While the lids of the Holostomata are rounded in shape, those that belong to the Siphonostomatous sh.e.l.ls are necessarily more or less modified so as to fit the mouth of the sh.e.l.l, and are consequently oval or even claw-like in shape. The Sting-winkle already spoken of, the common small whelk, _Purpura lapillus_, and the large whelk, _Buccinum undatum_, are common sh.e.l.l-fish in which the elongated lid may be studied. The lid is not, however, like the tongue-ribbon, an essential feature of the structure of every univalve mollusc.[D] Not only are there special instances in which it is greatly modified, but also there are whole groups of univalve molluscs in which it is absent.
[D] There are one or two exceptional cases of gasteropod molluscs that have no tongue-ribbon. The majority of these are parasitic forms, which can get their food without the trouble of filing it down.
A curious suggestion has been made with regard to the lids of univalve sh.e.l.l-fish; namely, that the snapping to of the lid is capable of producing a sound, which may perhaps be audible at a distance under the water. Various molluscs have been credited with producing sounds, either by muscular movements or by the grating of the sh.e.l.l as the animal walks. The common Tortoise-sh.e.l.l Snail, _Helix aspersa_, sometimes makes a most alarming noise when crawling over a window. It has been disputed whether the sounds thus made are produced by the grating of the creature"s tongue-ribbon on the gla.s.s, as it files off small particles of algae and vegetable moulds, which are invisible to our eyes: or whether they are sounds due to suction of the muscular surfaces, such as may be produced by drawing a wet finger across gla.s.s. The noise, however produced, is, as I can testify from experience, sufficiently loud and weird to be very startling, if heard in the dead of night.
Turn now to the Bivalves or Lamellibranchiate molluscs, which include the familiar oyster, c.o.c.kle, and mussel. These are also known as the Pelecypoda, and as the Aglossa, or molluscs without a tongue-ribbon. The name Lamellibranchiate refers to the shape of the gills--"plate-like,"
or flat; the name Pelecypoda to the shape of the foot, "hatchet-foot."
The animal usually chosen as a type of these, the fresh-water mussel, is rather a dull sort of creature, so we have chosen a prettier and more lively specimen as a representative of the cla.s.s; namely, one of the Scallops, _Pecten opercularis_, sometimes called the Quin, the sh.e.l.l of which is shown in the frontispiece of the book. This is one of the most beautiful, perhaps the most beautiful, of the English sh.e.l.ls. The generic name, _Pecten_, the Comb-sh.e.l.l, probably refers, not to the shape of the gills, which is somewhat peculiar, but to the marking of the sh.e.l.l, which presents raised ridges, side by side. Anyone familiar with sh.e.l.ls will see at once that this is an unusual pattern. There are plenty of bivalve sh.e.l.ls with concentric ridge markings, comparatively few with radiating ridges. We shall see presently that there is a good reason for this. The specific name "opercularis," lid-like, refers to the neat round shape of the sh.e.l.l. Each half of the sh.e.l.l has a pair of "ears," so-called. The person who first gave this name to these flaps of sh.e.l.l, three of which are three-cornered and the fourth nondescript, must have been familiar in his youth with books afflicted with the "dog"s-ear" disfigurement; for certainly there is no other kind of ear which greatly resembles these. The notch beneath the irregularly shaped ear is called the "byssal notch": many Pectens spin a byssus or thread, like that spun by the common Sea-Mussel, and thus anchor themselves to fixed objects for a time; this notch is the place where threads of this kind leave the sh.e.l.l.
The two valves of the sh.e.l.l differ in depth, one being flatter than the other; and the "ears" of the two valves differ in shape. The inside of the sh.e.l.l shows muscular impressions, but these cannot be seen in a photograph. The picture, however, shows the strong hinge-ligament which joins the halves of the sh.e.l.l, and the difference in depth and shape of the two valves. The valve on which the animal usually lies is the lighter in colour of the two, and has one ear much longer than the other.
The creature swims by means of the "mantle," or muscular margin of the body. It contracts this suddenly, after first opening the sh.e.l.l and taking in as much water as possible. Thus the water is squeezed out again, and the effect of this is to propel the animal in an opposite direction.
