[Sidenote: MOLLUSCA. CLEODORA.]
Twelfth Class--_Mollusca._
"As I have observed that you have been very diligent in studying
Lamarck since our last lesson," said Mr. Elliot, "I propose to make you
acquainted with the variations in the univalve genera.
"The animals of the _Mollusca_ are soft, without joints, generally
possessing a head, eyes, and tentacula, or feelers. They have also
a fleshy membrane, called a foot, which they use for climbing. The
orders, excepting the first, are named from the position of this foot.
They are five in number. The first order contains very few genera. One
genus, named _Cleodora_, contains a species brought from Africa. The
shell is curious, transparent, and shaped like the head of a halberd.
"The animals of the second order, _Gasteropoda_, have a muscular
foot, or disk, on which they rest. The families are seven. The first,
_Tritonia_, I shall pass over.
"_Phyllidiana_ includes the genera--
_Phyllidia._
_Chitonella._
_Chiton_ Coat of mail.
_Patella_ _Patella_, or limpet.
[Sidenote: CHITON. PATELLA.]
"The _Chiton_ moves like the _Patella_, upon a disk, or foot.
"The body of _Patella_ is entirely covered by the shell. You may have
many opportunities of examining the British species. _P. pellucida_ is
very transparent, with blue lines.
* * * * *
"The family _Semi-phyllidiana_ contains
_Pleurobranchus._
_Umbella_ _Patella_.
"The _Umbella_ shell is flat and white, and is sometimes four inches in
diameter. It is common in the Isle of France: there is another from the
Mediterranean.
* * * * *
"_Calyptracea_ is a larger family: it has many genera taken from
_Patella_:
_Parmorphorus_, Thracian-shield.
_Emarginula_, }
_Fissurella_, }
_Pileopsis_, } _Patella_.
_Calyptræa_, }
_Crepidula_, }
_Ancylus_, }
[Sidenote: FISSURELLA. PILEOPSIS. BULLA.]
_Parmophorus_ is found in the seas of New Holland and New Zealand.
The margin of the next genus is distinguished by a slit: the shell
is conic. (Plate 3.) _Fissurella_ has the top of the shell
perforated; it is called the _key-hole_ limpet, from the shape of the
aperture. _Pileopsis_ is obliquely conic. It was with this division
that the curious _Hipponyx_ ranked.
"_Calyptræa_ is very thin and brittle, with an internal lip. (See
Frontispiece.)
"_Crepidula_ has the shell partly covered, or arched: it looks like a
little slipper.
"_Ancylus spina-rosa_ is a pretty species from the south of France: the
genus is fluviatic. _A. lacustris_ and _fluviatilis_ are both natives
of our fresh-waters.
* * * * *
"The next family, _Bullæana_, has
_Acera_ _Bulla_.
_Bullæa_ _Bulla_.
_Bulla_ The same.
"_Acera_ and the following genus have each but one species, _Bulla
carnosa_ and _B. aperta_ of Linnæus. The original genus _Bulla_ was
composed of an assemblage of shells of various characters, having
little resemblance except in their gibbous form. _Bulla naucum_ is an
example of the genus of Lamarck's system; so is _B. lignaria_. (See
Frontispiece.) _B. ampulla_ and _B. striata_ are common in
collections.
* * * * *
[Sidenote: APLYSIA. DOLABELLA.]
"_Aplysiana_ is a small family, containing
_Aplysia_, or Sea-hare.
_Dolabella._
"One species of _Aplysia_ is found on the Devonshire coast: the name
Sea-hare marks the singularity of the two tentacula, which resemble the
ears of the hare. The body is folded up in a loose skin, or mantle:
upon the middle of the back it carries a circular shield, thin,
transparent, and yellowish, in which it resembles the slug. These
animals swim with ease.
"_Dolabella_ resembles the _Aplysia_ in some degree; the genus is
foreign, and one species is known to inhabit the bays of the Isle of
France, where it covers itself with a portion of mud."
"I cannot understand why animals related to the slugs should find a
place here," said Charles.
"Have not slugs the characteristics of the _Mollusca_ class?" asked his
father. "And are you quite sure that they are without a shell?
[Sidenote: ONCHIDIUM. LIMAX.]
"Our next family, the _Limacina_, has
_Onchidium._
_Parmacella._
_Limax_, slug.
_Testacella._
_Vitrina._
"_Onchidium_ is a genus from the shores of the Indian seas. The animals
have a shield: they live near the sea, and some are known to swim,
often coming to the surface to breathe the air.
"_Parmacella_ was found by an English traveller in Mesopotamia. It has
a shell covered by a shield. But you have not answered my question
respecting the _Limax_, or slug."
"I do not recollect," replied Charles; "yet how often we see slugs!"
"If I may be allowed to answer," said Lucy, "I think that the slug has
what I now understand to be a _shield_. I have often watched the animal
contract itself, and seen a broad, flat piece upon the back, which I
thought was a kind of shelter for it."
"The _Limax_, or slug," continued Mr. Elliot, "is, in fact, provided
with a coriaceous escutcheon, or shield, beneath which the animal
partly conceals itself. The _Limax agrestis_, or spinning-slug, has the
power of suspending itself by a kind of thread, formed of the viscid
substance that covers the body.
[Sidenote: TESTACELLUS. COLIMACEA.]
"_Testacellus_ is a very interesting genus, lately found in England:
the animal has a resemblance to the common slug: it carries the shield
on the hinder part of the body.
"_Testacellus scutellum_ feeds on earth-worms, and can so much lengthen
the body that it follows them under-ground.
