group of animals that are enclosed in bivalve shells, and which were
once included with the Mollusca, but are now made to form quite a
distinct group by themselves. We refer to the Brachiopods, at one time
very abundant, as proved by the immense number of fossil shells embedded
in various stratified rocks, but now represented by only a few living
species.
The shells of these animals are commonly known as Lamp Shells, on
account of their resemblance to an antique lamp; and although at first
sight they bear a general likeness to certain bivalve shells of
lamellibranchs, a close examination will show that not only the shell,
but also the animal residing within it, are both of a nature very
different from that of the molluscs with which they were at one time
supposed to be closely related.
[Illustration: FIG. 154.--_Terebratulina._ THE UPPER FIGURE REPRESENTS
THE INTERIOR OF THE DORSAL VALVE]
The valves of the shell are unequal, and are not placed respectively on
the right and left sides of the body of the animal, but rather on the
dorsal and ventral or upper and lower sides. The ventral shell is the
larger, and is produced into a beak which sometimes has a round hole
corresponding in position with the hole for the wick of an antique lamp,
and the dorsal or smaller valve is always imperforate. The hinge is a
perfect one, the junction of the two valves being so well secured by it
that it is impossible to separate them without injury. It is formed by
two curved teeth on the margin of the ventral valve that fit into
corresponding sockets on the dorsal. A few brachiopods, however, have no
hinge, the valves being secured by means of numerous muscles. The hole
in the shell serves for the protrusion of a pedicel or foot by means of
which the animal is enabled to attach itself.
Two long arms, covered with vibratile cilia, and capable of being folded
or coiled, are attached at the sides of the mouth. They are practically
processes of the lips, mounted on muscular stalks, and attached to a
delicate calcareous loop on the dorsal valve; and serve not only to
produce water currents for the conveyance of food to the mouth, but also
answer the purpose of gills.
The digestive system of a brachiopod includes an œsophagus that leads
into a simply formed stomach round which is a large digestive gland. The
heart has only one cavity, but the animal is provided with two smaller
and separate organs that assist in the propulsion of the blood, which
circulates through numerous blood spaces in the bristly mantle.
About two thousand fossil species of brachiopods are known, extending
over a vast range of time; and the living species, numbering less than a
hundred, are found from shallow water to the greatest habitable depths.
Since the reader is hardly likely to form any extensive acquaintance
with the Brachiopods, we shall illustrate our remarks by the
introduction of only one species--the Serpent’s Head Terebratula
(_Terebratulina caput-serpentis_), which is found in deep water in the
North Sea. The interior of the dorsal valve, showing the calcareous loop
above referred to, is represented in fig. 154, as is also the exterior
of the shell, which is finely striated. The latter represents the dorsal
aspect of the shell in order to show the hole in the upturned beak of
the ventral valve.
[Illustration: FIG. 155.--UNDER SIDE OF THE SHELL OF _Natica catena_,
SHOWING THE UMBILICUS; AND OUTLINE OF THE SHELL,
SHOWING THE RIGHT HANDED SPIRAL]
We have now to consider the large group of head-bearing molluscs
(_Cephalophora_), the study of which forms a very important part of the
work of the sea-side naturalist; and while we deal with the general
characteristics of this group, the reader will do well to have before
him a few living typical species in order that he may be able to verify
as many as possible of the descriptions here given by actual
observation. These types may include such creatures as the whelk,
periwinkle, and limpet; or if marine species are not at hand at the
time, the garden snail, fresh-water snail, and slug will serve the
purpose fairly well.
By far the large majority of Cephalopods are enclosed in a single shell,
though a few have a rudimentary shell or none at all.
As is the case with the lamellibranchs, the shell is composed of both
animal and mineral substance, the latter being a calcareous deposit
secreted by the mantle of the animal. The shell is usually spiral in
form, as in the whelk, but sometimes conical (limpet) or tubular.
Spiral shells are nearly always _dextral_ or right-handed; that is, if
we trace the direction of the spiral from the apex to the mouth, we
find that its turns or whorls run in the same direction as the hands of
a watch. A few, however, are _sinistral_, or left-handed, and
occasionally we meet with left-handed varieties of those species that
are normally of the right-handed type. The cavity of the shell is a
single spiral chamber which winds round a central pillar, and each whorl
of the shell generally overlaps the preceding one, the two being
separated externally by a spiral depression called the _suture_.
Sometimes the coils of a shell are not close together internally, so
that the central column of the spiral is hollow, and opens to the
exterior at the base of the shell. In this case the shell is said to be
_umbilicated_, and the opening referred to is the _umbilicus_. In others
the spiral winds round a solid central pillar which is spoken of as the
_columella_.
[Illustration: FIG. 156.--SECTION OF THE SHELL OF THE WHELK, SHOWING
THE COLUMELLA]
The apex of the shell, sometimes called the _nucleus_, is the oldest
part, and represents what was once the whole. It is generally directed
backwards as the animal crawls, and in adult shells is often more or
less worn away by constant friction. We speak of the whorls as first,
second, third, &c., taking them in the order of their growth, and it
will generally be found that the last whorl is much larger than the
others, so much so that it contains the greater part of the body of the
animal; hence this one is commonly spoken of as the _body-whorl_, and
the others make up the _spire_ of the shell.
The mouth of the shell is of different forms in different species, but
in the herbivorous kinds it is usually simple, while in the carnivorous
species it is notched or produced. The edge of the mouth (_peristome_)
is formed by an _outer lip_ which is usually sharp in young shells and
either thickened, reflected (turned outward), or inflected (turned
inward) in adults; also it may be considerably expanded, or ornamented
by a fringed margin. The _inner lip_ is that side of the peristome
adjacent to the central pillar of the shell.
If we examine the external surface of several different shells, we find
that they are usually more or less distinctly furrowed or sculptured,
and that they are often marked by lines or bands of a colour different
from that of the ground tint. These furrows, lines, or bands sometimes
pass directly from the apex, across the various whorls, to the base of
the shell, in which case they are said to be _longitudinal_. If they
follow the course of the whorls, they are described as spiral; and if
parallel with the peristome, so that they mark the former positions of
the mouth of the shell, thus denoting the _lines of growth_, they are
said to be _transverse_.
Most univalve shells are covered with epidermis, but in some instances
the animal, when extended, surrounds the exterior of the shell with its
mantle, as do the cowries, and then the outside of the shell is always
glazed. Other species keep their shells covered with the mantle, and in
these the shell is always colourless.
The body of the head-bearing mollusc is attached to the shell internally
by one or more muscles, and if we examine the interior surface we are
generally able to distinguish the impressions or scars denoting the
points of attachment.
