appears to be to lick the
surface upon which it is gliding. Around the foot and the head there
runs a frill which is really the creature’s breathing apparatus, and
between that and the shell there is, of course, the mantle by which the
shell was secreted, and is enlarged as occasion requires. The Limpet is
now in rapid motion, and we can see that it progresses in exactly the
same fashion as do the garden snails and slugs, that is, by a series
of muscular contractions, evidenced by the constant ripple along the
surface of the foot. The foot exudes a very tenacious slime, which
enables it to obtain perfect contact with the surface over which it is
gliding, or upon which it is resting. It is perfectly astonishing how
much nonsense is still written in books upon this subject by persons
who ought to know better, and who could easily test the correctness
of their views by occasionally studying Nature, instead of relying
so much upon academical teaching, and that of an antique character.
Their statement is, that the Limpet holds on so tightly by creating a
vacuum, some say under the foot, others under the shell. So ancient an
authority as Reaumur disproved these notions. He tested the matter by
cutting a Limpet in two, shell and all. According to the teaching of
the vacuumites, the animal’s hold should then have loosened; but no,
the two portions still adhered to their base. Anyone by observation
can testify to the truth of Reaumur’s explanation; there is the same
powerful hold in the foot of a garden snail on a damp surface, but in
that case it does not seem so great, because his shell affords a better
hold for the experimenter. The annoying feature of the Limpet is the
shape of his shell, which prevents our taking hold of it. Where the
surface of the rock is friable, as some of our Cornish Killas rocks,
and the chalk rocks of the Kentish coast, the Limpet’s foot, when
forcibly pulled up, brings with it particles of the surface, which have
separated from the parent rock more easily than from the glue of the
mollusk’s foot.
A wonderful thing about the Limpet is its power to sink a shallow pit
in the surface of the rock, corresponding to the shape of the shell;
and this, of course, has led to much theorising to explain how it is
accomplished. Patent solvents secreted by the animal, the carbonic
acid gas given off from the breathing apparatus (which strangely does
not destroy its own shell!), and so on. A little study of Nature would
show that the wonderful organ which enables them to scrape away the
surface in long zigzag lines, as they crop the minute vegetation,
would be equally effective if applied to the spot upon which they
prefer to roost, and to which they habitually return after their
pastoral wanderings. The action of this tongue on the rocks can be very
distinctly _heard_ on the shore, though possibly not in the library or
the museum, where only the empty shells are admitted. It is worth while
dissecting a Limpet, and getting out this remarkable tongue, which is a
ribbon-shaped organ, closely studded with minute hooks of flint, to the
number of nearly 2,000. A similar _lingual ribbon_, as it is termed,
will be found in most of the Gasteropods.
I have dealt at such length with the Limpet, because its structure will
enable us to understand the other mollusks we have to mention, widely
as they may appear to differ in the forms of their bodies and shells.
The Limpet’s shell is a low cone, and the shell of a Whelk is a greatly
elongated cone, coiled spirally upon itself; the animal adapting itself
to that form.
[Illustration:
SMOOTH LIMPET. SMOOTH LIMPET, THICK VARIETY.
]
In addition to the Common Limpet (_Patella vulgata_) we have the Smooth
Limpet (_Patella pellucida_), which must be sought at low-water on
the borders of the laminarian zone. It feeds upon the Great Oar-weed,
and a peculiar variation will be found between the specimens feeding
on the smooth flat fronds and those feeding on the great stems. The
shell of the first is coloured a pale brown, pellucid as its specific
name suggests, the apex set very far forwards, and from it there start
backwards from three to six exceedingly fine radiating lines of a
dazzling brilliant blue. The specimens that live upon the Oar-weed’s
stems look entirely different, for the shell becomes thickened,
and consequently much more opaque, and its shape alters to enable
it to sit close on a rounded surface. It was formerly considered a
distinct species, and was named _Patella lævis_. So, too, the little
Tortoise-shell Limpet (_Acmæa testudinalis_), changes its form when
feeding upon the leaves of the Grass-wrack (_Zostera marina_), and has
then had the name of _Acmæa alvea_ bestowed upon it.
