William S. Furneaux

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Before passing on to the univalve molluscs, we must refer briefly to a 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.

Fig. 154.—Terebratulina. The upper figure represents the interior of the Dorsal Valve