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.
Plate V.
MOLLUSCS
| 1. Solen ensis | 8. Tellina |
| 2. Trivia Europæa | 9. Capulus hungaricus |
| 3. Trochus umbilicatus | 10. Chrysodomus antiquus |
| 4. Trochus magnus | 11. Buccinum undatum |
| 5. Littorina littorea | 12 & 13 Scalaria communis |
| 6. Littorina rudis | 14. Pecten opercularis |
| 7. Haminea (Bulla) hydatis | 15. Pecten varius |
| 16. Pecten maximus | |
The mantle of the animal is free, with double margins, the inner of which forms a finely fringed curtain all round, and on this curtain are a number of black eyes surrounded by very fine tentacles. The gills are in the form of very thin crescents, and the foot is shaped like a finger.
Although the majority of scallops are inhabitants of tropical seas, several species are to be found off our coasts, where they range from depths of about four to forty fathoms, and the empty shells, often in the most perfect condition, are frequently found on the beach.
The Common Scallop (P. maximus) is largely used as food, and is therefore a common object in the fishmonger’s shop. Its colour is very variable, and the shell has equal ears and about twenty radiating ribs. The Quin (P. opercularis) is also an important article of diet in some parts.
Perhaps the prettiest of the British species is the Variable Scallop (P. varius), so called on account of the very variable colour of the shell, the ground tint of which may be almost anything between a very pale yellow and a dark reddish brown, and this is irregularly patched with some lighter colour. The chief distinguishing features of the species are the spiny projections of the numerous ribs, most prominent near the margin of the valves, and the presence of a permanent byssus, which, in other species, occurs only in the young. Three of the species named above are shown on Plate V.
We may also mention the Tiger Scallop (P. tigrinus), the radiating ribs of which are sometimes slightly formed, and which has only one ear in each valve; and P. pusio, in which the adult shell is often greatly altered in form.
It may be noted, in conclusion, that all the species of this genus have the power of swimming rapidly by flapping their valves—a mode of locomotion very common among the bivalves especially during an early stage of their existence.
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.
The valves of the shell are unequal, and are not placed respectively on the right and left sides of the body of the animal, but rather on the dorsal and ventral or upper and lower sides. The ventral shell is the larger, and is produced into a beak which sometimes has a round hole corresponding in position with the hole for the wick of an antique lamp, and the dorsal or smaller valve is always imperforate. The hinge is a perfect one, the junction of the two valves being so well secured by it that it is impossible to separate them without injury. It is formed by two curved teeth on the margin of the ventral valve that fit into corresponding sockets on the dorsal. A few brachiopods, however, have no hinge, the valves being secured by means of numerous muscles. The hole in the shell serves for the protrusion of a pedicel or foot by means of which the animal is enabled to attach itself.
Two long arms, covered with vibratile cilia, and capable of being folded or coiled, are attached at the sides of the mouth. They are practically processes of the lips, mounted on muscular stalks, and attached to a delicate calcareous loop on the dorsal valve; and serve not only to produce water currents for the conveyance of food to the mouth, but also answer the purpose of gills.
The digestive system of a brachiopod includes an œsophagus that leads into a simply formed stomach round which is a large digestive gland. The heart has only one cavity, but the animal is provided with two smaller and separate organs that assist in the propulsion of the blood, which circulates through numerous blood spaces in the bristly mantle.
About two thousand fossil species of brachiopods are known, extending over a vast range of time; and the living species, numbering less than a hundred, are found from shallow water to the greatest habitable depths.
Since the reader is hardly likely to form any extensive acquaintance with the Brachiopods, we shall illustrate our remarks by the introduction of only one species—the Serpent’s Head Terebratula (Terebratulina caput-serpentis), which is found in deep water in the North Sea. The interior of the dorsal valve, showing the calcareous loop above referred to, is represented in fig. 154, as is also the exterior of the shell, which is finely striated. The latter represents the dorsal aspect of the shell in order to show the hole in the upturned beak of the ventral valve.