There are other forms of Limpets (though not species of Patellidæ)
to which we wish to refer, but we are getting far away from our
illustration of the Purple (_Purpura lapillus_), on page 208, to which
we must now hark back. The Purple is often known as the Dog Winkle.
It abounds upon the rocks between tide-marks, whence it may be picked
without the formalities necessary in the case of the Limpet. It comes
off easily, for its foot is small, but the moment it is disengaged from
the rock it retires into its shell and closes its door. Now apart from
the difference in the shape of the shell, here is another departure
from molluskan arrangements as illustrated by the Limpet. It is called
an _operculum_ (Latin, a cover or stopper), and is so attached to
the foot, that when the Purple withdraws from public view this comes
last, and fits the mouth of the shell so accurately that there is no
getting inside. In this case it is a horny oval disk, but in some
species it is strengthened by the deposit of layers of shelly matter
until it becomes of considerable thickness and quite stony. If we mark
our disapproval of the Purple’s lack of courtesy in slamming his door
in our face, by pushing against his door, he retaliates by exuding
a purple fluid, which is said to permanently dye fabrics a similar
hue. The Purple is not a vegetarian like the Limpet. His mouth forms
a fleshy proboscis, which contains a marvellous boring apparatus--the
modified tongue. Often you may pick up bivalve shells on the beach,
of which one has been pierced with a very clean and smooth round hole
near the beak. If you did not know otherwise, you might suppose that
this was the work of a person who desired to make a shell-necklace or
other ornament, and had bored this hole with the greatest of care,
and then had unfortunately dropped it on the beach. The truth is,
it is the work of the Purple, or some other carnivorous sea-snail.
He has the reputation of being very destructive to mussel beds, by
boring these workmanlike holes in their shells, and literally eating
the poor mussel out of house and home. That is the style in which
the Purple gets his living; but he has a Nemesis in the shape of the
Star-fish, and I have seen one Star-fish eating or digesting three
Purples at once. It is a case of “diamond cut diamond,” for you would
think a Mussel or a Limpet would be safe enough with the shell closed
down, and so you might suppose the Purple’s operculum would shield
him from the Star-fish; but as I have already described in an earlier
chapter, the Star-fish knows well how to deal with obstinate victims
who won’t show their noses outside the door when their enemy calls--he
digests them first, and swallows them afterwards. Here is a complete
reversal of the Shakespearean motto, “May good digestion wait on
appetite;” to be complimentary to the Star-fish we should say, “May
appetite on good digestion wait!” In the bottom left-hand corner of
the purple-and-limpet illustration, is a baker’s dozen of nine-pins:
they are the egg-cases of the Purple, which may be found in larger or
smaller patches on any rock where these mollusks abound.
[Illustration: NETTED DOG-WHELK.]
Among the weeds on the rocks we are sure to find the Netted Dog-whelk
(_Nassa reticulata_), with a rather dirty-looking shell. It is covered
with broad grooves crossed by fine lines at right angles, producing the
appearance of network, which gives it the distinctive name, netted.
Its scientific name also is suggested by the same appearance, for
_Nassa_ is Latin for a special kind of fishing-net. Like the Purple,
the Dog-whelk is carnivorous. There is a prettier species, with a thick
lip, called _Nassa incrassata_.
The true Whelk (_Buccinum undatum_) only comes within our province in
the shape of empty shells cast up on the beach, for its range is from
low-water to a hundred fathoms. In deep water it is very plentiful,
and fishermen who want it for bait, let down baskets containing pieces
of fish, which attract a large number to their doom. Their remarkable
clusters of egg-nests are frequently washed ashore with seaweeds; each
capsule in the bunch contains about half-a-dozen eggs. The shell of
the Whelk, rubbed down on a smooth slab of stone, affords an admirable
vertical section illustrating the structure of gasteropods.
[Illustration: RED WHELK.]