Now we are in a position to understand a little more about the shape of the sh.e.l.l. These curious "ears" possessed by the two valves, together form a straight, strong edge, which cuts the water as the animal flies along. It reminds us of a ship"s prow, and not without reason, for the use of each is the same. A boat"s sharp prow, compared with the rounded front of a "tub," makes all the difference in the possibilities of straight steering, and favours the putting on of speed: the ears of the sh.e.l.l are not less useful to our Scallop. The following account of the swimming powers of this species of Scallop, quoted by Woodward, was given by the Rev. D. Landsborough, who observed young specimens, about the size of the small ones in our picture, swimming about in a pool of sea-water, left by the ebbing tide. "Their motion was rapid and zigzag; they seemed, by the sudden opening and shutting of their valves, to have the power of darting like an arrow through the water. One jerk carried them some yards, and then by another sudden jerk they were off in a moment on a different tack." To the sharp prow, the _Pecten_ owes this capability of arrow-like flight. Its eyes are situated on the fringe of its mantle, and consequently near the wide end of the sh.e.l.l; its peculiar mode of progression, therefore, enables it to back away instantly from any enemy it sees.
Something must be said regarding the interior of the sh.e.l.l. The majority of bivalve sh.e.l.ls have a complicated system of so-called "teeth," or interlocking projections, at the hinges of the sh.e.l.l: these exhibit great variety in different kinds of sh.e.l.l, and are therefore often a ready means of distinguishing one sh.e.l.l from another. The Scallop, however, is very deficient in this respect, as are also some of its near relations, for instance the oyster and its family group. The Fresh-water Mussel also gains its name, _Anodon_, or _Anodonta_, the Toothless One, from the same circ.u.mstance. The name often puzzles the beginner, who asks, bewildered, "But do Bivalves ever have any teeth?" True teeth, of course, they have none--it is the sh.e.l.l-hinge that has teeth, not the animal inside it. Not only have the bivalve sh.e.l.l-fish no teeth indeed, or tongue-ribbon, but furthermore they have no head. For this reason the group has not only received the name already mentioned, of Aglossa, the Tongue-less Ones, but also that of Lipocephala, _i.e._ Molluscs in which the head is not developed. The reason of its absence is not far to seek--a head would be no use inside such a sh.e.l.l. The snail-sh.e.l.l, so differently built, allows freedom for the head; the bivalve mollusc, squeezed in between its valves, has room only for a mouth.
We have referred above to the ridges on the outside of the sh.e.l.l. Now that we have learnt that the _Pecten_ is a very active animal, and moves in the manner described, we see that these ridges run parallel to the direction in which it moves as it darts away ears foremost. Let us try to realise what is the effect of this.
Take a mat with parallel stripes and move it along the floor or table in the direction of the stripes; then try moving it in an opposite direction across the stripes. It is easy to perceive that in the former case one"s eye does not detect the movement nearly so soon as in the latter case. To explain this would necessitate a lengthy digression on the subject of optical illusions: that the fact is so everyone may easily ascertain by experiment. The ridges, therefore, converging in the direction towards which the sh.e.l.l is going, are a protective decoration, enabling it to slip away more easily from under the eyes of its foes.
The reader will readily recall a parallel instance in the common c.o.c.kle.
This also is a very active creature; it takes leaps by means of a strong muscular foot; and the ridges on the sh.e.l.l, like those of the Scallop, converge towards the hinges, that is to say, in the direction in which the sh.e.l.l moves. Another instance of a very active sh.e.l.l-fish with similar markings is afforded by certain kinds of Lima, a near relative of the Scallops. It may be added that all Scallops are not equally active, nor all Limas; and various modifications of their form and colour might be pointed out which lead us to suspect that in the less active kinds the pattern of ridges is often somewhat obscured by means of these differences.
Now, take up a comb and draw it over your fingers, firstly _along_ the teeth, and secondly _across_ them, and you will be able to estimate the gain in speed and comfort to the comb-sh.e.l.l, Pecten, and to the common c.o.c.kle, from having its ridges set in the direction in which it is going. Were the ridges concentric, as is so often the case in bivalve sh.e.l.l-fish of a more sluggish disposition, the friction caused by the ridges would seriously delay the progress of the sh.e.l.l.
Something must be added regarding the colouring of the sh.e.l.l, which is vivid, corresponding with that of the animal within. It is capable of great variety, though perhaps not so great as in some of the smaller _Pectens_. The predominant shades are pink, crimson and yellow, either separately or mixed; that is to say, some sh.e.l.ls are pure pink, some almost pure yellow, some almost pure crimson, while others present every imaginable shade of pinkish yellow, reddish brown and brownish crimson.
Local variation of colour is so marked that we may suspect the variations in tint to be in some degree protective. The sh.e.l.l also varies considerably in size and strength according to the neighbourhood in which it has grown.
This scallop-sh.e.l.l is but one of many: a number of other species are found on our own sh.o.r.es, and many others again in foreign seas.