Fig. 155.—Under side of the Shell of Natica catena, showing the Umbilicus; and outline of the Shell, showing the Right handed Spiral
We have now to consider the large group of head-bearing molluscs (Cephalophora), the study of which forms a very important part of the work of the sea-side naturalist; and while we deal with the general characteristics of this group, the reader will do well to have before him a few living typical species in order that he may be able to verify as many as possible of the descriptions here given by actual observation. These types may include such creatures as the whelk, periwinkle, and limpet; or if marine species are not at hand at the time, the garden snail, fresh-water snail, and slug will serve the purpose fairly well.
By far the large majority of Cephalopods are enclosed in a single shell, though a few have a rudimentary shell or none at all.
As is the case with the lamellibranchs, the shell is composed of both animal and mineral substance, the latter being a calcareous deposit secreted by the mantle of the animal. The shell is usually spiral in form, as in the whelk, but sometimes conical (limpet) or tubular.
Spiral shells are nearly always dextral or right-handed; that is, if we trace the direction of the spiral from the apex to the mouth, we find that its turns or whorls run in the same direction as the hands of a watch. A few, however, are sinistral, or left-handed, and occasionally we meet with left-handed varieties of those species that are normally of the right-handed type. The cavity of the shell is a single spiral chamber which winds round a central pillar, and each whorl of the shell generally overlaps the preceding one, the two being separated externally by a spiral depression called the suture.
Sometimes the coils of a shell are not close together internally, so that the central column of the spiral is hollow, and opens to the exterior at the base of the shell. In this case the shell is said to be umbilicated, and the opening referred to is the umbilicus. In others the spiral winds round a solid central pillar which is spoken of as the columella.
The apex of the shell, sometimes called the nucleus, is the oldest part, and represents what was once the whole. It is generally directed backwards as the animal crawls, and in adult shells is often more or less worn away by constant friction. We speak of the whorls as first, second, third, &c., taking them in the order of their growth, and it will generally be found that the last whorl is much larger than the others, so much so that it contains the greater part of the body of the animal; hence this one is commonly spoken of as the body-whorl, and the others make up the spire of the shell.
The mouth of the shell is of different forms in different species, but in the herbivorous kinds it is usually simple, while in the carnivorous species it is notched or produced. The edge of the mouth (peristome) is formed by an outer lip which is usually sharp in young shells and either thickened, reflected (turned outward), or inflected (turned inward) in adults; also it may be considerably expanded, or ornamented by a fringed margin. The inner lip is that side of the peristome adjacent to the central pillar of the shell.
If we examine the external surface of several different shells, we find that they are usually more or less distinctly furrowed or sculptured, and that they are often marked by lines or bands of a colour different from that of the ground tint. These furrows, lines, or bands sometimes pass directly from the apex, across the various whorls, to the base of the shell, in which case they are said to be longitudinal. If they follow the course of the whorls, they are described as spiral; and if parallel with the peristome, so that they mark the former positions of the mouth of the shell, thus denoting the lines of growth, they are said to be transverse.
Most univalve shells are covered with epidermis, but in some instances the animal, when extended, surrounds the exterior of the shell with its mantle, as do the cowries, and then the outside of the shell is always glazed. Other species keep their shells covered with the mantle, and in these the shell is always colourless.
The body of the head-bearing mollusc is attached to the shell internally by one or more muscles, and if we examine the interior surface we are generally able to distinguish the impressions or scars denoting the points of attachment.
The reader will have observed that the periwinkle, whelk, and other univalves close their shells by a kind of lid when they retract their bodies. This lid is called the operculum, and is constructed of a horny material, often more or less calcified on the exterior, and is attached to the hinder part of the foot. It sometimes fits accurately into the mouth of the shell, but in some species it only partially closes the aperture. The operculum, like the shell itself, often exhibits distinct lines of growth which display the manner in which it was built up. If these lines are concentric we know that the operculum grew by additions on all sides; but if its nucleus is at one edge, and the lines of growth widest apart at the opposite side, the growth must have taken place on one side only. Some, even, are of a spiral form, denoting that the additions were made continuously at one edge, and such opercula may be right-handed or left-handed spirals.