The Red Whelk is the _Fusus antiquus_, so-called because it abounds
in a fossil condition in the Red Crag of Essex, where also occurs a
reversed form--that is, with the spire coiled the contrary way, and
hence called _Fusus contrarius_. In Scotland it is the Buckie, or
the Roaring Buckie, for this is the shell in which the roar of the
sea resides. It is more esteemed than the Common Whelk as food by
the poorer population of Scotland. It occurs, like _Buccinum_, from
low-water to a hundred fathoms.
There is a fairly common shell, similar in size and general form to
the Purple, but bristling all over with flattened recurved hooks, in
clusters of threes. It is generally known as the Sting-winkle (_Murex
erinaceus_), one of a genus from which the celebrated purple dye of
ancient Tyre was obtained. Its familiar name it owes to its sharing in
the hideous crime of destroying edible species for the sole purpose of
gratifying its own base appetite. The fishermen have actually noticed
it in the act, and seeing the peculiar boring apparatus at work, have
thought this a sting. It is far worse than that, for a sting may be
survived, but no mollusk, I believe, gets over the attack of the boring
tongue, which changes its function when the boring is finished, and
becomes an instrument for tearing and masticating its victim’s flesh.
The exotic representatives of the great Cone-family of shells are
familiar and admired objects in collections as well as on nick-nack
tables in the drawing-room. We have no native species of the genus
_Conus_ but we have a number of representatives of the _family_ in the
Pleurotomas and Mangelias, though they do not approach very closely
to the typical form of a Cone-shell, with which we commonly associate
the spotted Cone (_Conus marmoreus_) of Chinese seas. The Seven-ribbed
Conelet (_Mangelia septangularis_) is like a tiny Buckie-shell--it is
but half an inch long--with bold longitudinal ribs, of which you can
count seven in one revolution of the shell. The shell is thick, of a
dull pinkish hue, and unprovided with an operculum. The outer lip is
notched where it joins the previous whorl. There are several British
species.
[Illustration: COWRY.]
One of the most charming of our native shells is the little Cowry
(_Cypræa europea_), which is so plentiful on most of our shores. Most
of us who have visited the sea-shore in childhood have had the delight
of hunting for this shell, empty and clean, among the ingredients of a
fine beach; but probably some of those who are most familiar with it
as an empty shell would scarcely recognise it for the same species if
they saw the living Cowry gliding along with his shell on his back. He
carries a pair of tentacles, with eyes at their base, and the long
curved tubular tongue ready for service; but the most singular feature
is that his mantle is used not merely to clothe the delicate body,
but a portion of it comes outside, and closely wraps the greater part
of the shell. In its younger days it had not its present beautifully
arched lip, which almost closes up the doorway of the shell, and
leaves but a narrow slit, delicately denticulated, to allow the foot
and mantle to pass through. Before maturity it had a wide mouth, with
a sharp thin edge to the outer lip, but that, you see, has now grown
over towards the inner lip. The colour of the shell may be described
as a flesh-tint on the upper surface, varying in intensity to both
lighter and darker. Many specimens bear on the crown of the shell three
ill-defined blotches of a very dark brown. The under surface is white.
The whole shell is ornamented by very regularly disposed transverse
ribs, which are rounded and polished.
There are several other cowry-like shells to be found generally
distributed, but by no means so plentifully on our coasts. One of these
is the Smooth Margin-shell (_Erato lævis_), smaller than the Cowry, and
with the lip curved outward, instead of inward as in the Cowry: it has
thus an external margin, whence the name. It is white and exquisitely
smooth. The animal is very similar to _Cypræa_, and it envelops its
shell in the same fashion. Of similar habit is the Poached Egg (_Ovula
patula_), though the shell is very different. The mouth of the shell
gapes widely, and the lip is thin and sharp. Its colour is white with a
pink tinge, and its appearance is so suggestive of its name that there
is little likelihood of mis-identification. It is a South Coast form.