The reader will have observed that the periwinkle, whelk, and other
univalves close their shells by a kind of lid when they retract their
bodies. This lid is called the _operculum_, and is constructed of a
horny material, often more or less calcified on the exterior, and is
attached to the hinder part of the foot. It sometimes fits accurately
into the mouth of the shell, but in some species it only partially
closes the aperture. The operculum, like the shell itself, often
exhibits distinct lines of growth which display the manner in which it
was built up. If these lines are concentric we know that the operculum
grew by additions on all sides; but if its nucleus is at one edge, and
the lines of growth widest apart at the opposite side, the growth must
have taken place on one side only. Some, even, are of a spiral form,
denoting that the additions were made continuously at one edge, and such
opercula may be right-handed or left-handed spirals.
It will be noticed that in the above general description of univalve
shells we have introduced a number of technical terms which are printed
in italics, and this we have done advisedly, for the employment of these
terms is a very great convenience when giving descriptions of individual
shells, and we shall use them somewhat liberally in noting the
distinguishing characteristics of the families and genera; but before
entering into this portion of our work we must briefly note the general
features of the bodies of the _Cephalophora_.
[Illustration: FIG. 157.--DIAGRAM OF THE ANATOMY OF THE WHELK, THE
SHELL BEING REMOVED
_c_, stomach; _e_, end of intestine; _g_, gills; _h_, ventricle of
the heart; _a_, auricle; _f_, nerve ganglia; _b_, digestive gland;
_ft_, foot; _o_, operculum; _d_, liver]
Sometimes these bodies are bilaterally symmetrical, as we have observed
is the case with the worms, but more commonly the organs on one side are
aborted, while the growth proceeds apace on the opposite side. Thus the
animal assumes a spiral form, being coiled towards the aborted side,
with the gills and other organs developed on that side only. As a rule
this curvature is such that the body takes the form of a right-handed or
dextral spiral, as we have already observed in the shells which cover
them, the mouth being thus thrown to the right, but sometimes it takes
the opposite direction.
When one of these animals is extended and creeping, we observe that it
has a distinct head, furnished with a mouth below, and tentacles and
eyes above; also, if an aquatic species, the gills are more or less
prominent. Further, the exposed portion of the body is covered with a
leathery mantle, and the animal creeps on a broad, flattened surface
which is called the foot.
The tentacles or feelers are usually retractile, and, when retracted,
are turned outside-in. Each one is provided with a muscle that runs from
the body internally to the tip; and, by the contraction of this muscle
the tentacle is involuted just in the same way as the finger of a glove
could be by pulling a string attached to the tip inside. In addition to
these tentacles, and the eyes and mouth previously mentioned, the head
is furnished with ear-sacs, which are little cavities, filled with fluid
containing solid particles, with nerve filaments distributed in the
walls.
On the floor of the mouth there is a ribbon, supported on a base of
gristle, and covered with numerous minute teeth arranged regularly in
rows. The gristle is moved backwards and forwards by means of muscles in
such a manner that this 'lingual ribbon' acts like a rasp, and is
employed in scraping or tearing away portions of the substance on which
the animal is feeding. By this action the teeth are gradually worn away
in front, but this is of no consequence, for the lingual ribbon is
always growing forwards, the worn material being replaced by new growth
behind.
[Illustration: FIG. 158.--A PORTION OF THE LINGUAL RIBBON OF THE WHELK,
MAGNIFIED; AND A SINGLE ROW OF TEETH ON A MUCH LARGER SCALE
_b_, medial teeth; _a_ and _c_, lateral teeth]
The arrangement and form of the teeth are characteristic and important;
and since they afford one of the means by which we may trace the natural
affinities of similar species, they will be frequently referred to when
dealing with the principles of classification. For this reason the
student should be prepared to examine the lingual ribbons of molluscs
with the aid of a compound microscope as occasion requires. As a rule
the ribbon is easily stripped away from the floor of the mouth; and, if
placed in a drop of water and covered with a cover-glass, the teeth are
readily observed. Until a little experience has been gained the
observations may be confined to some of the larger species, in which the
ribbon is both large and easily obtained. In the common whelk, for
example, it often measures more than an inch in length.
It is difficult to understand how the univalve mollusc manages to glide
along so rapidly and gracefully on its expanded foot when we observe it
from above, but the difficulty is cleared away when we see it creeping
on the side of a glass aquarium, or when we place it on a sheet of glass
and observe its movements from the other side. We then see that the foot
is in complete contact with the glass, and that a steady but rapid
undulatory movement is produced by the successive expansions and
contractions of the disc, brought about, of course, by the action of
muscular fibres.
A few of the univalves are viviparous--that is, they produce their young
alive; but the majority lay eggs. The eggs are often enclosed in horny
cases, some of which may be commonly seen washed up on the beach, or
attached to rocks and weeds between the tide-marks. The larvæ are always
enclosed in a shell, though they are sometimes wholly or partially
concealed by the mantle. The shell is usually closed by an operculum;
but as the animal advances in age the shell sometimes disappears
altogether, or is reduced to a mere shelly plate, as is the case with
the land and marine slugs and sea lemons. The young of the
water-breathers always swim about freely by means of a pair of ciliated
lobes or fins, but these remain only for a brief period, after which the
animal settles to the bottom for a more or less sedentary existence.
[Illustration: FIG. 159.--EGG CASES OF THE WHELK]
The Cephalophora fall naturally into two fairly well-defined groups,
which we may describe as the air-breathers and the water-breathers. The
former breathe air direct from the atmosphere through an aperture on the
right side of the body, the air passing into a pulmonary organ or lung,
in the walls of which the bloodvessels ramify, and they include all the
land snails and slugs. The latter breathe by gills which are more or
less prominent on the sides of the body, and include all the fresh-water
snails, as well as the marine species which fall within our special
province.
We shall first consider the class _Pteropoda_ or Wing-footed Molluscs,
so called from the wing-like appendages that are attached to the side of
the mouth, or to the upper side of the foot, which is either very small
or altogether wanting.
These Pteropods are in many respects lowly organised as compared with
the higher molluscs; and as they spend the whole of their existence in
the open sea, they can hardly be considered as falling within the scope
of the sea-side naturalist’s work. Yet since their shells are
occasionally drifted on to the shore, and because a knowledge of them is
essential to the student of the mollusca, we shall briefly note their
principal characteristics.
The pteropods are extremely abundant in some seas, occurring in such
vast numbers that they discolour the water for miles. They swim about by
flapping the pair of wings already referred to. They are known to form
an important article of the diet of the whale, and are also devoured in
enormous numbers by various sea birds; and they are themselves
carnivorous, feeding on various smaller creatures that inhabit the open
waters.