It will be noticed that in the above general description of univalve shells we have introduced a number of technical terms which are printed in italics, and this we have done advisedly, for the employment of these terms is a very great convenience when giving descriptions of individual shells, and we shall use them somewhat liberally in noting the distinguishing characteristics of the families and genera; but before entering into this portion of our work we must briefly note the general features of the bodies of the Cephalophora.
Fig. 157.—Diagram of the Anatomy of the Whelk, the Shell being removed
c, stomach; e, end of intestine; g, gills; h, ventricle of the heart; a, auricle; f, nerve ganglia; b, digestive gland; ft, foot; o, operculum; d, liver
Sometimes these bodies are bilaterally symmetrical, as we have observed is the case with the worms, but more commonly the organs on one side are aborted, while the growth proceeds apace on the opposite side. Thus the animal assumes a spiral form, being coiled towards the aborted side, with the gills and other organs developed on that side only. As a rule this curvature is such that the body takes the form of a right-handed or dextral spiral, as we have already observed in the shells which cover them, the mouth being thus thrown to the right, but sometimes it takes the opposite direction.
When one of these animals is extended and creeping, we observe that it has a distinct head, furnished with a mouth below, and tentacles and eyes above; also, if an aquatic species, the gills are more or less prominent. Further, the exposed portion of the body is covered with a leathery mantle, and the animal creeps on a broad, flattened surface which is called the foot.
The tentacles or feelers are usually retractile, and, when retracted, are turned outside-in. Each one is provided with a muscle that runs from the body internally to the tip; and, by the contraction of this muscle the tentacle is involuted just in the same way as the finger of a glove could be by pulling a string attached to the tip inside. In addition to these tentacles, and the eyes and mouth previously mentioned, the head is furnished with ear-sacs, which are little cavities, filled with fluid containing solid particles, with nerve filaments distributed in the walls.
On the floor of the mouth there is a ribbon, supported on a base of gristle, and covered with numerous minute teeth arranged regularly in rows. The gristle is moved backwards and forwards by means of muscles in such a manner that this ‘lingual ribbon’ acts like a rasp, and is employed in scraping or tearing away portions of the substance on which the animal is feeding. By this action the teeth are gradually worn away in front, but this is of no consequence, for the lingual ribbon is always growing forwards, the worn material being replaced by new growth behind.
Fig. 158.—A portion of the Lingual Ribbon of the Whelk, magnified; and a single row of Teeth on a much larger Scale
b, medial teeth; a and c, lateral teeth
The arrangement and form of the teeth are characteristic and important; and since they afford one of the means by which we may trace the natural affinities of similar species, they will be frequently referred to when dealing with the principles of classification. For this reason the student should be prepared to examine the lingual ribbons of molluscs with the aid of a compound microscope as occasion requires. As a rule the ribbon is easily stripped away from the floor of the mouth; and, if placed in a drop of water and covered with a cover-glass, the teeth are readily observed. Until a little experience has been gained the observations may be confined to some of the larger species, in which the ribbon is both large and easily obtained. In the common whelk, for example, it often measures more than an inch in length.
It is difficult to understand how the univalve mollusc manages to glide along so rapidly and gracefully on its expanded foot when we observe it from above, but the difficulty is cleared away when we see it creeping on the side of a glass aquarium, or when we place it on a sheet of glass and observe its movements from the other side. We then see that the foot is in complete contact with the glass, and that a steady but rapid undulatory movement is produced by the successive expansions and contractions of the disc, brought about, of course, by the action of muscular fibres.
A few of the univalves are viviparous—that is, they produce their young alive; but the majority lay eggs. The eggs are often enclosed in horny cases, some of which may be commonly seen washed up on the beach, or attached to rocks and weeds between the tide-marks. The larvæ are always enclosed in a shell, though they are sometimes wholly or partially concealed by the mantle. The shell is usually closed by an operculum; but as the animal advances in age the shell sometimes disappears altogether, or is reduced to a mere shelly plate, as is the case with the land and marine slugs and sea lemons. The young of the water-breathers always swim about freely by means of a pair of ciliated lobes or fins, but these remain only for a brief period, after which the animal settles to the bottom for a more or less sedentary existence.