A solid-looking shell, with a highly-polished surface, over which three
lines of arrow-heads are chasing each other, a perforation of the shell
just outside the inner lip, a fairly wide mouth, closed when at rest by
an operculum: these are the principal features of the Necklace Natica,
so-called because it deposits a large number of eggs, so agglutinated
into a broad spiral band, that the whole has been likened to a
necklace. So it is called _Natica monilifera_, and _monilifera_ means
necklace-bearing. The animal is an odd creature, whose mantle laps
partly over the shell, and the large foot is furnished in front with
a broad fold, which is turned back as a protection to the head. It is
herbivorous, and crops the seaweeds on sandy and gravelly shores, from
low-water to about ninety fathoms.
There is a very thin, ear-shaped shell, clear and fragile, known as
_Lamellaria perspicua_. It is not sufficiently capacious to accommodate
the whole of the animal, so parts of it have to remain permanently
outside; the mantle, for instance, cannot be withdrawn, and it folds
over, completely wrapping up the shell and hiding it from view. It is
an awkward thing to have your house so small that you cannot get right
inside, because in the sea there are so many hungry creatures always
roving about, and snapping up any delicate morsel that is unprotected;
and even some that are protected get swallowed up in like manner. But
_Lamellaria_ has learned how to make up to some extent for Nature’s
stinginess in the matter of shell-stuff. About a quarter of a century
since, Giard showed that _Lamellaria_ was to be found in association
with compound ascidians, a group to which we shall call attention in a
later chapter. Quite recently[6] Prof. W. A. Herdman, Director of the
Port Erin Biological Station, added greatly to the interest of Giard’s
observation by one of his own, which shall be given in his own words:--
[6] “Conchologist,” 1893.
“_Lamellaria perspicua_ is not uncommon round the south end of the Isle
of Man, and is frequently found under the circumstances described by
Giard; but I met lately with such a marked case on the shore near the
Biological Station at Port Erin, that it seems worthy of being placed
on record. The mollusc was on a colony of _Leptoclinum maculatum_, in
which it had eaten a large hole. It lay in this cavity so as to be
flush with the general surface; and its dorsal integument was not only
whitish with small darker marks which exactly reproduced the appearance
of the _Leptoclinum_ surface with the ascidiozooids scattered over it,
but there were also two larger elliptical clear marks which looked like
the large common cloacal apertures of the Ascidian colony. I did not
notice the _Lamellaria_ until I had accidentally partly dislodged it
in detaching the _Leptoclinum_ from a stone. I then pointed it out to
a couple of naturalists who were with me, and we were all much struck
with the difficulty in detecting it when _in situ_ on the Ascidian.
[Illustration: HORN-SHELL.]
“This is clearly a good case of protective colouring. Presumably the
_Lamellaria_ escapes the observation of its enemies through being
mistaken for a part of the _Leptoclinum_ colony; and the _Leptoclinum_
being crowded like a sponge with minute sharp-pointed spicules is,
I suppose, avoided as inedible (if not actually noxious through
some peculiar smell or taste) by carnivorous animals which might
devour such things as the soft unprotected mollusc. But the presence
of the spicules evidently does not protect the _Leptoclinum_ from
_Lamellaria_, so that we have, if the above interpretation is correct,
the curious result that the _Lamellaria_ profits by a protective
characteristic of the _Leptoclinum_ for which it has itself no respect,
or to put it another way, the _Leptoclinum_ is protected against
enemies to some extent for the benefit of the _Lamellaria_ which preys
upon its vitals.”
Since the publication of Prof. Herdman’s note, I have frequently found
_Lamellaria_ on the undersides of large stones at low-water on the
Cornish coast. I have always found it on _Leptoclinum gelatinosum_, and
can quite endorse his remark as to the difficulty of distinguishing
it. On one occasion I found no less than four specimens feeding upon
one patch of the ascidians, and pointed them out to a friend, who,
however, failed to see them until they were absolutely touched by my
finger. The shell is exceedingly delicate, and in the hands of most
persons would be hopelessly ruined at the first touch. The ordinary
methods adopted by conchologists for getting the animal from the
shell will not answer in this case; but I have a plan which succeeds
admirably. I give a specimen of _Lamellaria_ to an anemone of refined
tastes, who will deal with it carefully. _Bunodes verrucosa_ is my
favourite assistant, and he returns the shell clean and sound in a day
or two.