One sh.e.l.l of the English coast is very annoying to the juvenile sh.e.l.l-collector who gathers specimens on the sh.o.r.e. This is _Pecten pusio_, a very small and delicate kind, with a raised pattern of fine markings upon the ridges, which are very narrow. A good specimen of the deeper valve is common enough, but the shallow valve, if of any size, is distorted into all manner of shapes, as if it had been squeezed and crumpled. The disappointing character of these specimens, from an aesthetic point of view, is explained when we learn that it not only lies on its shallow valve, but becomes fixed in this position, instead of hopping about freely like the _P. opercularis_. It therefore has frequently to adapt its shape to the nature of the ground where it has happened to fix itself. Thus arises the disfigurement of the sh.e.l.l.
So far we have only considered two great groups of the Mollusca, two which are represented by common sh.e.l.ls, familiar to everybody. We must not leave the subject of the Mollusca without referring to their most aristocratic group, the Cephalopoda. These are represented in museums by the sh.e.l.ls of the Pearly Nautilus, and of its not very near relative, the Paper Nautilus; and they are represented on English sh.o.r.es by the cuttle-fishes. All these agree with the Gasteropoda in the possession of a tongue-ribbon, and in cla.s.sification are therefore treated with them under the name Glossophora.
With the Pteropods, transparent forms found swimming over the surface of the deep sea, the reader is not likely to have much to do. In cla.s.sification they are now placed near the Sea-Slugs.
The Placophora, or Polyplacophora, wholly different from our usual idea of a sh.e.l.l-fish, may be named as creatures which the reader is quite likely to meet with. Though not very common, they are widely distributed over our coasts, and may be found near low-tide mark clinging to stones.
Imagine a wood-louse without any apparent head which has taken to clinging to the rock like a limpet, so that it cannot be removed without injury, and you have a rough idea of their general appearance. _Chiton_ is the name of these animals, which have received the group name of Polyplacophora, carriers of many plates, because their external covering consists of an armour of successive sh.e.l.ly plates. These also belong to the Glossophora or Tongue-ribbon Carriers, of which they present a comparatively primitive form.
Reference has already been made to the labours of the earthworm and of the insects, and to their important effects upon the vegetable world.
Although the Mollusca include but one terrestrial group, the Snails, they, too, have played an appreciable part in modifying plant life. If we owe our flowers to the insects, we have probably to thank the snail for our medicines. For the snail dislikes bitter-tasting leaves, and lets them alone, thus exercising an artificial selection in favour of the survival of medicinal plants. In the same way the snail has favoured the survival of hairy and th.o.r.n.y plants, upon which it cannot easily crawl.
The larval forms of the Mollusca differ considerably from the adult.
That of _Anodon_, the fresh-water mussel, at first received, in consequence, a different name, that of Glochidium, by which it is still known, although it has now been long identified as a larval form. It is exceptional in the fact that it is parasitic on fish.
The usual Molluscan larva is a ciliated creature which has been compared to a modified trochosphere. It is preceded by a gastrula stage, and it develops later on into what is called a "Veliger," or "veil-carrying"
larva, so called because it has in front a broad two-lobed ciliated expansion, the velum. This larva is adapted for swimming, which is accomplished by means of the velum. In terrestrial molluscs, the development is necessarily much more direct. It is worthy of note that the periwinkle mentioned above, which lives high and dry (_L. rudis_) has no larval form, while its relatives that live under water develop in the usual way.
The eggs of Mollusca are often enclosed in tough cases, calculated to resist waves and weather. Some of these are shown in miniature, in the group of eggs of various kinds, Fig. 35.
[Ill.u.s.tration: FIG. 35.--Eggs (reduced to half the natural size).
_A_, Egg-Capsules of _Murex_. _B_, Frog"s Eggs. _C_, Eggs of large Land-snail. _D_, Eggs of Snail placed on a leaf. _E_, c.o.c.kchafer"s Eggs.
_F_, Egg-case of c.o.c.kroach. _G_, Egg-cases of Locust. _H_, _I_, _J_, Eggs of Gasteropod Molluscs. _H_, _Sycotypus_ (_Pyrula_). _J_, _Fusus_.]
TABLE SHOWING THE CLa.s.sIFICATION OF THE MOLLUSCA
{ AGLOSSA: the LAMELLIBRANCHIATA, also called { CONCHIFERA, and PELECYPODA.
=MOLLUSCA.= { { { GLOSSOPHORA { GASTEROPODA.
{ CEPHALOPODA.