The Cephalophora fall naturally into two fairly well-defined groups, which we may describe as the air-breathers and the water-breathers. The former breathe air direct from the atmosphere through an aperture on the right side of the body, the air passing into a pulmonary organ or lung, in the walls of which the bloodvessels ramify, and they include all the land snails and slugs. The latter breathe by gills which are more or less prominent on the sides of the body, and include all the fresh-water snails, as well as the marine species which fall within our special province.
We shall first consider the class Pteropoda or Wing-footed Molluscs, so called from the wing-like appendages that are attached to the side of the mouth, or to the upper side of the foot, which is either very small or altogether wanting.
These Pteropods are in many respects lowly organised as compared with the higher molluscs; and as they spend the whole of their existence in the open sea, they can hardly be considered as falling within the scope of the sea-side naturalist’s work. Yet since their shells are occasionally drifted on to the shore, and because a knowledge of them is essential to the student of the mollusca, we shall briefly note their principal characteristics.
The pteropods are extremely abundant in some seas, occurring in such vast numbers that they discolour the water for miles. They swim about by flapping the pair of wings already referred to. They are known to form an important article of the diet of the whale, and are also devoured in enormous numbers by various sea birds; and they are themselves carnivorous, feeding on various smaller creatures that inhabit the open waters.
In appearance they much resemble the young of higher species of molluscs. The nervous system consists of a single ganglion situated below the gullet, and the eyes and tentacles are either rudimentary or absent. The digestive system includes a muscular gizzard provided with teeth for the mastication of food, and a digestive gland or liver for the preparation of a digestive fluid. The heart has two cavities, and respiration is effected by a surface covered with minute cilia. This surface is either quite external or is enclosed in a chamber through which water freely circulates.
The shell is very different from that of a typical head-bearing mollusc, for it generally consists of two glassy, semitransparent plates, situated dorsally and ventrally respectively on the body of the animal, with an opening for the protrusion of the body, and others at the sides for processes of the mantle; and it terminates behind in one or three pointed processes. Sometimes, however, its form is conical or spiral, with or without an operculum. We append illustrations of a few pteropods, selecting for our purpose species that have been found in the Atlantic.
It will have been noticed from the above short description that the pteropod is very unlike the typical Cephalophore as outlined in our general remarks on the group, especially in the symmetrical form of both body and shell and in the total or almost total absence of the foot; and this distinction is so marked that the pteropods are often separated from all the other Cephalophora into a class by themselves, while all the remainder are placed in a separate extensive class called the Gasteropoda, because they creep on the ventral surface of the body, the term signifying stomach-footed.
These gasteropods are divided into four orders: the Nucleobranchiata, in which the respiratory and digestive organs form a nucleus on the posterior part of the back; the Opisthobranchiata, with gills more or less exposed towards the rear of the body; the Pulmonifera, or lung-breathing order; and the Prosobranchiata, in which the gills are situated in advance of the heart. The third order includes all the land snails and slugs, and does not therefore fall within the scope of our work; but the remaining three consist either exclusively or principally of marine species, and will be dealt with in the order in which they are named.
The Nucleobranchs are not really gasteropods in the strictest sense of the term, for they do not creep along by means of their foot, but all swim freely in the open ocean, always at the surface, and sometimes adhere to floating weed by means of a sucker. In fact, the foot of these creatures is greatly modified in accordance with their habits, one part being often expanded into a ventral swimming fin, and provided with a sucking-disc for adhesion, and another produced into a posterior fin for locomotion.
Like the pteropods, the nucleobranchs are purely pelagic, so that we can hardly expect to meet with a specimen on or near the shore; and thus we shall content ourselves with a brief notice of their general characters.
The shell is very variable in size and form, and sometimes even entirely absent. Large-bodied species often possess but a very small shell, while some are able to entirely retract themselves and close the mouth of the shell by an operculum. These animals are generally provided with a large cylindrical proboscis, and the tongue has recurved teeth. The body is usually very transparent, often so much so that the blood may be seen circulating within it, and the nervous system is much more perfectly developed than in the pteropods. The eyes, too, are perfectly formed.