There are several species of Spire-shells (_Rissoa_) to be found
feeding in great numbers on Grass-wrack and Sea lettuce, and we shall
also find the empty shells in the sand. There are, however, other
forms that may be confused with them and with each other, that are
very plentiful in sand. These are the comparatively large Turret-shell
(_Turritella communis_), which is ornamented with spiral ridges, each
one running continuously from the apex to the mouth. In the Ruddy
Pyramid (_Chemnitzia rufescens_), which is much smaller, but of similar
form, the ridges run _across_ instead of _along_ the whorls, whilst in
the Horn-shell (_Cerithium reticulatum_), a similar effect is obtained
by several rows of very regularly arranged round dots in high relief.
A more distinct member of the family of Cerites is to be found in the
well-known Pelican’s-foot or Spout-shell (_Aporrhais pes-pelicani_), in
which the whorls are boldly tuberculated. When the shell has grown to
its full length, its annual stages of growth take a somewhat different
direction, and spread out in expansive lobes and corrugations until it
bears a fanciful resemblance in outline to the foot of the pelican. The
shell is about an inch and a half in length, and very thick. The animal
is carnivorous.
[Illustration: PELICAN’S-FOOT.]
Delicate specimens of the well-known Wentletraps (_Scalaria_) may
be found among fine sands. They are readily known by their dazzling
whiteness, the nearly round and flat-lipped mouth, and the bold curved
ridges that stand out across the whorls like cogs on a wheel. To this
genus belongs the Precious Wentletrap (_S. pretiosa_), from China, for
a single specimen of which as much as forty guineas has been paid. This
was in the days when shell-collecting without any scientific object
in view was a mania with some wealthy people; just as we have had the
tulip-mania, and now have the orchid-mania affecting persons who are
impelled by fashion rather than a love of knowledge or the beautiful in
Nature.
However we may be inclined to pass over the Periwinkle (_Littorina
littorea_) as a species too common to need any attention, it is bound
to thrust itself upon our vision at every turn among the rocks, where
it swarms. It appears strange that whilst this species is so largely
eaten by the poorer classes in towns as a “relish” for tea, the allied
and almost equally common species, _L. rudis_, should be let severely
alone. But the explanation is probably to be found in the fact that
whereas _littorea_ deposits her eggs in the ordinary way, _rudis_
retains hers until they have hatched out. Now seeing that the Winkles
of both species develop their hard stony shells before they hatch, it
would be impossible to eat _L. rudis_ without the great inconvenience
of having these hard gritty infants damaging one’s teeth. The smaller
red, or bright yellow shell, that may be found in abundance on the
rocks and weeds between tide-marks, is _Littorina littoralis_.
The seeker for shells on a sandy shore must do as the children
do--throw himself prone upon the beach, and hunt thoroughly, inch by
inch, examining the topmost layer first, then lightly scraping it off
and bringing fresh treasures to light. In this manner he will certainly
turn up the exquisite little Pheasant shells (_Phasianella pullus_),
that have the misfortune to be so small, or they would be greatly
esteemed for their rich colouring. They are very smooth, and of a white
or pale yellow hue, but so thickly covered with fine crimson lines
that at first sight this appears to be the colour of the shell. These
lines run parallel with each other, but with many curves, some flowing
gently, others short and acute. These lines vary much in thickness
throughout their length, here being very fine, there thickening
gradually and thinning off again. The shells are less than a quarter
of an inch in length, and the mouth is closed with an operculum. The
animal has the peculiar habit of moving first one half, then the other,
of its foot in progressing.
[Illustration: THE COMMON TOP.]
One of the handsomest of our common rock-shells is the so-called Common
Top (_Trochus zizyphinus_), though it is scarcely as plentiful as the
much smaller Grey Top (_Trochus cinereus_). It is pyramidal in form,
with an almost flat, broad base; the mouth closed by a spiral, horny
operculum. In some species there is an umbilicus, in others it is
wanting. The animal has two small fringed lobes between the tentacles,
and similarly fringed lappets to the neck. The sides, too, are lobed,
and several tentacle-like processes are given off from them.