"Our next order will show great alterations in the very large genus
_Helix_. I shall name to you those of Lamarck.
"The third order of _Mollusca_, _Trachelipoda_, begins with a
well-known genus, the snail, _Helix_. The term signifies that the
_foot_ is situated under the neck, or anterior part of the body.
The families in this order are numerous: they are divided into two
sections; the first includes those that breathe only in the air; the
second those that can exist only in the water, and are furnished with a
syphon.
* * * * *
[Sidenote: HELIX. CUROCOLLA. ANOSTOMA.]
"First section: family _Colimacea_; genera numerous; animals live
upon land only; tentacula generally four; during winter they enclose
themselves in their shells, with a false operculum.
_Helix_, snail _Helix_.
_Carocolla_ _Helix_.
_Anostoma_ _Helix_.
_Helicina_ _Helix_.
_Pupa_ _Helix_.
_Clausilia_ _Helix_.
_Bulimus_ _Helix_.
_Achatina._
_Succinea_ _Helix_.
_Auricula_ _Voluta_.
_Cyclostoma._
"What a number of new genera!" said Lucy. "I see the forms of the
shells vary very much; and how beautiful these little transparent
shells are!"
"They will find a place shortly. Here is a well-known species, _H.
aspersa_, in most of its varieties; _H. pomatia_, the apple-snail, now
naturalized in the county of Surrey; _H. ericetorum_, white with brown
bands, very frequent on chalky soils; _H. citrina_, transparent, pale
yellow, sometimes with one dark band; _H. muralis_, from the walls of
Rome; _H. bidentalis_, from Teneriffe; and the little _Helix hispida_,
which you may search for in your own garden; it is small, dark brown,
and rough.
"_Carocolla_ has the shells more flattened than _Helix_.
"_Anostoma depressa_ is a rare and curious shell.
"_Helicina_ is a West Indian genus. You saw them just now. We shall
find Helix in two other families.
[Sidenote: PUPA. BULIMUS. AURICULA.]
"_Pupa_ is a curious genus. The shells resemble a chrysalis. A few
minute species are found in Britain. _P. muscorum_ I have found buried
among damp moss. The larger species are natives of tropical regions.
These shells are often found _decollated_.
"_Clausilia papillaris_ is a pretty shell. (Plate 9.)
"_Clausilia rugosa_ is found in some parts of Britain, under old
hedges, at the foot of old trees, and similar places. It is a tapering
shell, with the aperture reversed, or left-handed, and bi-dentated:
the colour red-brown. It is to be found in the vicinity of Dorking, in
Surrey.
"_Bulimus_ is a large genus. A common small species is the _Gaudaloupe
Bulimus_.
"The largest land-shells are found in the genus _Achatina_. The greater
number are African.
"_Succinea_ contains a few species. One of them, _S. amphibia_, is
common near fresh-water.
"_Auricula_ has some resemblance to a _Voluta_. The aperture is
longitudinal: the columella has one or more folds.
"The forms of the species in _Cyclostoma_ are variable; but the
aperture is circular, and the margin revolute, or rolled back. _C.
elegans_ is often to be found on hedge-banks or chalk soils. It is a
pretty shell, sometimes white, often tinted with purple.
"In the family _Lymænana_, the species are amphibious; inhabiting
fresh-water; but rising to the surface to breathe the air. They have
but two tentacula.
"As several species are British, you may have the satisfaction of
examining them for yourselves.
* * * * *
[Sidenote: PLANORBIS.]
"The genera are,
_Planorbis_ _Helix_.
_Physa_ _Bulla_.
_Lymnæa_ _Helix_.
"_Planorbis_ is a discoid shell, and one peculiarity of the genus is,
that they are all reverse shells. In a discoid shell the spire is
depressed; when held up, the whorls turn from right to left, and the
aperture is left-handed. The largest species is _P. cornu-arietis_,[B]
which is a native of Brazil. _P. corneus_ is common in ponds and
ditches. Empty shells are to be found at the edge of the water.
(Plate 9.) If you take the animal to examine, and study its
habits, remember that you have no right to injure it, and that you have
already promised me that no kind of cruelty shall take place.
[B] Ram's-horn.
"_P. vortex_ is a smaller species. The outer valve is carinated.
"_Physa_ is found in fresh-water upon aquatic plants. They are small
shells.
[Sidenote: LYMNÆA. MELANOPSIS. VALVATA.]
"The animal of the _Lymnæa_ genus has two flat tentacula. _L.
stagnalis_ is a very pretty spiral shell, common in ponds.
"_L. auricularia_ is also frequent. It is much smaller than the first
species. The last whorl is swelling, and the aperture very wide. They
are both thin and brittle.
* * * * *
"The family _Melaniana_ are chiefly foreign. The shells are covered
with a dark-coloured epidermis. They are operculated.
_Melania_ _Helix_.
_Melanopsis._
_Pirèna._
"A species of _Melanopsis_ inhabits the river Orontes, in Syria.
* * * * *
"There is yet another family connected with _Helix_, the _Peristomata_,
containing
_Valvata._
_Paludina_ _Helix_.
_Ampullaria_ _Helix_, partly.
[Sidenote: PALUDINA.]
"Some of the _Valvata_ genus are found in fresh-water in Britain and
other European countries. The shells are small; they are discoid or
conoid, and have an operculum. In the shells of this family the margin
of the aperture is carried completely round. In _Paludina_ the whorls
are convex. They generally inhabit fresh-waters.
"_P. vivipara_ is found in rivers. Quantities of empty shells may be
taken from the sand of the Thames.
"Fossil species abound--Petworth marble is full of them."