_MARINE MOLLUSCS_
The sub-kingdom _Mollusca_ includes a great variety of soft-bodied
animals which differ from the members of the last division in the fact
that they are never segmented, and in the possession of a thick outer
covering, of a leathery nature, which completely envelops the body, and
which usually secretes a calcareous shell of one or more parts. A
general idea of the extent of the group may be formed when we state that
it contains the Octopus and the Cuttlefish; all Snails and Slugs, and
animals of a similar nature; and all those numerous ‘bivalves’ which are
represented by the well-known Oysters, Mussels, Scallops, &c.
By far the greater number of the molluscs are aquatic in habit; and of
these such a large proportion are marine that the group provides plenty
of occupation for the sea-side naturalist. This being the case, we shall
devote the present chapter to a description of the general
characteristics of these animals, and to the principles of their
classification, illustrating our remarks by a few selections from all
the chief divisions.
Although, as we have already hinted, the body of a mollusc generally
bears but little resemblance to that of the typical elongated and
segmented worm, yet the study of the earliest stages of the former shows
that a certain relationship exists between the two sub-kingdoms, the
newly hatched mollusc being often a minute free-swimming creature with
expanded lobes fringed with cilia, and bearing a resemblance to certain
of the Rotifers, Moss Polyps, and other animals that are included among
the _Vermes_. But in the adult molluscs this resemblance is lost, these
creatures being generally easily distinguished from all others by
certain well-marked external features, as well as by internal characters
that are peculiar to them and fairly constant throughout the group.
The external shell, where it exists, is usually composed of one or of
two parts, and therefore we speak of univalve and bivalve molluscs; and
no internal skeleton of any kind is to be found except in the division
containing the Cuttlefishes, the ‘bone’ of which is one of the common
objects washed up on our shores by the breakers.
In all the molluscs there is a well-formed digestive tube, and often a
complex arrangement of small teeth which sever the food by a rasp-like
action. There is also a well-formed heart, consisting of two or more
cavities, by means of which the blood is forced through the body; but,
as a rule, blood vessels are either few or absent, the blood being
driven through spaces between the tissues that serve the same purpose.
[Illustration: FIG. 126.--LARVÆ OF MOLLUSCS
_v_, ciliated ‘velum’; _f_, rudimental foot]
The nervous system consists of a few masses of nerve substance
(_ganglia_), connected by nerve cords, and sending off fibres to various
parts of the body, the principal ganglion being one situated close to
the mouth, and often surrounding the first portion of the digestive
tube.
The animals of this sub-kingdom are grouped into three principal and
well-marked divisions--the _Lamellibranchs_, or Plate-gilled molluscs,
the gills of which are composed of plate-like layers, and the headless
bodies enclosed in a bivalve shell; the _Cephalophora_, or head-bearing
molluscs, protected by a univalve shell; and the _Cephalopoda_, or
Head-footed molluscs, so called because the mouth is surrounded by
tentacles or arms by which the animal can cling to objects or seize its
prey.
We shall deal with these three divisions in the above order, taking
first the bivalves, the shells of which are found in great variety along
our shores.
The general nature of a lamellibranch is easily made out by the
examination of one of the common species that may be obtained alive on
any part of the coast, such as the Edible Mussel, the Cockle, or the
Oyster, and the reader will do well to secure a few specimens and
examine them with the aid of the following description of the principal
distinguishing features.
The shell is formed of two valves, united by a hinge which is sometimes
of the simplest possible description, but which often exhibits a
beautiful arrangement of interlocking teeth. A _ligament_ of flexible
and elastic substance often holds the two valves together.
[Illustration: FIG. 127.--SHELL OF THE PRICKLY COCKLE (_Cardium
aculeatum_) SHOWING UMBO AND HINGE; ALSO THE INTERIOR SHOWING THE
TEETH]
The reader has probably observed that the valves of a dead lamellibranch
usually gape. This is due either to the pull exerted by a ligament that
is attached to the valves outside the hinge, or to the pressure of an
internal cartilage which unites the valves within, and which is
compressed when the shell is closed. When the animal is alive, it has
the power of closing its shell by the contraction of the adductor
muscles, to be presently described, and when the valves are brought
together by this means the external ligament is more or less stretched,
or the cartilage within, which is also an elastic material, is
compressed.
Examining the shell from the exterior we observe that each valve has a
nucleus (the _umbo_) close to the hinge, round which are usually a
number of more or less distinct concentric lines, extending to the lower
or ventral margin. This nucleus represents the whole shell of the young
mollusc, and the lines are the lines of growth, each one marking the
extreme limit of the valve at a particular period of the animal’s
existence. Further it will be observed that the lines of growth are
often wider apart in some directions than in others, thus denoting the
unequal rate of growth that determined the form of the adult shell.
[Illustration: FIG. 128.--INTERIOR OF BIVALVE SHELL, SHOWING MUSCULAR
SCARS AND PALLIAL LINE]
The shell of a bivalve is often made up of two very distinct layers, the
outer one called the prismatic layer because, when examined
microscopically, it is seen to consist of minute vertical prisms of
calcareous matter; and the inner one presenting a beautiful pearly
iridescence, due to the fact that it is made up of a number of extremely
thin and finely waved layers of calcareous substance that have the power
of decomposing light. This latter layer is secreted by the whole surface
of the mantle that lies in contact with it, while the outer, prismatic
portion of the shell is formed only by the free edge of the mantle; and
we often find a distinct line (the _pallial line_), some little distance
from the ventral margin that marks the junction of the muscle of the
mantle with the shell. The shape of this line is a very important
feature of the shell, since it is of great value in the determination of
relationships.
Further, the inner surface of each valve is marked by the impressions or
scars of other muscles, the number and position of which vary
considerably in different species. They include the _adductor_ muscle or
muscles (one or two in number) that pull the valve together; the muscle
or muscles that withdraw the foot, called the _retractor pedis_, and the
_protractor pedis_ that pulls the foot out. Not only are these scars
often very distinct in themselves, but we may frequently observe lines
running tangentially from their circumferences towards the umbo, to
which they all converge. These lines enclose the areas previously
occupied by the muscular impressions; in other words, they show the
directions in which the muscles named above shifted their positions as
the animal grew.