The Grey Top (_T. cinereus_) is variable in colour. Usually it is
a dull yellowish grey, with inconspicuous dark zigzag marks upon
it; sometimes the ground colour is pinkish-white, with decided pink
markings, which present a checkered appearance. There is a deep and
wide umbilicus. In _Trochus zizyphinus_ there is no umbilicus, and in
the large Painted Top (_T. magus_), again, there is a very wide one.
This last-mentioned species lacks the smoothness of outline exhibited
by the other two, its whorls being more boldly ridged at their
junctions (_suture_). The animal has the head-lobes largely developed,
and it is brilliantly and variously coloured; hence its name. The Tops
are vegetable feeders.
[Illustration:
VIOLET-SHELL. RAFT OF VIOLET-SHELL.
]
On our South-western shores, when strong winds have blown from the S.W.
for days together, there are borne to us on the waves, and wrecked
upon our beaches, singular sea-snails from the mid-Atlantic. There the
Violet-shells (_Janthina_) float in myriads, and consume the still more
plentiful “Sallee-man” (_Velella_), a Jelly-fish we have mentioned
in a previous chapter. There are many singular features about this
_Janthina_. Like a shipwrecked mariner, it constructs a raft, secreting
glutinous material from the foot, in the form of many air-chambers
cemented together, and bearing beneath a large number of egg-capsules.
The shell is of somewhat similar shape to that of the Tops, but with
a much larger mouth. Its material, too, is so thin it can almost be
seen through; and on the upper part it is white, whilst beneath it is
coloured violet, whence its names. The animal has its head produced
into a thick muzzle, with a pair of tentacles and a pair of eye-stalks,
_but no eyes_. The breathing organs are two plume-like gills which
protrude from the shell.
[Illustration: SLIT LIMPET.]
We must return now to certain limpet-like forms, of which one, the
Keyhole Limpet (_Fissurella græca_), might be easily passed by as a
Common Limpet that has got damaged. In form and appearance the shell
is not unlike the common kind; the peculiarity consists in a short and
narrow slit at the summit, which has suggested the name. As a living
mollusk it must be sought in the laminarian zone, but the empty shells
are to be found between tide-marks. A smaller, but not very dissimilar
shell, has the keyhole not on the apex, but a little in advance of it.
This is the Perforated Limpet (_Puncturella noachina_). It is rarer
than the last, and is to be sought on the North coasts, where it lives
below the twenty-fathom line.
Yet another species is depicted in this cut. It is the Slit Limpet
(_Emarginula reticulata_), in which the notch or slit is in the fore
edge of the white shell; length of slit variable. Internally the shell
is thickened near the notch, and outside it is deeply grooved, so that
strong ribs radiate from the summit, and are themselves partly cut up
by lighter grooves transversely to the others. It comes up to low-water
mark, so may be taken alive from the shore. There is a second British
species, the Rosy Slit Limpet (_E. rosea_), much smaller, and sometimes
with the slit rosy, but this is not a reliable character.
On our Southern shores may be found--frequently on oysters--a shell
that may be said to be the highest development of the limpet type. Seen
from the point of view taken for our illustration, there is a long
gentle curve from the mouth to the beak, which is spirally twisted. The
general effect is to remind one of the conventional representations
of the Cap of Liberty. Owing to this cap-like form, it is known as
the Hungarian Cap, and the Torbay Bonnet (_Pileopsis hungaricus_). In
colour it varies from brown to an indefinite dirty-white hue.
[Illustration:
HUNGARIAN CAP. TUSK-SHELL.
]
The miniature elephant’s tusks represented in the illustration with
the Hungarian Cap are really shells, called Tusk or Tooth-shells
(_Dentalium_). They are represented of the natural size. The shell is
open at each end, and is tenanted by a strange little animal who is
attached to it near the small end. The Dentalium is not a very highly
developed creature, for though it has a head, it is quite a rudimentary
one, without eyes. But though it lacks eyes, it has a mouth, surrounded
by eight tentacles and into this go foraminifera and other minute
creatures it picks up on the sands and mud in deep water. We have two
British species, of which we may occasionally find the empty shells
washed up on the sands. Of these the Elephant’s-tusk (_Dentalium
entalis_) is very smooth and quite white throughout; whereas the
Grooved-tusk (_D. tarentinum_) is delicately grooved at the larger or
fore end, and tinged with pink at the small end.