[Illustration: FIG. 160.--PTEROPODS]
In appearance they much resemble the young of higher species of
molluscs. The nervous system consists of a single ganglion situated
below the gullet, and the eyes and tentacles are either rudimentary or
absent. The digestive system includes a muscular gizzard provided with
teeth for the mastication of food, and a digestive gland or liver for
the preparation of a digestive fluid. The heart has two cavities, and
respiration is effected by a surface covered with minute cilia. This
surface is either quite external or is enclosed in a chamber through
which water freely circulates.
The shell is very different from that of a typical head-bearing mollusc,
for it generally consists of two glassy, semitransparent plates,
situated dorsally and ventrally respectively on the body of the animal,
with an opening for the protrusion of the body, and others at the sides
for processes of the mantle; and it terminates behind in one or three
pointed processes. Sometimes, however, its form is conical or spiral,
with or without an operculum. We append illustrations of a few
pteropods, selecting for our purpose species that have been found in the
Atlantic.
It will have been noticed from the above short description that the
pteropod is very unlike the typical Cephalophore as outlined in our
general remarks on the group, especially in the symmetrical form of both
body and shell and in the total or almost total absence of the foot; and
this distinction is so marked that the pteropods are often separated
from all the other _Cephalophora_ into a class by themselves, while all
the remainder are placed in a separate extensive class called the
_Gasteropoda_, because they creep on the ventral surface of the body,
the term signifying stomach-footed.
These gasteropods are divided into four orders: the _Nucleobranchiata_,
in which the respiratory and digestive organs form a nucleus on the
posterior part of the back; the Opisthobranchiata, with gills more or
less exposed towards the rear of the body; the _Pulmonifera_, or
lung-breathing order; and the _Prosobranchiata_, in which the gills are
situated in advance of the heart. The third order includes all the land
snails and slugs, and does not therefore fall within the scope of our
work; but the remaining three consist either exclusively or principally
of marine species, and will be dealt with in the order in which they are
named.
The Nucleobranchs are not really gasteropods in the strictest sense of
the term, for they do not creep along by means of their foot, but all
swim freely in the open ocean, always at the surface, and sometimes
adhere to floating weed by means of a sucker. In fact, the foot of these
creatures is greatly modified in accordance with their habits, one part
being often expanded into a ventral swimming fin, and provided with a
sucking-disc for adhesion, and another produced into a posterior fin for
locomotion.
Like the pteropods, the nucleobranchs are purely pelagic, so that we can
hardly expect to meet with a specimen on or near the shore; and thus we
shall content ourselves with a brief notice of their general characters.
The shell is very variable in size and form, and sometimes even entirely
absent. Large-bodied species often possess but a very small shell, while
some are able to entirely retract themselves and close the mouth of the
shell by an operculum. These animals are generally provided with a large
cylindrical proboscis, and the tongue has recurved teeth. The body is
usually very transparent, often so much so that the blood may be seen
circulating within it, and the nervous system is much more perfectly
developed than in the pteropods. The eyes, too, are perfectly formed.
The presence of special breathing organs may seem to be superfluous in
such delicate and soft-bodied creatures as these, for it may be supposed
that all the oxygen required could be absorbed directly from the water
through their soft structures, as is really the case with many aquatic
creatures; and as a matter of fact some of the nucleobranchs possess no
gills, but others have these organs fully formed.
Passing now to the true gasteropods, we shall first consider the
_Opisthobranchs_, which are commonly known as Sea Slugs and Sea Lemons.
Some of these have no shell at all, and even where one exists it is very
rudimentary, usually very small and thin, and concealed within the
mantle. The gills are either branched and tree-like, or are composed of
tufts or bundles of filaments; and, as the name of the order implies,
are situated towards the posterior part of the body. They are also
retractile, and when the animal is alarmed it will conceal its gills,
thus reducing its body to a shapeless, slimy mass, inviting neither to
sight nor to touch.
The sea slugs are principally animal feeders, subsisting on small
crustaceans, other molluscs, &c.; the food being first reduced by the
rasping action of the teeth, and then masticated in a gizzard which is
provided internally with horny spines or hard, shelly plates.
It will not be necessary to enumerate all the different families of this
order, especially as the species are mostly to be found beyond the
tide-marks, and are therefore obtained only with the aid of the dredge;
but we shall describe a few of the British species with a view of
showing the general characteristics of the animals.
They are usually divided into two sections, those with exposed or naked
gills (_Nudibranchiata_) forming the first, and those in which the gills
are covered either by the shell or the mantle (_Tectibranchiata_)
comprising the second.
In the Nudibranchs the shell exists only during the embryonic stage, and
the external gills are arranged on the back or along the sides. The
tentacles are not employed as organs of touch, but are probably
connected only with the sensation of smell, being provided with
filaments of the olfactory nerve; the eyes are small dark-coloured spots
embedded in the skin behind the tentacles. Various species are to be
found on all rocky coasts, where they range from low-water mark to a
depth of fifty or sixty fathoms, but a few are pelagic, living on the
surface of floating sea weeds.
It is almost impossible to identify the species of nudibranchs from dead
specimens, for the classification of the section is based largely on the
arrangement of the gills, which are almost always retracted in the dead
animals. This is also the case even with living specimens when disturbed
or removed from the water; hence they should always be examined alive in
sea water, while the animals are extended and moving.
[Illustration: FIG. 161.--NUDIBRANCHS
1. _Doto coronata._ 2. _Elysia viridis._ 3. _Proctonotus
mucroniferus._ 4. _Embletonia pulchra_]
It will be understood from the above statements that special methods
will be necessary when it is required to preserve specimens for future
study, the gills being always retracted when the animal is killed for
this purpose by any rapid process. We have found two methods, however,
that are fairly satisfactory in the majority of instances.--Place the
living animals in a suitable vessel of sea water, and leave them quite
undisturbed till they are fully extended, and then either _gradually_
raise the temperature till they are dead, or introduce into the water,
cautiously, a solution of corrosive sublimate. In the latter case a much
larger proportion of the sublimate will be required than when used for a
similar purpose with freshwater molluscs. When the animals are dead it
will be found that their gills are more or less extended, sometimes
fully so, and they may then be transferred to diluted spirit or a two
per cent. solution of formaldehyde.
[Illustration: FIG. 162.--NUDIBRANCHS
1. _Dendronotus arborescens._ 2. _Tritonia plebeia._ 3. _Triopa
claviger._ 4. _Ægirus punctilucens_]
In fig. 162 we represent four species. Two of these--_Triopa claviger_
and _Ægirus punctilucens_--belong to the family _Doridæ_, the members of
which are popularly known as Sea Lemons, and are distinguished by the
presence of plume-like gills situated on the middle of the back. Another
family (_Tritoniadæ_), characterised by the arrangement of the gills
along the sides of the back, and by tentacles that can be retracted into
sheaths, is represented by _Tritonia plebeia_ and _Dendronotus
arborescens_ in the same figure, and by _Doto coronata_ in fig. 161. The
family _Æolidæ_ also have their gills arranged along the sides of the
back, but they differ from the last in that their tentacles are not
retractile. They include the two species numbered 3 and 4 on fig. 161.