CHAPTER X
THE BRACHIOPODA OR LAMP-Sh.e.l.lS
These were at one time included under the Mollusca, on account of their possession of a bivalve sh.e.l.l. This sh.e.l.l, however, is placed practically back and front of the animal, not to the right and left of it, as is the case with the sh.e.l.ls of the bivalve Mollusca.
The name, arm-footed, was given them in reference to a pair of special structures called the arms, bearing a large number of tentacles; it is now more frequently spoken of as the lophoph.o.r.e (see p. 122), and regarded as comparable to the lophoph.o.r.e of the Polyzoa, spread out into two portions. With the latter group the Brachiopods were formerly united by Huxley, under the name of Molluscoidea. This name is now obsolete, because it is understood that all these creatures are widely different from Molluscs; but the theory of relationship of the Brachiopoda to the Polyzoa, implied in it, still holds good.
The chief importance of this group lies in its fossil forms, which are exceedingly numerous, particularly in the Mountain Limestone of the Carboniferous Period; it is crowded with their sh.e.l.ls, especially a form named, from its elongated shape, Productus. The sh.e.l.ls of Brachiopods are equal-sided; that is to say, the right and left valves match; but they are inequivalve, the ventral valve being much the biggest. It often contains a foramen, or hole, at the beak, for the pa.s.sage of the pedicle, or stalk, by which the animal is attached to the ground (_e.g._ _Terebratula_, _Rhynchonella_). Sometimes, however, the pedicle pa.s.ses out between the valves (_Lingula_, _e.g._), in which case there is no foramen; or it may be arranged in other ways. Sometimes the sh.e.l.l is merely attached to the ground by its side, like an oyster. Some forms are enormously widened in a lateral direction, _e.g._ _Spirifera_, and the _Productus_ above named. _Lingula_, among others, is remarkable as being a form that has survived from the earliest geological period to the present day.
TABLE SHOWING THE CLa.s.sIFICATION OF THE BRACHIOPODA
{ TESTICARDINIS.--Sh.e.l.l Calcareous, with hinges.
{ Skeleton present in the arms.
=BRACHIOPODA.= { { ECARDINES.--Sh.e.l.l comparatively soft, composed { of Chitin, only strengthened by deposits { of lime, without hinges. No skeleton in { the arms.
The larva, in its best known forms, pa.s.ses through the typical larval stages of the animal kingdom. It is first a one-layered larva, then a two-layered form, and then becomes a ciliated animal. In this three regions may be distinguished, representing respectively the head, body, and pedicle.
The sh.e.l.ls of the Brachiopoda, including the kinds above named, may be seen by the reader in any geological museum.
CHAPTER XI
THE POLYZOA; MOSS-CORALS AND SEA-MATS
We have already described the creatures which are popularly known as Corallines. Modern zoologists have long separated off from the Corallines of the older writers, a group of animals known as the Sea-Mats, which also are colonies made up of unit individuals. The common Sea-Mat, _Fl.u.s.tra foliacea_, may be picked up on almost any part of the English coast, being often torn up "by the roots" and washed in by the tide. When fresh it has a pleasant scent, which has been compared to that of Lemon Verbena, and a pinkish colour, due to the presence of the little inhabitants in their cells. When dry it has no odour, the cells are empty, and the colour a pale drab like that of a dead Coralline. Its texture is, however, much more crisp and brittle, and less h.o.r.n.y, than that of a dead Coralline: it grows in flat, forked expansions, much resembling in outline the fronds of several common seaweeds; and each side of these is covered with a diamond pattern of little cells. This crowded arrangement of the cells, with a tendency to a.s.sume a geometrical pattern, is the readiest feature by which the beginner may distinguish a Sea-Mat from a Coralline. The latter arrange their cells in a free-growing, tree-like or fernlike form, without any crowding of the units into a geometrical pattern. The division of the flat leaf-like colony by two, resulting in bifurcated branches, is another obvious feature of the Sea-Mat.
Covering--and to the botanist"s eye disfiguring--the branches of many sea-weeds, and growing upon oyster-sh.e.l.ls, tangle-roots, and other fixed objects, we may find many little incrustations which remind us of the lichens of the land: the diamond pattern of little cells shows us, however, that these things are relations of the Sea-Mats. The name of Bryozoa, Moss-Corals, was formerly given to these growths. Many of them bear long hair-like processes at regular intervals; these, which are large enough to be plainly seen with the naked eye, afford a ready means of recognising these creatures.
TABLE SHOWING THE CLa.s.sIFICATION OF THE POLYZOA