[Illustration: SMOOTH MAIL-SHELL.]
In chipping off fragments of rock at low-water, upon which anemones
and other specimens are sitting, you may often get more than you
had thought, for sometimes when the piece of rock is placed in an
aquarium, other creatures will make their appearance, which were
unobserved before, owing to their colour, and the closeness with
which they attach themselves. One of these is the Bristly Mail-shell
(_Chiton fascicularis_), distinguished from other British species
by the possession of little bunches of short bristles, which are
arranged along the shell-border opposite each plate of mail. There is
considerable resemblance between these creatures and limpets, though
there are also important differences. Instead of the shell being in
one piece, it is composed of eight transverse plates, which overlap
at their edges, and allow it to be rolled up like a woodlouse. Each
plate is attached to the mantle by its front margin, and the mantle
forms a narrow border all round the shell. The animal, like the limpet,
has a broad foot upon which it creeps, mostly at night, so far as my
observations of _C. fascicularis_ go. Its head chiefly consists of its
mouth and jaws, eyes and tentacles being dispensed with as unnecessary
to its manner of life. The breathing organs are similar to those of the
limpet, but are arranged round the posterior end of the body only. The
shell is very flexible in all directions, so that the animal is not
constrained, like the limpet, to return to the same roosting spot each
time it wishes to rest.
There are a number of British species; the one figured is known as the
Smooth Mail-shell (_C. lævis_). It has a glossy shell of a reddish hue,
with a central ridge. The largest of the native forms is the Marbled
Mail-shell (_C. marmoreus_), whose delicately sculptured shell is
further ornamented with a mottling of browns and yellows. It is about
an inch and a quarter in length. The British species is almost as long,
but of much more slender proportions. The most plentiful form is the
Grey Mail-shell (_C. cinereus_), which does not greatly exceed half an
inch in length. It is not entirely grey, though this is the prevailing
tint, but there are delicate mottlings and streaks of many colours upon
it.
We now reach what we may very fitly term the Sea-slugs, for they
are creatures that externally have considerable resemblance to the
land-slugs, though structurally they are very different, and they are
far removed from each other in classification. The land-slugs (_Limax_)
carry a little shell embedded in their back, and their breathing organs
are internal; the Sea-slugs are entirely shell-less, except in the
embryo-stage, and their breathing apparatus is always exposed, and
situated on the back or sides. In consequence of this characteristic,
the Sea-slugs, as a group or section of the Gasteropods, are called
the Nudibranchiata, or naked-gilled mollusca. They are plentiful on
rocky coasts, where they range from half-tide to a great depth. The
best plan is to seek for them at low spring-tides, turning over stones
at the edge of the laminarian zone, when the slugs will be found at
rest on the under surfaces, in a more or less collapsed condition. They
will readily respond, however, to the attention paid them by putting
them in the calm clear water of our collecting bottles, and extending
their tentacles and branchial plumes, will explore their new quarters.
One of the most striking of these sea-slugs is the Sea Lemon (_Doris
tuberculata_), which is about three inches in length, broad, and with
the upper surface thickly studded with tubercles; this, in conjunction
with its colour, gives it a very close likeness to the half of a lemon
adhering to the rocks. As will be seen in the illustration, there are
two tentacles, and these are retractile within special cavities. The
branchial plumes are arranged in a crown-like circle in the middle
of the back, but near to the posterior end; and these also can be
withdrawn at the creature’s will. There are several British species,
some of them very small, and they range from low-water to twenty-five
fathoms, feeding upon zoophytes, sponges, anemones, and their own kind.
[Illustration: SEA LEMON.]