[Illustration: FIG. 129.--DIAGRAM OF THE ANATOMY OF A LAMELLIBRANCH
_f_, mouth, with labial palps; _g_, stomach; _i_, intestine,
surrounded by the liver; _a_, anus; _r_, posterior adductor muscle;
_e_, anterior adductor muscle; _c_, heart; _d_, nerve ganglion;
_m_, mantle (the right lobe has been removed); _s_, siphons; _h_,
gills; _ft_, foot]
Now let us obtain a few species of live lamellibranchs, put them in a
vessel of sea water, and observe them after they have been left
undisturbed for a time. The shell will be seen to gape slightly,
exposing the edges of the two lobes of the mantle which lie closely on
the inner surface of the valves, thus completely enveloping the body of
the animal; and at one end, usually the narrower end in the case of
irregular shells, we shall observe two openings--the _siphons_,
sometimes enclosed within a tube formed by a prolongation of the united
mantle lobes, and protruding from between the valves, and sometimes
formed by the mere contact of the mantle lobes at two adjacent points.
If now we introduce a little carmine or other colouring matter by means
of a glass tube, setting it free near the lower siphon--the one more
remote from the umbo of the shell, we observe that it enters the body of
the mollusc through this opening, and reappears shortly afterwards
through the upper or dorsal siphon. Thus we see that water currents are
incessantly circulating in the body of the animal, entering by the
_inhalent_ or _ventral siphon_, and leaving by the _exhalent_ or _dorsal
siphon_. These currents are maintained by the vibratile action of
thousands of minute cilia belonging to cells that line the cavities of
the body, and serve to supply the animal with both air and food; for
lamellibranchs, being gill-breathers, derive the oxygen necessary for
respiration from the air held in solution by the water, and their food
consists entirely of the minute living creatures that always abound in
natural waters.
Again, we shall find that some of our live bivalves have protruded a
thick, conical, fleshy mass--the _foot_, from the opposite end of the
body. This organ is the means of locomotion in the case of the burrowing
and other free-moving bivalves, but is developed to a less extent in
those species that lead a sedentary life. Thus, the common Edible Mussel
secretes a tuft of strong silky fibres (_byssus_) by means of which it
fixes itself to a rock or other body, and therefore does not need the
assistance of a muscular foot; and an examination of its body will show
that the foot is very small in proportion to the size of the animal, as
compared with that of the wandering and burrowing species. The same is
true of the oyster, which lies fixed on its side, the lower valve being
attached to the surface on which it rests.
[Illustration: FIG. 130.--_Mytilus edulis_, WITH BYSSUS]
We have made use of the terms _dorsal_ and _ventral_ in speaking of the
shell of a bivalve, and it is important that these and a few other
similar terms be well understood by those who are about to read the
descriptions of the animals, or who may desire to describe them
themselves. To do this, take a bivalve in your hand, and hold it before
you in such a position that the hinge is uppermost, and the siphons
turned towards you. The foot of the animal is now pointing in the
direction you are looking, and the mouth, situated at the base of the
foot, is also directed the same way. You have now placed the shell, and,
of course, also the animal, in such a position that its _dorsal_ side is
uppermost, the _ventral_ side below, the _anterior_ end turned from you,
the _posterior_ (often narrower) end towards you, the _right valve_ on
your right, and the _left valve_ on your left. Knowing the exact uses of
these few terms you are in a better position to understand the
descriptions of bivalves, and to locate the exact situations of the
various internal organs named in such descriptions.
A great deal of the internal anatomy of a bivalve mollusc may be made
out by easy dissections, and although the structure of the different
species varies in several details, the general characteristics of the
group are practically the same in all and may be gathered by the
examination of a few specimens.
[Illustration: FIG. 131.--A BIVALVE SHELL (_Tapes virgineana_)
_a_, anterior; _p_, posterior; _l_, left valve; _r_, right valve;
_u_, umbo, on dorsal side]
For this purpose the shell should be prised open by means of some
flattened but blunt implement, such as the handle of a scalpel, and
then, after inserting a piece of cork to keep the valves apart, gently
remove the mantle lobe from the valve which is held uppermost with the
same implement, being careful to separate it from the shell without
doing any damage to the soft structures. Separating the mantle from the
shell in this way we meet with one or more hard masses of muscle that
are joined very firmly to the latter. These are the adductor muscles
that pass directly from valve to valve, and on cutting them through
close to the uppermost valve, the latter can be raised so as to expose
the body of the animal, mostly hidden by the overlying mantle lobe.
Before raising the upper mantle lobe we observe the heart, on the dorsal
margin of the body, near the hinge of the shell, situated in a
transparent cavity (the _pericardium_) containing a colourless fluid. It
consists of at least two cavities--a thick-walled ventricle and a
thin-walled auricle, and its slow pulsations may be watched with or
without the use of a hand lens. On opening the pericardium the heart is
still better seen, and if we carefully cut into the thick-walled
ventricle we find a tube running completely through its cavity. This is
the _rectum_--the last part of the digestive tube, that commences at the
mouth, and terminates in a cavity at the posterior end communicating
with the exhalent siphon.
After noting the nature and position of the one or two adductor muscles
previously cut through, we turn the upper mantle lobe upwards, laying
it back over the hinge of the shell, cutting it through at the bases of
the siphons if we find it is united with the opposite lobe at those
points; or, if not united, we observe two points at which the lobes
touch each other in order to form the siphonal openings.
Several organs are now exposed to view. The lower mantle lobe is seen in
close contact with the valve below it, and if we touch its edge we shall
probably observe that it is retracted slightly by the contraction of its
own muscular fibres. The tip of the foot is also seen projecting towards
the anterior end, its base being hidden between the two sets of
plate-like gills that extend along the length of the body. On touching
the tip of the foot we find it retract by the contraction of the
muscular fibres of which it is composed, aided, perhaps, by the action
of one or more _retractor pedis muscles_ with which it is supplied. On
raising the upper gill-plates we may observe the dark colour of the
digestive gland (liver) at the base of the foot, and also see two or
more _tentacles_ or _labial palpi_ on the anterior side of the same.
Between the labial palpi is the mouth, which leads into the stomach by a
short, wide tube, and then into a convoluted tube which finally passes
through the heart, and terminates near the exhalent siphon as above
described. The whole length of this tube may be followed by careful
dissection, its direction being determined at short intervals by probing
it with a bristle that has been tipped with a little melted sealing wax.
It will be seen to wind through the base of the foot, surrounded through
the greater part of its course by the digestive gland, from which a
digestive fluid enters it through small ducts.
The diagram on p. 194 shows the general internal anatomy of a
lamellibranch, parts of which have been removed to reveal the underlying
structures. The animal lies in its left valve, the right valve, the
right mantle lobe, and the right set of gill-plates having been
completely dissected away. The whole course of the digestive tube has
also been exposed, and the positions of the three nerve ganglia, with
their connecting nerve cords, constituting the central portion of the
nervous system, are also indicated.
