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.

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.

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.

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.

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.

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 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.

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.

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 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.

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.

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.

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.