The presence of special breathing organs may seem to be superfluous in such delicate and soft-bodied creatures as these, for it may be supposed that all the oxygen required could be absorbed directly from the water through their soft structures, as is really the case with many aquatic creatures; and as a matter of fact some of the nucleobranchs possess no gills, but others have these organs fully formed.
Passing now to the true gasteropods, we shall first consider the Opisthobranchs, which are commonly known as Sea Slugs and Sea Lemons. Some of these have no shell at all, and even where one exists it is very rudimentary, usually very small and thin, and concealed within the mantle. The gills are either branched and tree-like, or are composed of tufts or bundles of filaments; and, as the name of the order implies, are situated towards the posterior part of the body. They are also retractile, and when the animal is alarmed it will conceal its gills, thus reducing its body to a shapeless, slimy mass, inviting neither to sight nor to touch.
The sea slugs are principally animal feeders, subsisting on small crustaceans, other molluscs, &c.; the food being first reduced by the rasping action of the teeth, and then masticated in a gizzard which is provided internally with horny spines or hard, shelly plates.
It will not be necessary to enumerate all the different families of this order, especially as the species are mostly to be found beyond the tide-marks, and are therefore obtained only with the aid of the dredge; but we shall describe a few of the British species with a view of showing the general characteristics of the animals.
They are usually divided into two sections, those with exposed or naked gills (Nudibranchiata) forming the first, and those in which the gills are covered either by the shell or the mantle (Tectibranchiata) comprising the second.
In the Nudibranchs the shell exists only during the embryonic stage, and the external gills are arranged on the back or along the sides. The tentacles are not employed as organs of touch, but are probably connected only with the sensation of smell, being provided with filaments of the olfactory nerve; the eyes are small dark-coloured spots embedded in the skin behind the tentacles. Various species are to be found on all rocky coasts, where they range from low-water mark to a depth of fifty or sixty fathoms, but a few are pelagic, living on the surface of floating sea weeds.
It is almost impossible to identify the species of nudibranchs from dead specimens, for the classification of the section is based largely on the arrangement of the gills, which are almost always retracted in the dead animals. This is also the case even with living specimens when disturbed or removed from the water; hence they should always be examined alive in sea water, while the animals are extended and moving.
Fig. 161.—Nudibranchs
1. Doto coronata. 2. Elysia viridis. 3. Proctonotus mucroniferus. 4. Embletonia pulchra
It will be understood from the above statements that special methods will be necessary when it is required to preserve specimens for future study, the gills being always retracted when the animal is killed for this purpose by any rapid process. We have found two methods, however, that are fairly satisfactory in the majority of instances.—Place the living animals in a suitable vessel of sea water, and leave them quite undisturbed till they are fully extended, and then either gradually raise the temperature till they are dead, or introduce into the water, cautiously, a solution of corrosive sublimate. In the latter case a much larger proportion of the sublimate will be required than when used for a similar purpose with freshwater molluscs. When the animals are dead it will be found that their gills are more or less extended, sometimes fully so, and they may then be transferred to diluted spirit or a two per cent. solution of formaldehyde.
Fig. 162.—Nudibranchs
1. Dendronotus arborescens. 2. Tritonia plebeia. 3. Triopa claviger. 4. Ægirus punctilucens
In fig. 162 we represent four species. Two of these—Triopa claviger and Ægirus punctilucens—belong to the family Doridæ, the members of which are popularly known as Sea Lemons, and are distinguished by the presence of plume-like gills situated on the middle of the back. Another family (Tritoniadæ), characterised by the arrangement of the gills along the sides of the back, and by tentacles that can be retracted into sheaths, is represented by Tritonia plebeia and Dendronotus arborescens in the same figure, and by Doto coronata in fig. 161. The family Æolidæ also have their gills arranged along the sides of the back, but they differ from the last in that their tentacles are not retractile. They include the two species numbered 3 and 4 on fig. 161. The remaining one on fig. 161—Elysia viridis—is a member of the family Phillirhoidæ, characterised by a pair of tentacles on the dorsal side of the head and by the foot being either very narrow or absent, the latter feature denoting that the animals are not adapted for creeping on the bottom. In fact, several of the species of this family swim freely by means of flattened tails.