It will be interesting, finally, to learn the direction taken by the
water currents which supply the animal with air and food in their course
through the system. Passing in through the inhalent siphon, the water
immediately enters a large cavity between the mantle lobes. This cavity
(the _branchial cavity_) contains gills, as we have already seen, and
also extends to the mouth. The water, urged on by the motion of myriads
of minute ciliated cells in the walls of the cavity, passes in part
through the digestive tube, and in part around, between, and through the
gill plates, which are perforated by numerous holes. After thus
completely bathing the gills, and supplying the oxygen necessary for
respiration, this latter current passes into a second cavity above the
gills, and thence into the exhalent siphon, where it mingles with the
fluid from the digestive tube as well as with other excretory matter.
Lamellibranchs are, as a rule, exceedingly prolific, a single individual
of some species discharging more than a million ova in one season. The
larvæ swim freely in the water, and are provided with eyes that enable
them to search for their food, but the eyes always disappear when the
young settle down to a more sedentary life. It is true that adult
bivalves sometimes possess visual organs, often in the form of
conspicuous coloured spots on the edge of the mantle, these, however,
are not the same that existed during the larval stage, but are of a more
recent development.
Lamellibranchs are classified in various ways by different authorities,
the arrangement being based principally on the number and position of
the adductor muscles, or on the nature of the gills. For our present
purpose we shall look upon them as consisting of two main divisions--the
_Asiphonida_ and the _Siphonida_, the former including those species
which do not possess true tubular siphons, the inhalent and exhalent
openings being formed merely by the touching of the mantle lobes; and
the latter those in which the mantle lobes are more or less united and
tubular siphons formed. Each of these divisions contains a number of
families, most of which have representatives that inhabit the sea; and
we shall now note the principal characteristics by which the more
important families are distinguished, and take a few examples of each,
starting with the _Siphonida_.
Examining the rocks that are left exposed at low tide we frequently find
them drilled with holes that run vertically from the surface, seldom
communicating with each other within, and varying in diameter from less
than a quarter of an inch to half an inch or more. Some of these holes
are the empty burrows of a boring mollusc, while others still contain
the living animal _in situ_.
The molluscs in question belong to the family _Pholadidæ_, which
contains a number of species that exhibit very remarkable features both
as regards structure and habit. The shell is very thin and fragile, but
yet composed of hard material, and its surface is relieved by a series
of prominent concentric ridges that bear a number of little rasp-like
teeth. It gapes at both ends, has neither true hinge nor ligament, and
is often strengthened externally by two or more extra or accessory
valves. The _hinge-plate_ is a very peculiar structure, for it is
reflected over the exterior of the umbones, above which they are
supported by about ten thin shelly plates, the whole thus forming a
series of chambers. The accessory valves are supported by these bridged
structures, and a long, straight, calcareous plate also fills the space
along the dorsal side of the shell in some species. The muscular scars
and the pallial line are distinctly seen on the inner surface, and a
peculiar curved shelly plate projects from under the umbo of each valve.
[Illustration: FIG. 132.--_Pholas dactylus_
1, ventral aspect, with animal; 2, dorsal side of shell showing
accessory valves]
The animal inhabiting the shell is somewhat wormlike in general form,
and the mantle lobes are united in front--that is at the lower end of
the shell as it lies in the burrow--except that an opening is left for
the protrusion of the short foot. The siphons are united and much
elongated, so that they protrude beyond the mouth of the burrow when the
animal is active; the gills are narrow, and extend into the exhalent
siphon; and the anterior adductor muscle, being very near the umbones,
serves the double purpose of adductor and ligament.
Such are the general distinguishing features of this family, all the
species of which burrow into stone or other material. Those more
commonly met with on our coasts belong principally to the genus
_Pholas_, and are popularly known as Piddocks.
It was long a puzzle as to how the fragile piddocks could excavate the
tubular burrows in which they live, and, since their shells are so thin
that it seemed almost impossible for hard stones to be ground away by
them, it was suggested that the rocks were excavated by the action of an
acid secretion. This, however, would not account for the formation of
holes in sandstone and other materials which are insoluble in acids;
and, as a matter of fact, no such acid secretion has ever been
discovered. The boring is undoubtedly done by the mechanical action of
the rasp-like shell, which is rotated backwards and forwards, somewhat
after the manner of a brad-awl, though very slowly, by the muscular
action of the foot of the animal.
Piddocks are found principally in chalk and limestones, though, as
before hinted, they are to be seen in sandstones and other rocks, the
material in any case being, of course, softer than the shell that bores
it. The largest holes and the largest specimens are to be found in chalk
and other soft rocks; while the piddocks that burrow into harder
material are unable to excavate to the same extent and are, as a
consequence, more stunted in their growth. The burrowing is continued as
long as the animal grows, the hole being always kept at such a depth
that the shell is completely enclosed; and not only this, for when the
rock is soft, and the surface is worn down by the sea, the piddock has
to keep pace with this action, as well as to allow for its increase in
size.
As a result of the rasping action of the pholas shell on the surrounding
rock the space hollowed out becomes more or less clogged with débris.
This is ejected at intervals by the sudden contraction of the foot of
the animal, which brings the shell quite to the bottom of the burrow,
thus causing the water with its sediment to shoot upwards,
It is not usually an easy matter to obtain perfect specimens of the
pholas by simply pulling them from their burrows, the shells being so
thin and fragile, and the mouth of the burrow being often narrower than
the widest part of the shell. The best plan is to chip away the rock
with the aid of a mallet and chisel, or to break it into pieces with a
hammer, thus laying open the burrows so that the molluscs fall from
their places.
The Common Piddock (_Pholas dactylus_) may be identified by the
illustrations, and the other members of the family may be recognised at
once by the similarity in structure and habit. The principal species are
the Little Piddock (_P. parva_), the shell of which is wider in
proportion to the length, with only one accessory valve; and the White
Piddock (_P. candida_), also with a single accessory. In all the above
the foot is remarkable for its ice-like transparency.
[Illustration: FIG. 133.--_Pholas dactylus_, INTERIOR OF VALVE; AND
_Pholadidea_ WITH ANIMAL]
There is another genus--the _Pholadidea_--the species of which are very
similar to _pholas_ both in structure and habit. The shells are,
however, more globular in form, and are marked by a transverse furrow.
The gape at the anterior (lower) end is also very wide, and covered over
with a hardened plate in the adult. Also, at the posterior (upper) end
of the shell is a horny cup through which the siphons protrude, and the
latter, which are combined throughout their length, terminate in a disc
that is surrounded by a fringe of little radiating appendages.