The remaining one on fig. 161--_Elysia viridis_--is a member of the
family _Phillirhoidæ_, characterised by a pair of tentacles on the
dorsal side of the head and by the foot being either very narrow or
absent, the latter feature denoting that the animals are not adapted for
creeping on the bottom. In fact, several of the species of this family
swim freely by means of flattened tails.
The Tectibranchs are similar in general structure, but are very
different in appearance, inasmuch as the gills, so prominent in the last
division, are here covered by the mantle, or by the shell, which is
often well developed. The latter is very variable in form, being of a
globular, twisted, spiral, or other shape, but is sometimes absent in
the adult. In fig. 163 we give a few examples of the shells of British
species; and one (_Bulla hydatis_) is shown on Plate V.
[Illustration: FIG. 163.--SHELLS OF TECTIBRANCHS]
We now pass on to the largest and last order of gasteropods--the
_Prosobranchiata_--so called because the gills are situated in front of
the heart. This group is an important one to the sea-side naturalist,
since it contains nearly all the univalve molluscs that are common
between the tide-marks of our shores, as well as some abundant species
that are protected by a shell of several distinct parts. In nearly all
of them the abdomen is well developed, and the shell is sufficiently
large to cover the whole animal when the latter is retracted; and the
gills, which are either pectinated (comb-shaped) or plumed, are lodged
in the chamber formed over the head of the animal by the mantle.
The order is often divided into two sections--the _Holostomata_ or Sea
Snails, in which the margin of the aperture of the shell is entire, and
the _Siphonostomata_, in which the margin of the mantle is prolonged
into a siphon by which water passes into the gill chamber. This division
does not seem to be very satisfactory, as the sections are not separated
by very prominent natural characteristics, but it becomes convenient on
account of the great extent of the order.
In the _Holostomata_ the shell is either spiral, conical, tubular, or
composed of several valves, and the spiral forms are usually closed by
a horny or shelly operculum of the spiral kind. The head is provided
with a proboscis that is generally non-retractile, and the gills usually
extend obliquely across the back, or are attached to the right side
behind the head.
We shall first consider the lower forms, starting with the family
_Chitonidæ_, the animals of which, as the name implies, are covered with
a shell that resembles a coat of mail.
Some of these creatures are very common on our rocky coasts, and yet
their nature is such that they are liable to be overlooked by those who
are not acquainted with their appearance and habits. The shell is oval
or oblong, often so coloured as to closely resemble the rocks and stones
over which they crawl; and the animal is so inactive when left exposed
by the receding tide, and its flat under surface so closely applied to
that on which it rests, that it looks merely like a little convexity of
the rock. But after a few have been discovered the eye becomes
accustomed to their appearance, and large numbers may be obtained in a
short space of time.
The shell will be seen to consist of eight transverse, curved plates,
overlapping each other at their edges, and all enclosed in a leathery
mantle, which also forms a projecting margin all round. The middle six
plates are different from the first and last in that they are grooved in
such a manner that each one displays a dorsal and two lateral areas.
The animal holds on tightly to the rocks by its large creeping disc-like
foot, but may be removed without injury by forcing a knife-blade under
the margin of its shell. When examined it will be found that it has not
a well-formed head like the majority of the gasteropods, and both eyes
and tentacles are wanting. The gills form a series of lamellæ round the
posterior end of the body, between the edge of the foot and the mantle;
and it is interesting to note that the Chitons further justify the low
position assigned to them among the gasteropods by their possession of a
simple, central, tubular heart, similar to that of worms.
Perhaps the commonest of the British species is _Chiton cinereus_. Its
colour is a dull grey, but the ground is variously mottled, often in
such a manner as to give it a protective resemblance to its
surroundings. _C. ruber_ is the largest of our species: its shell is
variously mottled with shades of yellow and brown; _C. fascicularis_ is
bristled. Another rather common species (_C. lævis_) is distinguished by
the glossy appearance of the dorsal portion of the shell.
It will have been observed that the chitons differ from the majority of
gasteropods in that their shells and bodies are both bilaterally
symmetrical, and the same is true of the next family--_Dentaliadæ_,
which derive their name from the tooth-like form of their conical
shells. They are popularly known as the Tooth Shells, and although they
generally live beyond low-water level, they may sometimes be seen alive
on the beach, and the empty shells are often washed up by the waves.
The shells (fig. 165) are curved, and open at both ends, the narrower
extremity being the posterior. The mouth is circular, and the outer
surface is quite smooth or grooved.
[Illustration: FIG. 164.--CHITON SHELLS]
[Illustration: FIG. 165.--SHELLS OF _Dentalium_]
In these animals, too, the head is imperfectly formed, without eyes or
tentacles. The foot is conical and pointed, with two symmetrical side
lobes; and the gills, also two in number, are symmetrically disposed.
The margin of the mouth is fringed, and the animal is attached to the
shell near the posterior end.
The _Dentaliadæ_ are carnivorous, subsisting on minute molluscs,
foraminifera, &c., and generally live on sandy or muddy bottoms, in
which they sometimes bury themselves.
Our next family includes the familiar Limpets, and is designated
_Patellidæ_ on account of the resemblance of the conical shell to a
little dish. In these the apex of the cone is not central, but situated
more or less towards the anterior; and the muscular impression within is
shaped like a horseshoe, with its open end turned to the front.
Unlike the members of the preceding families, the limpets have a
well-formed head furnished with both eyes and tentacles, the former
situated at the bases of the latter. They have a horny upper jaw, and
the tongue, which is very long, is supplied with numerous hooked teeth.
The foot is a very large disc, as large as the shell, and the gills
consist either of one or two branched plumes, or of a series of lamellæ
almost or entirely surrounding the animal between the shell and the
margin of the mantle.
The reader has probably experienced the difficulty of detaching a limpet
from its hold on the rocks. The tenacity of the grip is not due to the
mere adhesive power of the foot itself, but to atmospheric pressure, the
effect of which is complete on account of the total exclusion of air
from under the disc of the foot; and when we remember that this pressure
amounts to fifteen pounds on every square inch of surface, we can
readily understand the force required to raise a large limpet from its
position.
[Illustration: FIG. 166.--_Patellidæ_
1. _Patella vulgata._ 2. _P. pellucida._ 3. _P. athletica._
4. _Acmæa testudinalis_]
The Common Limpet (_Patella vulgata_) is found on all our rocky coasts
between the tide-marks, often at such a level that it is left exposed to
the air for eight or nine hours at a time. The apex of the shell of this
species is nearly central, and the exterior is sometimes nearly smooth,
but more commonly relieved by radiating ribs.