The Tectibranchs are similar in general structure, but are very different in appearance, inasmuch as the gills, so prominent in the last division, are here covered by the mantle, or by the shell, which is often well developed. The latter is very variable in form, being of a globular, twisted, spiral, or other shape, but is sometimes absent in the adult. In fig. 163 we give a few examples of the shells of British species; and one (Bulla hydatis) is shown on Plate V.
We now pass on to the largest and last order of gasteropods—the Prosobranchiata—so called because the gills are situated in front of the heart. This group is an important one to the sea-side naturalist, since it contains nearly all the univalve molluscs that are common between the tide-marks of our shores, as well as some abundant species that are protected by a shell of several distinct parts. In nearly all of them the abdomen is well developed, and the shell is sufficiently large to cover the whole animal when the latter is retracted; and the gills, which are either pectinated (comb-shaped) or plumed, are lodged in the chamber formed over the head of the animal by the mantle.
The order is often divided into two sections—the Holostomata or Sea Snails, in which the margin of the aperture of the shell is entire, and the Siphonostomata, in which the margin of the mantle is prolonged into a siphon by which water passes into the gill chamber. This division does not seem to be very satisfactory, as the sections are not separated by very prominent natural characteristics, but it becomes convenient on account of the great extent of the order.
In the Holostomata the shell is either spiral, conical, tubular, or composed of several valves, and the spiral forms are usually closed by a horny or shelly operculum of the spiral kind. The head is provided with a proboscis that is generally non-retractile, and the gills usually extend obliquely across the back, or are attached to the right side behind the head.
We shall first consider the lower forms, starting with the family Chitonidæ, the animals of which, as the name implies, are covered with a shell that resembles a coat of mail.
Some of these creatures are very common on our rocky coasts, and yet their nature is such that they are liable to be overlooked by those who are not acquainted with their appearance and habits. The shell is oval or oblong, often so coloured as to closely resemble the rocks and stones over which they crawl; and the animal is so inactive when left exposed by the receding tide, and its flat under surface so closely applied to that on which it rests, that it looks merely like a little convexity of the rock. But after a few have been discovered the eye becomes accustomed to their appearance, and large numbers may be obtained in a short space of time.
The shell will be seen to consist of eight transverse, curved plates, overlapping each other at their edges, and all enclosed in a leathery mantle, which also forms a projecting margin all round. The middle six plates are different from the first and last in that they are grooved in such a manner that each one displays a dorsal and two lateral areas.
The animal holds on tightly to the rocks by its large creeping disc-like foot, but may be removed without injury by forcing a knife-blade under the margin of its shell. When examined it will be found that it has not a well-formed head like the majority of the gasteropods, and both eyes and tentacles are wanting. The gills form a series of lamellæ round the posterior end of the body, between the edge of the foot and the mantle; and it is interesting to note that the Chitons further justify the low position assigned to them among the gasteropods by their possession of a simple, central, tubular heart, similar to that of worms.
Perhaps the commonest of the British species is Chiton cinereus. Its colour is a dull grey, but the ground is variously mottled, often in such a manner as to give it a protective resemblance to its surroundings. C. ruber is the largest of our species: its shell is variously mottled with shades of yellow and brown; C. fascicularis is bristled. Another rather common species (C. lævis) is distinguished by the glossy appearance of the dorsal portion of the shell.
It will have been observed that the chitons differ from the majority of gasteropods in that their shells and bodies are both bilaterally symmetrical, and the same is true of the next family—Dentaliadæ, which derive their name from the tooth-like form of their conical shells. They are popularly known as the Tooth Shells, and although they generally live beyond low-water level, they may sometimes be seen alive on the beach, and the empty shells are often washed up by the waves.
The shells (fig. 165) are curved, and open at both ends, the narrower extremity being the posterior. The mouth is circular, and the outer surface is quite smooth or grooved.