In the same family are the molluscs popularly known as ship worms, which
are so destructive to the woodwork of piers and jetties, or which burrow
into masses of floating timber. Some of these, belonging to the genus
_Xylophaga_--a word that signifies ‘wood eaters’--have globular shells
with a wide gape in front, and burrow into floating wood, nearly always
in a direction across the grain. The burrows are about an inch deep, and
are lined with a calcareous deposit. The siphons, combined except at the
ends, are slender and retractile; and the foot, which is thick, is
capable of considerable extension.
[Illustration: FIG. 134.--THE SHIP WORM]
[Illustration: FIG. 135.--1. _Teredo navalis._ 2. _Teredo norvegica_]
Other ship worms belong to the genus _Teredo_, and are very similar in
general characters. The shell is small and globular, with a wide gape at
both ends, and consists of two three-lobed valves with concentric
furrows. It is so small in proportion to the size of the animal that it
encloses but a small portion of the body, and lies at the bottom of the
burrow, which is of considerable length--often from one to two feet. The
animal is very wormlike in form; and although the shell is so small, yet
all the internal organs are enclosed by it. The mantle lobes are united
in front, except where the sucker-like foot passes through them; the
gills are long and narrow, and extend into the siphonal tube; and the
two very long siphons are united almost throughout their length. It is
also interesting to note that in these animals the rectum does not pass
through the heart, as it does in nearly all molluscs, and that a pair of
horny or calcareous 'styles' or 'pallets' project from the place where
the two siphonal tubes begin to diverge.
Several species of _Teredo_ are to be met with on our coasts, but they
are so similar in general structure that the above brief description
applies almost equally well to all.
Other boring molluscs frequent the British shores, but they belong to
quite a distinct family called the _Gastrochænidæ_ because their shells
gape widely on the ventral side. Their valves are equal in size and very
thin, the hinge has no teeth and the pallial line is sinuated. The
margins of the mantle lobes are thickened and united except where a
small aperture is left for the protrusion of the finger-like foot. The
siphons are very long and retractile, and the gills extend into the
inhalent tube. These animals burrow into mud, shells, or stone, often
dwelling together in such numbers that their galleries cross one another
and form a most intricate network, and the different species are to be
found from low-water mark to a depth of a hundred fathoms or more.
[Illustration: FIG. 136.--_Gastrochæna modiolina_
1, Animal in shell; 2, shell; 3, cell]
The British species belong to two genera--the typical genus
_Gastrochæna_, and the _Saxicava_ or stone-borers.
The former contains the Common Flask shell (_G. modiolina_) which
burrows into limestone and shells, in the latter case passing generally
through the shells into the ground below, and completing its home by
cementing together any fragments of hard material that come in its way
into a flask-shaped cell. The opening of the burrow is shaped like an
hour-glass, the two expansions serving for the protrusion of the
siphonal tubes, and the neck of the flask-shaped abode is usually
lined with a calcareous layer that projects slightly to afford further
protection to the extended siphons. Although this species is very common
on some parts of our coast, it is seldom obtained without the aid of a
dredge, for it usually lives at a depth of from five to ten fathoms; and
when found it is generally no easy matter to extricate them from their
holes, to the sides of which they often cement their shells.
The genus _Saxicava_ contains a few species that drill holes, often
several inches deep, in shells and stone, and frequently do great damage
to breakwaters and other artificial structures. The foot is usually
provided with a byssus by which the animal fixes itself to a little
projection on the side of its burrow. The species are to be found from
low-water mark to a depth of one hundred fathoms or more.
The next family, named _Anatinidæ_, contains a number of molluscs that
burrow in mud or sand or live in seclusion in the crevices of rocks.
Their shells are thin, with a granulated outer surface, and the valves
are united by a thin external ligament. The inner surface is pearly, the
pallial line usually sinuated, and both valves are pitted for the
reception of the somewhat stout internal cartilage. The mantle lobes are
united, as are also the siphons to a greater or lesser extent; and there
is only one gill on each side.
[Illustration: FIG. 137.--1. _Thracia phaseolina._ 2. _Thracia
pubescens_, SHOWING PALLIAL LINE]
Some of the common species of this family are popularly known as Lantern
shells, and perhaps the most familiar of these is _Thracia phaseolina_,
the specific name of which is given on account of a fancied resemblance
of the shell to a bean. The shell is very fragile, and although large
numbers may often be seen stranded on sandy beaches, but few of them are
perfect specimens.
The family _Myacidæ_ may be recognised by the thick, strong, opaque
shells, usually gaping at the posterior end; the wrinkled epidermis
which covers the whole or part of the shell; and the united siphons,
which are more or less retractile. The mantle cavity is also closed with
the exception of a small hole left for the protrusion of the small foot.
The pallial line of the shell is sinuated.
[Illustration: FIG. 138.--1. _Mya truncata._ 2. INTERIOR OF SHELL.
3. _Mya arenaria._ 4. _Corbula nucleus_]
In the above illustration we represent the Common Gaper (_Mya
arenaria_), which burrows to a considerable depth in the sand or mud,
especially in the estuaries of rivers, from between the tide-marks to a
depth of twenty fathoms or more. It may be readily distinguished, in
common with the other species of the same genus, by the characteristic
wrinkled, membranous tube that encloses its fringed siphons, the
membrane being a continuation of the epidermis that extends over the
shell. Another characteristic feature of the genus is the large, flat
process inside the left valve for the attachment of the internal
cartilage. An allied species, _Mya truncata_, is often found abundantly
in company with the above, and may be known by the abruptly squared
posterior end.
Other species of the _Myacidæ_ inhabit our shores, including the little
Basket shell (_Corbula nucleus_), the left valve of which is much
smaller than the right, which overlaps it. The latter, also, is covered
with epidermis, while the former, which is flat, is quite naked.
[Illustration: FIG. 139.--_Solen siliqua_
The valves have been separated and the mantle divided to expose the
large foot]
We now come to the interesting family of Razor shells (_Solenidæ_),
specimens of which are washed up on almost every sandy beach, while the
living molluscs may be dug out of their burrows at low-water mark. The
shells are elongated, gaping at both ends with an external ligament; and
the hinge has usually two teeth in one valve and three in the other. The
foot of the animal is cylindrical, large and powerful; and the siphons
are short and united in the long species, but longer and only partially
united in the shorter ones. The gills are long and narrow, and are
prolonged into the inhalent siphon.
These molluscs lie vertically in their deep burrows at low-water mark,
the opening of the burrow having a form resembling that of a keyhole.