Although the shell itself is not a particularly pretty object, it is
often rendered very beautiful and interesting by the various animal and
vegetable organisms that settle on it. Those shells that are left dry
for hours together are commonly adorned with clusters of small acorn
barnacles, while the limpets that have found a home in a rock pool and
are perpetually covered with water, often resemble little moving gardens
in which grow beautiful tufts of corallines or other weeds, as well as
polyzoa and other animal forms.
It appears that limpets are not great travellers, the appearance of the
rock from which they have been removed being such as to point to a very
long period of rest. Those on hard rocks are generally situated on a
smooth surface just the size of the shell and generally worn slightly
below the surrounding level by the constant friction of the shell; while
others that have settled on very rugged spots have their cones adapted
to the irregular surface. It has been suggested that the animals make
occasional short excursions from their chosen spot, but return again to
it; and whether or not this is the case, it is evident that they
frequently keep to one small spot for a considerable length of time.
Limpets on chalk and other soft rocks are sometimes in circular pits so
deep that even the apex of the shell is below the general level around;
and though it is possible that the abrasion is produced entirely by the
friction of the shell as the animal turns, yet, in the case of chalk,
the action may be partly due to the carbonic acid gas given off by the
animal as a product of respiration, for it is a well-known chemical fact
that this gas, in solution, has the power of dissolving calcareous
material.
The other British Limpets include _P. pellucida_, which lives on the
fronds and stalks of the tangle, the form of the shell varying according
to that of the surface on which it rests; also the Horse Limpet (_P.
athletica_), the bold radiating ribs of which are irregularly notched;
and _Acmæa testudinalis_--the Tortoiseshell Limpet, with reddish-brown
mottlings on the exterior, and a dark-brown patch at the apex within.
The last-named species lives principally on sea weeds, and has a single
pectinated gill in the cavity between foot and mantle, which is
protruded on the right side when the animal is extended. This latter
feature is interesting since it shows a tendency to that one-sided
development already referred to as characteristic of the typical
gasteropod, resulting in the spiral form of the adult.
In the limpets the lingual ribbon is proportionately long, and is easily
removed for examination. In _P. vulgata_ it may exceed an inch in
length, and the teeth are arranged in rows each of which contains four
central, with laterals on either side, while in _Acmæa_ there are only
three laterals on each side of the central line.
Other so-called limpets belong to separate families. Thus we have the
Cup-and-Saucer Limpet and the Bonnet Limpet in the _Calyptræidæ_. Both
these differ from Patella in that the apices of their shells show a
tendency to assume a spiral form, thus denoting a somewhat closer
relationship to the more advanced univalves. They have distinct heads,
with prolonged muzzles, and well-formed antennæ and eyes. The teeth of
the lingual ribbon are single, with dentated laterals on either side.
The Cup-and-saucer Limpet (_Calyptræa sinensis_) is so called on account
of a curved plate that projects from the interior of the shell, at the
apex; and though this plate takes the form of a half-cup rather than of
a cup, the whole shell has suggested the popular name, while the generic
name is derived from _calyptra_, which signifies a cap. This mollusc is
occasionally found among stones at low tide, but usually lives beyond
this line, thus necessitating the use of a dredge. The Bonnet Limpet
(_Pileopsis hungaricus_) is of similar structure and habit, but the
nucleus of the shell is a more decided spiral (see Plate V.). Both these
animals adhere to stones and rocks, and, like the common limpet, seldom
or never move from their selected sites; hence their shells are variable
in form, being adapted to the rock below, and the movements of the shell
often cause a little hollow to be scooped out of the softer materials.
[Illustration: FIG. 167.--_Calyptræa sinensis_]
Yet other limpets belong to the next family _Fissurellidæ_, which is
characterised by a perforation or a notch in the shell. In these, too,
the shell is conical, with a tendency to assume the spiral form, but the
curve of the nucleus, which is always apparent in the young shell,
frequently disappears as the growth proceeds.
[Illustration: FIG. 168.--_Fissurellidæ_
1. _Puncturella noachina._ 2. _Emarginula reticulata._
3. _Fissurella reticulata_]
In the Keyhole Limpet (_Fissurella reticulata_) which is found chiefly
on our southern shores, the perforation is at the summit of the shell;
but as the animal grows the hole increases in size, encroaching on the
curved nucleus until the latter quite disappears. In the genus
_Puncturella_ the perforation is just in front of the recurved apex, and
is surrounded by a rim internally; while in the Notched Limpets (genus
_Emarginula_) it is represented by a fissure on the anterior _margin_ of
the cone. In all, however, the hole or notch serves the same purpose,
for it is the means by which water enters the siphon.
It is doubtful whether we ought to claim the beautiful Ear shell
(_Haliotis tuberculata_) as one of our own, but it is generally included
among the British molluscs on the ground that it is abundant on the
coast of the Channel Islands, where it is called the Omar; and it is
certainly too beautiful an object to be excluded from the British
species without ample cause.
[Illustration: FIG. 169.--_Haliotis_]
It belongs to the family _Haliotidæ_, and our illustration will show
that the shell is less elevated than that of limpets, and that the
spire, though not prominent, is a fairly well-formed spiral. All along
the outer lip of the very large aperture is a series of perforations,
occupying the summit of a prominent, spiral ridge, and becoming
gradually smaller and smaller towards the spire. The whole shell is
pearly in structure, and displays a great variety of rich colouring. It
is used largely for inlaying and other ornamental purposes, and for
making the so-called pearl buttons. The animal is used largely as an
article of food in the Channel Islands, but it is of so tough a nature
that it requires a vigorous beating previously to being cooked.
[Illustration: FIG. 170.--_Ianthina fragilis_]
The same family contains the beautiful violet _Ianthina_, which also is
not a British species, but a free-swimming oceanic snail. It is,
however, occasionally drifted to our shores, though generally in an
imperfect condition. In the Atlantic and the Mediterranean it sometimes
abounds in such multitudes as to distinctly colour the surface of the
sea.
It will be seen that the shell is round, with a well-formed spiral. The
spire is white, but the base is of a deep violet colour. The animal is
very remarkable in some respects. In the first place, though it has
pedicels similar to those on which the eyes of the higher univalves are
placed, yet it has no eyes. Then the foot, which is in itself small,
secretes a float or raft so large that it cannot be retracted into the
shell, with numerous air vesicles to render it light, and the
egg-capsules of the animal are attached to the underside of this. The
animal has no power of sinking, but lives exclusively at the surface;
and, when disturbed, it exudes a violet fluid that colours the
surrounding water. It is apparently the only gasteropod that lives in
the open sea and has a large and well-formed spiral shell.
