CHAPTER IX.

[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."