While covered with water they occupy the upper portion of their abode,
but sink to a depth of a foot or more when the tide goes out. As we walk
along the water’s edge at extreme low tide we may observe jets of water
that are shot into the air before us. These are produced by the sudden
retreat of the ‘Razor-fish’ to the bottom of its burrow when alarmed by
the approaching footsteps. Owing to this wariness on the part of the
mollusc, and to the considerable depth of its burrow, specimens cannot
be obtained by digging without much labour; but if a little salt or some
other irritant be dropped into the hole, the animal will soon rise to
eject it, and may then be shut out from the lower part of the burrow by
sharply driving a spade below it. This is undoubtedly the best method of
securing perfect specimens for study or preservation, but fishermen
often obtain large numbers, either for food or for bait, by suddenly
thrusting a long hook down into the gaping shells, and then pulling them
out. This method always does injury to the soft body of the animal, and
often damages the shell, but answers the fisherman’s purpose exactly.
We give illustrations of two shells belonging to the typical genus
(_Solen_), including one on Plate V.; also a British representative of
each of two other genera of the family--_Cerati-solen_ and _Solecurtus_,
the latter of which, as the name implies, contains shorter species.
[Illustration: FIG. 140.--1. _Solen ensis._ 2. _Cerati-solen legumen._
3. _Solecurtus candidus_]
The next family--the _Tellinidæ_--contains a number of well-known
molluscs that burrow into sand or mud, and are enclosed in shells that
are often very prettily marked; and although the family includes several
genera, all may be recognised by the following general features. The
shell is compressed, composed of two equal valves, with little or no
gape, and the ligament situated on the shortest side. The central or
_cardinal_ teeth never exceed two in number in each valve, and the
adductor impressions are round and polished. The mantle is quite open at
the anterior end, and its margins are fringed; the foot is flattened and
tongue-shaped; and the siphons, which are quite separate, are generally
long and slender.
In the typical genus (_Tellina_), of which we represent two very common
British species, the ligament is very prominent, and the slender siphons
are often much longer than the shell. The members of this group move
very freely, travelling about by means of a broad, flattened foot.
[Illustration: FIG. 141.--_Tellinidæ_
1. _Psammobia ferroensis._ 2. _Donax anatinus._ 3. _Tellina
crassa._ 4. _Tellina tenuis._ 5. _Donax politus_]
The shells of the genus _Psammobia_ are popularly known as Sunset
shells, being prettily marked with radiating bands of pink or other
tint, reminding one of the beams of the sun when setting in a cloudy
sky. In these, too, the ligament is very prominent, and the shell gapes
slightly at both ends.
The same family contains the pretty little Wedge shells, which are so
called on account of their triangular form, and constitute the genus
_Donax_. These shells, which are seldom much over an inch long, are very
common on some of our sandy beaches, being washed up in considerable
numbers after the animals have died, but the specimens are seldom
perfect. The molluscs themselves are burrowers, and live in the sand, at
and just below low-water mark; and, as they usually burrow to a depth of
only a few inches, are easily obtained alive.
The shells are rather thin, closed at both ends, blunt and rounded at
the anterior end, but straight and more pointed at the shorter
posterior end; and the margins of the valves are very finely grooved in
such a manner as to resemble the milling of a coin. Each valve has two
central hinge teeth, with one long lateral tooth on each side; and the
ligament is external and prominent. The lobes of the mantle are fringed;
the siphons are separate and diverging, but shorter and thicker than in
most of the other _Tellinidæ_, and the foot is comparatively large,
flattened, and pointed.
The genus contains many species, the commonest being, perhaps, _D.
anatinus_, the colour of which is yellowish, banded with brown, and
marked by a number of radiating white lines. This colour, however, is
due entirely to the thin, shining epidermis that completely covers the
valves; and if this is rubbed off the shell itself will exhibit a pale
pinkish tint. Another common species (_D. politus_) may be recognised by
the broad patch of white running from the hinge to the margin, on the
posterior side of the middle of each valve.
The family _Mactridæ_ contains some British shells popularly known as
Trough shells, and the family name itself is derived from the word
_mactra_, which signifies a kneading trough. In this group the shells
are all more or less triangular in form, with the valves equal, and are
either closed or very slightly gaping. The ligament, perhaps more
correctly designated the cartilage, is generally internal, and contained
in a deep triangular hollow; and the shell is covered with epidermis.
The mantle of the animal is open in front, and the siphonal tubes are
united and fringed. The foot is usually large and flattened.
The typical genus, _Mactra_, contains some common molluscs that bury
themselves just beneath the surface of sandy beaches; and these are so
abundant in some parts of Great Britain that they are used largely for
feeding pigs. Some of the mactras are remarkable for the great power and
extensibility of the foot, which, in some cases, is used so vigorously
that the animal turns itself quickly over, or even leaps on the ground.
Our example of this genus is _M. stultorum_, which is a very common
object of the shore. Its colour is very variable, usually some shade of
grey or brown, and marked by radiating white lines.
The Otter shells (_Lutraria_), of which we figure one species, are much
like the _Mactræ_ in structure, and are usually included in the same
family, but in some respects they resemble the _Myacidæ_ or Gapers. The
shell is oblong rather than triangular, and gapes at both ends; and the
animal buries itself deep in sand or mud, principally in the estuaries
of rivers, from low-water mark to a depth of about ten fathoms. The
shells are not very common objects of the shore, for they are found only
in muddy places, and those of the commonest species (_L. elliptica_) are
too large and heavy to be washed ashore in the sheltered estuaries where
they abound.
[Illustration: FIG. 142.--1. _Lutraria elliptica._
2. PART OF THE HINGE OF _Lutraria_, SHOWING THE CARTILAGE PIT.
3. _Macra stultorum._ 4. INTERIOR OF SAME SHOWING PALLIAL LINE]
We now leave the burrowers, to consider a family of molluscs that move
about somewhat freely by means of a flattened tongue-shaped foot, and
which only rarely fix themselves in any way. The shells of the group are
popularly known as Venus shells, probably on account of the beauty of
some of the species, and the family in question as the _Veneridæ_.
The shells of the various species are usually of a graceful oval or
oblong form, frequently marked by chevron-shaped lines in pretty
colours, and distinctly grooved along the lines of growth. The ligament
is external, the hinge has usually three diverging teeth in each valve,
and the pallial line is sinuated.
The principal genus is _Venus_, in which the shells are ovate in form,
thick, and smooth, and the margins of the valves are minutely
crenulated. The genus is a very large one, and contains several British
species, two of which we represent in the accompanying illustrations.
Allied to these is the larger but pretty shell _Cytherea chione_, which
inhabits deep water off the southern coasts, to about one hundred and
fifty fathoms. It is much like the _Venus_ shells in form, but the
margins are not crenulated.