Passing now to the family _Turbinidæ_ we meet with turbinated or
pyramidal shells that are of a brilliant pearly lustre within, and
frequently without also when the epidermis is removed. The animals
inhabiting them have well-formed heads with a short muzzle, long and
slender tentacles, and eyes mounted on peduncles. The sides are
ornamented with fringed lobes and several tentacle-like filaments, and
the aperture of the shell is closed, when the animal is retracted, by a
spiral operculum. They are all vegetable feeders; and, as is usual with
the plant-eating molluscs, the teeth on the lateral portions of the
lingual ribbon are very numerous.
We have a few common species belonging to this group, mostly members of
the typical genus _Trochus_ and commonly known as Top Shells. In these
the shell is a pyramid formed of numerous flat whorls, with an oblique
and rhomboidal aperture. Of the three species figured (including two on
Plate V.) _T. umbilicatus_ and the Large Top (_T. magnus_) are
umbilicated, the umbilicus being very large in the latter; and the
former is characterised by the zigzag greyish or reddish markings that
run radially across the whorls. The other (_T. zizyphinus_) is usually
of a yellowish or pink colour and has no umbilicus.
The same family contains the pretty little Pheasant Shell (_Phasianella
pullas_), which is richly coloured with red, brown, and yellow on a
light ground; and _Adeorbis subcarinatus_, shown in the same group.
The well-known Periwinkle (_Littorina littorea_) and the species to the
right of it on Plate V., belong to the family _Littorinidæ_, the
members of which are similar in structure and habit to _Trochus_, but
the shell is usually more depressed, and is never pearly. The shell of
the Periwinkle is thick, having but few whorls, and is not umbilicated;
and the lingual ribbon, which is coiled up on the gullet, contains no
less than about five hundred rows of teeth; but only a little more than
twenty of these rows are in action at any one time, the remainder being
a reserve stock to come into active service as the ribbon grows forward.
In the genus _Lacuna_ there is a narrow umbilicus, and the aperture of
the shell is semilunar in form; and the species of _Rissoa_ are very
small, with white or horny shells, much more pointed and having more
whorls than those of the _Littorina_.
<div class="figleft" style="width: 175px;">
<a name="fig172" id="fig172"></a>
<img src="images/i_273b.jpg" width="175" height="202" alt="" />
FIG. 172.--_Rissoa labiosa_ AND _Lacuna pallidula_
</div>
Our next illustration shows three shells of the family _Turritellidæ_,
so named from the resemblance of the shells to a tower or spire. The
form indeed is so characteristic that they can hardly be mistaken. It
will be seen that _Turritella communis_ is striated spirally, while the
surface of _Scalaria communis_ (Plate V.) is relieved by strongly marked
transverse ribs. Both these species are very common, and the latter is
peculiar for its power of ejecting a dark purple fluid when molested.
The other representative of the family--_Cæcum trachea_--has a shell
something like that of _Dentalium_ (p. 238), being cylindrical and
tubular, but it differs in being closed at one end.
[Illustration: FIG. 173.--SECTION OF SHELL OF _Turritella_]
[Illustration: FIG. 174.--_Turritella communis_ AND _Cæcum trachea_]
In the succeeding shells, of the family _Cerithiadæ_, the spire is also
considerably produced, so much so that some of the species closely
resemble the Turret shells, but they are distinguished by usually having
an expanded lip, at least in the adult form; and the mouth is channelled
in front, and sometimes also behind. The animals of the group have short
muzzles that are not retractile, the tentacles are wide apart, and the
eyes are mounted on short pedicels. The median teeth are arranged in a
single row, with three laterals on either side of each.
[Illustration: FIG. 175.--_Cerithium reticulatum_ AND _Aporrhais
pes-pelicani_]
_Cerithium reticulatum_ receives its generic name from its appearance to
a small horn, and the specific name refers to the netted appearance of
its surface due to the presence of numerous little tubercles arranged in
rows--a feature that serves to distinguish it from the small Turret
shells. It is a common shell, as is also the other representative of the
family illustrated, but the latter is rendered conspicuous by the
enormously expanded lip that has earned for it the popular name of Spout
Shell. Its scientific name is _Aporrhais pes-pelicani_, and the
application of the specific term will be understood when the shell is
viewed from above, for the expanded lip is drawn out into long
finger-like lobes that suggest the foot of a bird. This is a very solid
shell, sometimes reaching a length of two inches; and the animal
inhabiting it is carnivorous.
[Illustration: FIG. 176.--_Aporrhais pes-pelicani_, SHOWING BOTH SHELL
AND ANIMAL]
We have yet some turreted shells to deal with, belonging to the family
_Pyramidellidæ_, but they need not be confused with the preceding groups
if carefully examined. In the first place, the aperture of the shell is
very small; and the operculum, instead of being spiral, as in the
turreted shells before mentioned, is imbricated or made up of parallel
layers denoting that the growth took place on one side only. Another
distinguishing feature is seen in the nucleus--that small portion of the
spire that was developed within the egg--which is sinistral or
left-handed. In addition to this, the animal has broad, ear-like
tentacles, a retractile proboscis, and a lingual ribbon without teeth.
The British species of this family belong principally to the genera
_Odostomia_, characterised by a tooth-like fold of the columella;
_Eulima_, containing small, white, polished shells with numerous level
whorls; and _Aclis_, with little polished shells not unlike
_Turritella_.
[Illustration: FIG. 177.--1. _Odostomia plicata._2. _Eulima polita._
3. _Aclis supranitida_]
The last family of the _Holostomata_ is the _Naticidæ_, the shells of
which are almost globular, with only a few whorls, and a small, blunt
spire. The mouth is semilunar in form, and the lip sharp. The proboscis
of the animal is long and retractile, and the foot large; but perhaps
the most characteristic feature is the presence of large mantle lobes
which hide some of the shell when the animal is crawling. In _Natica_
(fig. 155), the typical genus, the shells are somewhat thick and smooth,
with a large umbilicus. As the animal crawls a large fold of the mantle
is reflected back over the head, completely covering it, and apparently
obstructing its view; but this is not the case, for the creature has no
eyes. _Natica_ is very abundant on some sandy beaches, where it devours
small bivalves and other animals; and it is frequently washed up alive
by the waves. Its shell is also a favourite one with hermit crabs. Its
eggs, all connected together in a spiral band, may often be seen
stranded on sandy coasts. Several species of Natica are found on our
shores. An allied mollusc--_Velutina lævigata_, so called on account of
the velvety epidermis that clothes the shell, completely surrounds the
shell by its mantle folds when creeping.
The _Siphonostomata_ form a much smaller section than the last, and its
members are distinguished mainly by the presence of a true siphon,
formed by the prolongation of the mantle margin, and serving to convey
water into the gill chamber. In all these the shell is spiral, usually
without an umbilical opening, and the margin of the mouth is prolonged
into a canal or distinctly notched. The operculum is horny, and lamellar
or imbricated. The animal has a retractile proboscis, and the eyes or
eye-pedicels are joined to the tentacles. All the species of this
division are marine.