_Doris johnstoni_ is a smaller species than _tuberculata_, but
is worthy of attention on account of a certain resemblance to
_Lamellaria_. It is “got up” to mimic a sponge. There are no tubercles
on its surface, which is very finely roughed, so that it is sponge-like
to the touch. In colour it is creamy, wonderfully speckled with larger
and smaller spots of pale brown, that produce the effect of the porous
surface of a sponge, and the large spots are touched up with a darker
brown, to give depth to these false pores. When it is explained that
_D. johnstoni_ feeds on sponges like _Halichondria panicea_, this
colouring is easily understood, but its marvellous nature is not
lessened.
Some species of allied genera are quite remarkable, one might almost
say eccentric, in their ornamentation. _Ægirus punctilucens_, a species
found between tide-marks, is elaborately covered with large tubercles
and shining points; the branchial tufts assuming quite a tree-like
growth in miniature, around the orifice, which is placed further
forward than in _Doris_.
[Illustration: CROWNED EOLIS.]
The Crowned Eolis (_Eolis coronata_) has a slender body, long slender
tentacles, that cannot be withdrawn, and the back is covered with long
papillæ, gathered into a dozen spreading bunches. The two _erect_
tentacles behind the long pointed pair, if examined with a lens, will
be found to be beautifully ornamented by a series of annular plates.
It may be sought among the rocks at low-water, feeding chiefly on
the sertularian zoophytes. It is an active species, gliding over the
rocks, or swimming at the surface with its back downwards. They are
constantly waving their tentacles and moving their papillæ, from which
they exude a milky fluid when irritated, and even throw them off, as
a crab “shoots” his lesser limbs under similar circumstances. If kept
in an aquarium without suitable food, they become cannibals. _Eolis
papillosa_ is a similar species, the upper surface almost completely
covered with papillæ. It will be found under stones at low-water,
feeding on _Botrylli_ and other ascidians. If on a white species, it
will be wholly white, for like _Lamellaria_ and _Doris_, it goes in for
protective colouring. Introduce a specimen from a white ascidian into a
vessel containing, say, a crimson or brown Beadlet Anemone, and after a
few hours you will find the Anemone has disappeared, whilst the Eolis
has changed to the colour that the Beadlet was of. The papillæ of the
Eolis are really continuations of its digestive apparatus, and by this
simple arrangement a protective harmony is set up as often as it may
change its diet. Scientifically these papillæ are termed _cerata_.
[Illustration: SEA-HARE.]
[Illustration: SEA-HARE, FRONT VIEW.]
The last of our Sea-slugs does not belong to the Nudibranchiata,
for its branchiæ are concealed, and it possesses a shell--a thin,
flexible, translucent, convex plate, that covers the branchial plume,
and is itself covered by the mantle. My first Sea-Hare (_Aplysia
depilans_), was taken in ignorance. A hurried glance at a globular mass
of purple-brown jelly, among some small weeds, as I was hunting for
anemones, assured me I had something new to me, and I put it down at
the moment as a colony of compound ascidians; but on putting it into an
aquarium, I saw my mistake at once. The bundle unrolled, and some loose
wraps, shaking themselves out, resolved themselves into tentacles and
marginal lobes. The foot lengthened out, and I saw the creature had a
distinct neck, with a broad muzzle between the first pair of tentacles.
The second pair were folded, so as to present a strong suggestion
of the ears of a hare, and this is precisely the idea suggested to
fishermen in many countries, by whom the _Aplysia_ was first called
Sea-hare, or _Lepus marinus_. When it is viewed from the front, as in
the smaller illustration, the illusion is strengthened. It has the
habit of pouring out a violet fluid from the edge of the mantle when
handled, which is probably intended, like the Sepia’s ink, to produce a
cloud, under cover on which the Sea-hare can safely retreat. In other
days, this fluid was regarded with horror as a poison, and an indelible
stain. From this last notion the creature got its name, _Aplysia_,
which is from two Greek words, meaning unwashable, filthy. Its second
name, _depilans_, is also reminiscent of those old notions, for it was
thought that mere contact with the dreaded creature would cause the
hair to fall off. The Sea-hare of the present generation, however, is
quite harmless, as I can testify, whatever may have been the real or
assumed character of his ancestors.