[Illustration: FIG. 143.--_Veneridæ_
1. _Venus fasciata._ 2. _Venus striatula._ 3. _Tapes virgineana._
4. _Tapes aurea_]
The same family (_Veneridæ_) contains the large genus _Tapes_, so called
because many of its shells are marked in such a manner as to recall the
patterns of tapestry. The general form of these shells is oblong, and
the margins are quite smooth. They are frequently washed up on the
beach, especially during storms, but the animals may be found alive at
low water, buried in sand, or hiding in the crevices of rocks or among
the roots of the larger sea weeds. The mantle is open at the anterior
end, and the siphons are either quite distinct or only partly united.
Some of the shells are very prettily coloured. One (_T. aurea_) receives
its name from the yellow ground, which is variously marked by deeper
tints; another (_T. decussata_) is so called on account of the cross
grooves with which the shell is sculptured; and a third (_T.
virgineana_), which inhabits the muddy bottoms of deep water, is
prettily marked by radiating bands that run from the umbones to the
ventral margins.
We now come to the family _Cyprinidæ_, in which the shell is regular in
form, oval or elongated; and the valves, which are equal in size, are
thick and solid, and fit closely. The teeth are beautifully formed, the
central ones numbering from one to three in each valve, and the pallial
line is not sinuated. The mantle lobes are united on the posterior side
by means of a kind of curtain that is pierced by two siphonal openings.
There are two gills on each side, united posteriorly, and the foot is
tongue-shaped and thick.
The typical genus--_Cyprina_--contains a large mollusc (_C. islandica_),
which is moderately common round our shores, especially in the north,
but is not often seen above low-water mark, except when washed up by
storms. The shell is oval and thick, with the umbones prominent and
turned towards the posterior side, and the ligament is strong and
prominent. It is entirely covered with a thick epidermis, of a rich
brown colour, often exhibiting a fine silky gloss, especially near the
margins. The interior of the shell is white, and the adductor
impressions oval and polished.
The same family includes some smaller shells that inhabit deep water,
and are therefore not commonly seen on the beach. Among these are two
species of the genus _Astarte_, one of which is deeply furrowed in a
direction parallel with the margins; also _Circe minima_, which seldom
exceeds half an inch in length. Although so small compared with
_Cyprina_, these shells may be identified by their clothing of
epidermis, together with the family characteristics given above.
The _Cyprinidæ_ also contains the interesting Heart Cockle (_Isocardia
cor_), the form of which is so characteristic that identification is
easy. The heart-shaped shell is thick and strong, and is swollen out in
such a manner that the umbones are wide apart. These latter are also
curved into a spiral form, and the ligament between them is prominent.
The colour of the shell is variable, the epidermis being of any shade
from a yellow to a dark brown. The foot is small and pointed, and the
siphons fringed.
The Heart Cockle burrows in sand by means of its foot, going down just
far enough to bury the whole of its shell, and always leaving its
siphons exposed at the surface. It inhabits deep water, and is not
likely to be obtained without the use of the dredge or trawl.
[Illustration: FIG. 144.--_Cyprinidæ_
1. _Cyprina islandica._ 2. _Teeth of Cyprina._ 3. _Astarte
compressa._ 4. _Circe minima._ 5. _Isocardia cor_]
The molluscs of the family _Lucinidæ_ are found principally in tropical
and sub-tropical seas, ranging from the shore to a very great depth, but
a few are moderately common in our own waters. They are closely allied
to the _Cyprinidæ_, but the shell is round rather than oval, and is
obliquely grooved inside. The mantle lobes of the animal are not united
on the ventral side, but at the posterior end they are continuous,
except where they form one or two siphonal openings. The foot is long
and of almost the same thickness throughout when extended; and the
gills, numbering either one or two on each side, are large and thick. In
all the members of this family, as in the last, the pallial line of the
shell is simple. None of the shells are really common objects of our
shores, since the animals inhabit deep water, some of them moving about
freely on the bottom, while others moor themselves by means of a byssus.
We shall take only one example of the family--_Galeomma Turtoni_--the
generic name of which means ‘weasel eye.’ This pretty little mollusc may
be found on our southern coasts, where it often moors itself to the
rocks or weeds by means of its silken byssus; or, having broken itself
away from its temporary place of rest, creeps freely on the bottom by a
long, flattened foot, applied closely to the surface over which it
travels, and used much in the same way as the broad foot of a snail or
whelk, its valves being all the time spread out nearly in the same
plane.
[Illustration: FIG. 145.--_Galeomma Turtoni_]
The shell itself is oval, with central umbones, and is covered with a
thick epidermis. The mantle lobes are united behind, where they form a
single siphonal opening; and the margins are double, with a row of
eye-like spots on the inner edge of each.
The true Cockles, some few species of which are known to almost every
one, constitute the family _Cardiadæ_, so called on account of the
cordate or heart-shaped form of the shell as viewed from the anterior or
posterior side. The shell is regular, or nearly so, and the valves,
which are equal, are ornamented with prominent rays that run from the
umbones to the margin. The ligament is short, strong and prominent, and
the valves fit closely by the interlocking of their crenulated margins,
or gape slightly on the posterior side. There are two central teeth in
each valve, and a long lateral tooth both on the anterior and posterior
sides. The mantle lobes are open in front, with the margins plaited, and
the siphons, which are usually short, are provided with a number of
little tentacles. The foot is large and powerful, and is usually curved
into the form of a sickle.
Although the general nature of the common edible cockle (_Cardium
edule_) is so well known even to the inhabitants of inland towns that a
description may seem out of place here, yet it is possible that but few
of our readers have ever taken the trouble to place the animal in a
vessel of sea water, either obtained direct from the sea or artificially
prepared, for the purpose of studying its movements or other habits;
and it will be well to remember that this and several other species of
edible molluscs which reach our towns alive may be very conveniently
studied at home, and often at times and seasons when work at the
sea-side is undesirable or impossible.
[Illustration: FIG. 146.--1. _Cardium pygmæum._ 2. _Cardium fasciatum._
3. _Cardium rusticum_]
The edible species referred to lives in banks of sand or mud, buried
just below the surface, and frequently in spots that are exposed for
several hours between the tides. They are usually obtained by means of a
rake similar to that used in our gardens.
[Illustration: FIG. 147.--_Cardium aculeatum_]
On the coasts of Devon and Cornwall we find a much larger species, also
valued as an article of diet, and known locally as the Prickly Cockle
(_C. aculeatum_). Its shell is beautifully formed, the rays being very
prominent, each bearing a number of calcareous spines arranged in a
single row. We give an illustration of this species, together with two
sketches to show the nature of the teeth of the shell.