[Illustration: FIG. 178.--_Cypræa (Trivia) europæa_]
We will first take the family _Cypræidæ_, which contains the familiar
Cowries, these forming the lowest group of the division. An examination
of the shells may at first seem rather puzzling, for the spire is
concealed, and the whole is convoluted in such a manner as to make the
mouth long and narrow, with a channel at either end. The outer lip is
also thickened and bent inward, and there is no operculum.
The animal itself is particularly interesting, for, as it creeps along
on its broad foot, abruptly shortened in the front, the mantle lobes
bend over the top, meeting along the middle line, where they are usually
fringed with little tentacle-like processes; and, as a result, the whole
shell is beautifully enamelled on the outer surface. In all the Cowries
the central teeth are single, and the laterals are arranged either in
twos or threes.
Perhaps the commonest representative of this family is the pretty little
_Cypræa_ (_Trivia_) _europæa_ (Plate V.), the shells of which are
sometimes washed up in large numbers on sandy beaches. The animal lives
mainly below low-water level, but it may often be found in the larger
rock pools, creeping rapidly over the tangles, and may be easily secured
with the aid of a net.
In the same family we have the little _Erato_ (_Marginella_) _lævis_,
the white shell of which is minutely furrowed along the lips; and also
_Ovulum patulum_ (_Calpurna patula_), so called on account of its
fancied resemblance to a poached egg.
We have also several species of Cone shells (family _Conidæ_) on our
coasts, readily recognised by their form, which is a cone, with a long,
narrow aperture, partially closed by a minute operculum. As in the last
family, the foot is abruptly shortened in front. The head is very
prominent, with eyes situated on the tentacles. There are two gills, and
the teeth are arranged in pairs.
[Illustration: FIG. 179.--1. _Ovulum patulum._ 2. _Erato lævis_]
[Illustration: FIG. 180.--_Mangelia septangularis_ AND _Mangelia
turricula_]
The Conidæ are principally inhabitants of tropical seas, where some very
large species exist. Two of the British representatives, both common
shells, are shown in fig. 180.
Our next family (_Buccinidæ_) is so well distributed on our coasts, that
it would be difficult, we imagine, to find a spot quite free from its
familiar forms. It contains all those creatures commonly known as
Whelks, Dog Whelks, and Dog Winkles, ranging from deep water almost to
high-water mark.
In all these the shell is notched in front, or the canal is turned
abruptly upward. The foot of the animal is broad, the eyes are situated
either on the tentacles or at their bases, and there are two gill
plumes.
All the species are carnivorous, and some are said to be very
destructive to mussels and young oysters.
The Common Whelk (_Buccinum undatum_, Plate V.) lives in deep water,
whence it is dredged up largely for the market. Its clusters of egg
cases are washed up in large numbers on the beach, where they form one
of the commonest materials among the refuse at high-water mark. It is
not uncommon, also, especially after storms, to find the unhatched eggs
stranded by the waves, and these are so transparent that the embryos,
several in each capsule, may be seen within. The hole through which the
young escape may also be seen on the inner side.
[Illustration: FIG. 181.--1. _Purpura lapillus._ 2. EGG CASES OF
_Purpura_. 3. _Nassa reticulata_]
The Dog Periwinkle (_Purpura lapillus_) abounds on all our coasts and is
remarkable for the production of a dull crimson or purple fluid that may
be obtained from it by pressing on the operculum. This fluid turns to a
brighter colour on exposure to air, and is said to have been used
largely in former times as a dye. It will be seen from our figure that
the spire of this shell is shorter in proportion than that of
_Buccinum_; but both are alike in that the operculum is made up of
layers with a nucleus on the external edge.
The other species figured is _Nassa reticulata_, popularly known as the
Dog Whelk, and characterised by a tooth-like projection of the inner lip
close to the anterior canal. It is very common near low-water mark,
where it may be seen crawling over the rocks on its broad foot, from
which project two hornlike appendages in front and two narrow tails
behind.
[Illustration: FIG. 182.--_Murex erinaceus_]
From the last family of the gasteropods (the _Muricidæ_) we select two
common species--_Murex erinaceus_ and _Fusus antiquus_ (Plate V.). In
both these the anterior canal of the shell is straight and the posterior
wanting. The eyes are on the tentacles, and there are two plumed gills.
Both are carnivorous species, feeding on other molluscs; and the former
is said to bore through the shells of its prey with the prominent beak
of its shell.
_Murex_ may be readily distinguished by the prominent longitudinal
ridges of the thick shell, its rounded aperture, and by the partly
closed canal running through the beak. It is known to fishermen as the
Sting Winkle; the other species is called the Red Whelk in some parts,
and in Scotland is known as the Buckie. Like the common whelk, it is
dredged largely for the market, and is said to be far more esteemed than
the former, from which it may be distinguished by the fusiform shape of
the shell and the long straight canal.
We now pass to the last and highest class of the mollusca, called the
_Cephalopoda_ because they have a number of arms attached to the head,
round the mouth. Unlike the majority of molluscs they are bilaterally
symmetrical: and are much more highly organised, in some respects even
making an approach to the vertebrates. Thus they generally have an
internal hard structure, either horny or calcareous in structure,
representing the vertebral column, and the circulatory system consists
of arteries and veins, connected by minute capillaries. The corpuscles
of the blood are also similar in form to those of the vertebrates.
Externally they are all naked, with the exception of the nautilus and
argonaut of the warmer seas.
The arms, so characteristic of the class, are eight or ten in number,
long and muscular, and provided with numerous suckers by which the
animal can cling with remarkable tenacity. These suckers are situated on
the inner surface of the arms, and the disc of each one displays a
series of muscular fibres, all converging from the circumference towards
the centre, which is occupied by a softer structure that works inwards
and outwards like the piston of a pump. Thus the suckers form a system
of exhausting air-pumps by which a vacuum can be produced, and the
tenacity of the grip, maintained by atmospheric pressure, is so great
that the arms, strong as they are, may be torn asunder by attempting to
pull them from their hold; and yet the animal can release its grip with
the greatest of ease by simply releasing the pistons of its pumps.
The cephalopods are further distinguished by their very large, glaring
eyes, situated on the sides of the well-formed head, and by powerful
jaws that work in a vertical plane, like those of the vertebrates, but
somewhat resembling the beaks of certain birds. The tongue is also very
large and fleshy, and in part armed with numerous hooked spines or
teeth.