In addition to the two species named, we have the red-footed, _C.
rusticum_, which can suddenly turn itself over by the action of its
powerful pedal organ; the Banded Cockle (_C. fasciatum_), a very small
species distinguished by the brown bands of the shell; and a still
smaller one (_C. pygmæum_), with a triangular shell, occurring on the
Dorset and Devon coasts (fig. 146).
Passing now to the _Asiphonida_, we deal first with the family _Arcadæ_.
These include a number of shells which, though very variable in general
form and appearance, may all be recognised by the long row of similar
comb-like teeth that form the hinge. The shells of this group are
regular in form, with equal valves, and are covered with epidermis. The
mantle of the animal is open, the gills are united by a membrane behind,
and the foot is large, curved, and grooved.
[Illustration: FIG. 148.--_Pectunculus glycimeris_, WITH PORTION OF
VALVE SHOWING TEETH, AND _Arca tetragona_]
One of the prettiest shells in the family is _Pectunculus glycimeris_,
which reaches a length of about two inches. The shell is grooved in the
direction of the lines of growth, and there are also very delicate
striations running radially from umbones to margin; and the ground
colour of white or pale yellowish is beautifully mottled with reddish
brown. We give a figure of this species, together with a drawing of the
peculiar and characteristic teeth, but a more typical shell of this
family may be seen in the Noah’s Ark (_Arca tetragona_). This shell is
almost quadrate in form, swollen, and strongly ribbed. The hinge is
straight, with many comb-like teeth--increasing in number with the age
of the shell; and the umbones are separated by a diamond-shaped
ligament. The foot of the animal is heeled--that is, it has a creeping
surface that extends backwards as well as forwards; the mantle is
furnished with minute eyes (_ocelli_), and the animal has two distinct
hearts. We give a figure of this peculiar shell, and the other British
members of the same genus, though varying more or less in form, may be
recognised at once by the same general characteristics.
In the same family we have the small nutshells (genus _Nucula_), which
are often dredged up from deep water in large numbers; and the elongated
shells of the genus _Leda_, also inhabitants of deep water; and, as
before stated, the affinities of all may be readily established by the
characteristic nature of the teeth.
We now pass on to the family of Mussels (_Mytilidæ_), of which the
common Edible Mussel (_Mytilus edulis_) is a typical species. In this
interesting group the shell is oval or elongated, with equal valves, and
is covered with a dark-coloured epidermis which is often distinctly
fibrous in structure. The umbones are at the anterior end of the shell,
which end is usually very narrow and pointed, while the posterior is
broad and rounded. The hinge has small teeth or none, and the ligament,
which is long, is internal. The shells of mussels consist of two
distinct layers; on the inner, which is often of a most beautiful pearly
lustre, may be traced the simple pallial line and the impressions of the
small anterior and the large posterior muscles.
The mantle lobes of the animal are united only at a point between the
two siphonal openings. There are two elongated gills on either side, and
the foot is thick and more or less grooved.
[Illustration: FIG. 149.--_Mytilus edulis_]
Mussels inhabit salt, brackish, and fresh waters, generally attaching
themselves by means of a silken byssus, but sometimes concealing
themselves in ready-made holes, or in burrows of their own; and some
even hide themselves in a nest which they prepare by binding together
fragments of shells or sand.
The edible mussel, which forms such an important article of diet,
especially among the poorer classes in our large towns, may be easily
distinguished from similar species of another genus by the very pointed
umbones, and the coarse and strong fibrous byssus by which it clings to
any solid object. It is found most abundantly on muddy coasts, and on
mud banks in the estuaries of rivers, generally in such situations as
are uncovered at low tide. The fry abound just below low-water level,
and grow so rapidly that they reach their full size in a single year.
It is well known that a diet of mussels occasionally produces very
unpleasant and even dangerous symptoms in the consumer, and this result
has been attributed to the action of a particular organ of the animal
which has not been carefully removed before eating. This, however, is
not the case, as proved by the fact that the eating of these edibles is
usually perfectly safe when no such precautions have been taken. It is
highly probable that the deleterious character referred to is due to a
disease which sometimes attacks the mussels themselves, but the exact
nature of this has not been thoroughly made out.
[Illustration: FIG. 150.--1. _Modiola modiolus._ 2. _Modiola tulipa._
3. _Crenella discors_]
There is another genus (_Modiola_) containing several species commonly
known as Horse Mussels, and these may be distinguished from _Mytilus_ by
their habit of burrowing, or of constructing a nest by spinning together
various fragments. The shell, also, is more oblong in form, and much
swollen near the anterior end; and the umbones are not so pointed. The
epidermis covering the shell is of fibrous structure, and often extends
beyond the edges of the valves in the form of a fringe.
Several species of Horse Mussels inhabit our shores, from low-water mark
to a depth of fifty fathoms, but none of them is used for food. The
commonest species is _Modiola modiolus_, which has a particularly strong
byssus, and its fibres generally bind together such a number of stones
&c. that the shell is completely hidden in the entangled mass. Other
British species include _M. barbata_, so called on account of the
peculiar fringed threads of the epidermis; _M. phaseolina_, in which
the epidermis threads are not fringed; and _M. tulipa_, named from the
streaks of crimson or purple that radiate from the umbones of the shell
and remind us of the colouring of the tulip flower.
An allied sub-genus (_Crenella_) includes a few small British molluscs
the shells of which are crenulated on the dorsal margin behind the
ligament. The shells are short and swollen, and lined by a brilliant
pearly layer. One species (_C. discors_) is pale green, with radiating
lines from umbo to margin. It is common on many of our shores, but is
not easily found, as it hides at or below low water mark, in a nest
formed by binding together small stones. Other species, one of which is
black, are less abundant, and are not readily obtained except by the use
of the dredge.
Before leaving this family we must refer to the remarkable _Dreissena
polymorpha_, sometimes called the Chambered Mussel, on account of the
chamber which is formed in the beak of the shell by means of a pearly
plate that stretches across it. This animal is not indigenous to
Britain, but was introduced from the East by trading vessels, either
attached by its silken byssus to timber that had been left floating in
water previous to being shipped, or to the bottoms of the ships. It
seems to thrive almost equally well in salt, brackish, and fresh waters,
and has spread very rapidly since its introduction. It is more commonly
found, however, in docks, canals, and rivers, and is on that account
usually described with the fresh-water species.
[Illustration: FIG. 151.--_Dreissena polymorpha_]
The form of the shell is very similar to that of _Mytilus_, but has no
internal pearly layer, and the valves are bluntly keeled. The mantle is
closed, the siphons short, and the foot small.
Our next family--the _Aviculidæ_--contains those shells that are
distinguished by peculiar flat processes on each side of the umbones,
one of which, the posterior, is generally wing-like in form. They are
popularly known as Wing Shells, and the family includes the so-called
Pearl Oysters. Most of the species are natives of tropical seas, but
several are common on our own shores.
[Illustration: FIG. 152.--_Avicula_, AND _Pinna pectinata_]
One species of the typical genus is sometimes found off the coasts of
Cornwall and Devon. The shell is very oblique, and the valves are
unequal, the right one, on which the animal rests, being somewhat
smaller than the left; and the epidermis is very scanty. The hinge is
long and straight, without teeth, and the cartilage is contained in
grooves. The interior of the shell is pearly. The posterior adductor
impression is large, and not far from the middle of the shell, while the
anterior, which is small, is close to the umbones. The mantle of the
animal is open, and the margins of the lobes fringed; and the small foot
spins a powerful byssus.
Most of the British species of the family belong to the genus _Pinna_,
so called on account of the fins or wings on the dorsal side of the
shell. In this group the shell is more or less wedge-shaped, with equal
valves, and the umbones are quite at the anterior end, while it is
blunted and gaping at the other end. The hinge has no teeth. The margins
of the mantle are doubly fringed, and the byssus is extremely powerful.
The Common Pinna (_P. pectinata_) is a very large mollusc, sometimes
measuring a foot in length, and is very abundant off the south-west
coast, where it moors itself vertically at the bottom of the water with
the pointed end buried, and the broad end gaping widely so as to expose
its body. It has been stated that fishes are frequently tempted to
intrude into the open shell for the purpose of devouring the animal
within, and that they are immediately crushed by the sudden closing of
the valves, which are pulled together by two large and powerful
adductors.
We have already referred to the little Pea Crab that inherits the shell
of the Pinna, living permanently in the mantle cavity of the animal.
The last family of the Lamellibranchs is the _Ostreidæ_ or Oysters, of
which the edible oyster may be taken as a type. In this group the shells
are frequently unequal, and they lie on one side either free or adherent
to the surface below them; the hinge is usually without teeth. The
mantle is quite open, the gills number two on each side, and the foot is
either small or absent.
The Edible Oyster is a type of the typical genus _Ostrea_, its
scientific name being _Ostrea edulis_; and as this mollusc may be
readily obtained at any time, it is a convenient species for the study
of the general characteristics of its family. Its shell is irregular in
form, and the animal always rests on its left valve, which is convex,
while the upper or right valve is either flat or concave. The lower
valve is also thicker and laminated in structure, and is attached to the
surface on which it rests. On examining the interior we find that the
shell is somewhat pearly in appearance, and that the edges of the mantle
lobes are finely fringed. The gills, too, are united with each other and
with the mantle on the posterior side, thus forming a distinct branchial
chamber.
Oysters are found on banks at the depth of several fathoms, where they
spawn in early summer, and the fry or spats are collected in large
numbers and transferred to artificial beds or tanks, where they are kept
in very shallow water so as to be easily obtainable when required for
food. It is interesting to note, however, that their growth is slow on
these artificial grounds, the full size being attained in about seven
years, while, in the natural beds, they are full grown in a little more
than half that time.
Native oysters--those that are reared on artificial beds--are of course
removed as soon as they are ready for the market, but those that live on
natural banks are often left undisturbed till their shells are thick
with age. The latter, too, are often destroyed in large numbers by the
boring sponge (p. 124), which so completely undermines the substance of
the shell that it finally breaks to pieces.
In the genus _Anomia_ the lower valve is concave, and perforated with a
large oval hole very near the hinge, while the upper one is very convex,
but the shell is very variable in shape, since the animal sometimes
clings permanently to an object, and the shell, during its growth,
accommodates itself to the surface of that object. The use of the hole
is to allow of the protrusion of a set of muscles which proceed from the
upper valve, and give attachment to a plug or button, more or less
calcified, by which the animal clings.
[Illustration: FIG. 153.--1. _Anomia ephippium._ 2. _Pecten tigris._
3. _Pecten_,
ANIMAL IN SHELL]
One species (_A. ephippium_), known as the Saddle Oyster, is common on
some parts of our coast. It is seldom found on the beach at low water,
but the empty shells are often washed up by the waves.
The same family includes the Scallops, which constitute the genus
_Pecten_. In these the shell is nearly round, with ears on each side of
the umbones, those on the anterior side being generally much more
prominent than the others, and both valves are ornamented by prominent
radiating ribs. The shell is often very prettily coloured, and the
animal rests on the right valve, which may be distinguished from the
left by its greater convexity, and by the presence of a notch under
the anterior ear. The hinge is straight, with a very narrow ligament,
and the internal cartilage is situated in a central pit.
[Illustration: PLATE V.
MOLLUSCS
1. Solen ensis
2. Trivia Europæa
3. Trochus umbilicatus
4. Trochus magnus
5. Littorina littorea
6. Littorina rudis
7. Haminea (Bulla) hydatis
8. Tellina
9. Capulus hungaricus
10. Chrysodomus antiquus
11. Buccinum undatum
12 & 13 Scalaria communis
14. Pecten opercularis
15. Pecten varius
16. Pecten maximus]
The mantle of the animal is free, with double margins, the inner of
which forms a finely fringed curtain all round, and on this curtain are
a number of black eyes surrounded by very fine tentacles. The gills are
in the form of very thin crescents, and the foot is shaped like a
finger.
Although the majority of scallops are inhabitants of tropical seas,
several species are to be found off our coasts, where they range from
depths of about four to forty fathoms, and the empty shells, often in
the most perfect condition, are frequently found on the beach.
The Common Scallop (_P. maximus_) is largely used as food, and is
therefore a common object in the fishmonger’s shop. Its colour is very
variable, and the shell has equal ears and about twenty radiating ribs.
The Quin (_P. opercularis_) is also an important article of diet in some
parts.
Perhaps the prettiest of the British species is the Variable Scallop
(_P. varius_), so called on account of the very variable colour of the
shell, the ground tint of which may be almost anything between a very
pale yellow and a dark reddish brown, and this is irregularly patched
with some lighter colour. The chief distinguishing features of the
species are the spiny projections of the numerous ribs, most prominent
near the margin of the valves, and the presence of a permanent byssus,
which, in other species, occurs only in the young. Three of the species
named above are shown on Plate V.
We may also mention the Tiger Scallop (_P. tigrinus_), the radiating
ribs of which are sometimes slightly formed, and which has only one ear
in each valve; and _P. pusio_, in which the adult shell is often greatly
altered in form.
It may be noted, in conclusion, that all the species of this genus have
the power of swimming rapidly by flapping their valves--a mode of
locomotion very common among the bivalves especially during an early
stage of their existence.