The class is usually divided into two orders, one characterised by the
possession of two gills, and the other of four; but the British species
belong to the former, known technically as the _Dibranchiata_. This
order is subdivided into two sections according to the number of arms;
and the divisions are called the _Octopoda_ and _Decapoda_ respectively.
[Illustration: FIG. 183.--OCTOPUS]
The former section includes the Octopods, of which some species inhabit
our seas. They all have eight arms, of unequal size, with the suckers
arranged in two rows, and their round or oval bodies seldom have any
fins, locomotion being effected by means of the arms, and by the sudden
expulsion of water from the siphon. The shell is rudimentary, being
represented merely by two short ‘styles’ within the mantle. The species
vary considerably in size, some being only about an inch long when fully
grown, while others measure two feet or more, and are looked upon as
formidable creatures by man. Sometimes they are washed up on our
beaches, but the best way to make their acquaintance is to examine the
contents of the fishermen’s drag nets as they are hauled on the beach.
In the same manner we may secure various species of the Decapods or
Ten-footed Cephalopods, which comprise the Calamaries, Squids, and
Cuttlefishes. These, too, properly speaking, have but eight arms, the
other two appendages being really tentacles, which are usually longer
than the arms, and more or less retractile; they are also expanded at
the ends. The decapods are also to be distinguished from the octopods
by their elongated bodies, and a flattened, fin-like appendage on either
side. Their eyes, also, are capable of being rotated within the orbits,
while those of the octopods are fixed; and the shell consists of one or
more horny ‘pens,’ or of a calcareous ‘bone,’ contained in a cavity so
loosely that it drops out of its place when the cavity is opened.
[Illustration: FIG. 184.--_Loligo vulgaris_ AND ITS PEN]
[Illustration: FIG. 185.--_Sepiola atlantica_]
The Common Calamary (_Loligo vulgaris_) may be recognised by the
accompanying illustration, from which it will be observed that the body
tapers behind, bearing two rhomboidal fins in the rear. The suckers are
arranged in two rows on the arms, but in fours on the expanded tips of
the tentacles. The animal is a good swimmer, and sometimes crawls, head
downwards, on the disc surrounding the mouth, pulling itself along by
means of its arms. Its shell is a horny pen, lanceolate in form, but it
divides as the age of the animal advances, so that two or more may be
found in the same specimen.
Belonging to the same family we have the Common Squid (_Sepiola
atlantica_), also a very abundant species. Here the body is shorter and
purse-like, and the fins are dorsal and rounded. It seldom exceeds four
or five inches in length, and, like the Calamary, is used largely as a
bait by fishermen.
Another family--the _Sepiadæ_--contains the Cuttlefish (_Sepia
officinalis_), the ‘bone’ of which is such a common object on the beach.
This latter is a broad, curved plate of carbonate of lime, made up of a
number of regular layers, and having a cavity hollowed out at the
posterior end. It is exceedingly light and porous in structure, and at
one time was used largely as an antacid as well as a dentifrice. It is
also proportionately large, being both as long and as broad as the body
of the animal.
[Illustration: FIG. 186.--_Sepia officinalis_ AND ITS ‘BONE’]
Cuttlefishes live principally in the shallow water close to shore, where
they swim backwards by the sudden propulsion of water from their
siphons; and their eggs, which look like clusters of black grapes, are
frequently thrown up on the beach, generally attached to the stems and
fronds of sea weeds.
As a rule the cephalopods swim slowly by the aid of their fins or by a
rhythmic contraction by which water is expelled from their siphons, but
when in danger the muscular contraction is so violent that they dart
through the water with great speed, and even leap into the air to avoid
their enemies. But they have another and much more remarkable way of
escaping from their foes:--They possess a gland, the duct of which opens
into the base of the funnel or siphon, that prepares an inky fluid; and
when the animal is disturbed it suddenly ejects this fluid, rendering
the surrounding water so cloudy that it is often enabled to retreat
unobserved. The 'ink' of the _Sepia_ was used for writing in former
times, and is still employed in the preparation of the artist’s pigment
that bears the same name. Fishermen are well acquainted with this
peculiar characteristic of the animal, for they are frequently
bespattered with the contents of the ink bag of the _Sepia_ when the
creature is included in the contents of their draw-nets, and have learnt
to handle it cautiously until the objectionable fluid has been all
discharged.
[Illustration: FIG. 187.--EGGS OF _Sepia_]
We will conclude this chapter by giving a tabular summary of the
classification of the molluscs which will probably be useful to the
collector of marine objects.
CLASSIFICATION OF THE MOLLUSCA
Class =LAMELLIBRANCHIATA=--Plate-gilled. Headless, usually enclosed
in bivalve shell.
Section =SIPHONIDA=--Mantle lobes more or less united to form
tubular siphons.
Families--_Pholadidæ_, _Gastrochænidæ_, _Anatinidæ_,
_Myacidæ_, _Solenidæ_, _Tellinidæ_, _Mactridæ_,
_Veneridæ_, _Cyprinidæ_, _Lucinidæ_, _Cardiadæ_, &c.
Section =ASIPHONIDA=--Mantle lobes free or nearly so. No true
siphons.
Families--_Arcadæ_, _Mytilidæ_, _Aviculidæ_, _Ostreidæ_,
&c.
Class =CEPHALOPHORA=--Head-bearing. Usually enclosed in a univalve
shell.
Section =PTEROPODA=--Wing-footed molluscs.
Section =GASTEROPODA=--Stomach-footed molluscs.
Order =Nucleobranchiata=--Viscera form a nucleus on the back.
Order =Opisthobranchiata=--Shell generally absent. Gills more
or less exposed.
Section NUDIBRANCHIATA--Naked gills.
Section TECTIBRANCHIATA--Gills covered by shell or mantle.
Order =Pulmonifera=--Lung-breathers. Terrestrial.
Order =Prosobranchiata=.
Section HOLOSTOMATA--Aperture of shell entire (sea snails).
Families--_Chitonidæ_, _Dentaliadæ_, _Patellidæ_,
_Calyptræidæ_, _Fissurellidæ_, _Haliotidæ_,
_Turbinidæ_, _Littorinidæ_, _Turritellidæ_,
_Cerithiadæ_, _Pyramidellidæ_, _Naticidæ_, &c.
Section SIPHONOSTOMATA--Possess a true siphon. Carnivorous.
Families--_Cypræidæ_, _Conidæ_, _Buccinidæ_, _Muricidæ_,
&c.
Class =CEPHALOPODA=--Sucker-bearing arms round the mouth.
Order =Dibranchiata=--Two gills.
Section OCTOPODA--Eight arms.
Families--_Argonautidæ_, _Octopodidæ_.
Section DECAPODA.
Families--_Teuthidæ_ (Calamaries, Squids), _Sepiadæ_, &c.
Order =Tetrabranchiata=--Four gills (containing _Nautilidæ_).