Chapter V: Preface (5)

To the Greeks and Romans snakes were not such very terrible creatures, since the kinds found in South Europe are small and harmless—only the viper being poisonous—and they regarded the serpent as a beneficent creature, the familiar of Esculapius the god of medicine, companion of the household gods (the Lares), and guardian of sacred places, tombs, and concealed treasure (Fig. 27). The snake was the special earth-god, subterranean in habit, cunning, subtle, and gifted with powers of divination. The conception of the serpent as an avenging monster kept continually thrusting itself from the East into the popular mythology of the Greeks, and finally led to the building up of the dragon as a winged and clawed creature distinct from the harmless but cunning snake familiar to them. Even in India there arose a sort of double attitude towards the snake (as is not uncommon in regard to deities). On the one hand he was regarded as all that was terrible, destructive, and evil, and on the other as amiable, kindly, and wise. The services of the beautiful rat-snake in destroying house rats rendered him and his kind welcome and valued guests. In Egypt we find representations of small winged snakes without legs, and the ancient traveller, Herodotus, believed that they represented real creatures, as did the Roman naturalist, Pliny. Very probably the belief in winged snakes is due to the similarity of the snake and the eel in general form, since the paired fins of the eel close to the head (see Figs. 24 and 25) correspond in position with the wings shown in the Egyptian drawings of winged serpents. The particular form of winged snake pictured on Egyptian monuments (see Figs. 26, 27) appears to me to be a realisation of stories and fancies based on real experience of the locust. It was the terrible and destructive locust of which Herodotus tells—calling it “a winged serpent.” The Egyptian pictures of winged serpents have wings resembling those of an insect (see Figs. 26 and 27), and sometimes they are represented with one and sometimes with two pairs.

Aristotle says that, as a matter of common report in his time, there were winged serpents in Africa. Herodotus, on the contrary, says there were none except in Arabia, and he went across the Red Sea from the city of Bats in order to see them. He did not, however, succeed in doing so, though he says he saw their dead bodies and bones. He says that they hang about the trees in vast numbers, are of small size and varied colour, and that they are kept in check by the bird known as the Ibis, which on that account is held sacred, since they increase so rapidly that unless devoured they would render it impossible for man to maintain himself on the earth. They invade Egypt in swarms, flying across the Red Sea. All this agrees with my suggestion that the winged “serpents” heard of by Herodotus were really locusts; and the creature drawn in Fig. 27 may well be a locust transformed by fancy into a winged snake.

It would be a very interesting but a lengthy task to trace out the origin and history of the various traditional monsters, such as the basilisk, the gorgon, the cockatrice, the salamander, and the epimacus, which have come into European legend and belief, and to give some account of the special deadly qualities of each. St. Michael and St. George slaughtering each his dragon and rescuing a lovely maiden from its clutches are only appropriations by the new religion of the similar deeds ascribed to Greek heroes, such as Hercules, Bellerophon, and Perseus. Often a belief in the existence of a monster has arisen by a misunderstanding, on the part of a credulous people, of a drawing or carving showing a strange mixture of the leading characteristics of different animals, which was meant by the man who made it to be only symbolic of a combination of qualities. Just as the Latins and mediæval people credulously accepted Greek symbolic monsters as real, and transmuted Greek heroes into Christian saints, so were the Greeks themselves deluded by strange carvings and blood-curdling legends which reached them at various dates from mysterious Asia into a belief in the actual existence of a variety of fantastic monsters. “The Greeks,” says M. E. Pottier, a distinguished French writer on Greek mythology, “often copied Oriental representations without understanding them.” The conventional dragon probably came from Indian sources through Persia to China, on the one hand, spreading eastwards, and to the Latins of the early Roman Empire, on the other hand, spreading westwards; but at what date exactly it is difficult to make out.

In mediæval, as well as in earlier times, marvellous beasts were brought into imaginary existence by the somewhat unscrupulous enterprise of an artist in giving pictorial expression to the actual words by which some traveller described a strange beast seen by him in a foreign land. Thus the “unicorn,” which was really the rhinoceros, was seen by travellers in the earliest times, and was described as an animal like a horse, but with a single horn growing from its forehead. The heraldic draughtsman accordingly takes the spirally twisted narwhal’s tusk, brought from the northern seas by adventurous mariners (the narwhal being called “the unicorn fish”) as his unicorn’s horn, and plants it on the forehead of a horse, and says, “Behold! the unicorn.” Meanwhile the real “unicorn,” the rhinoceros, became properly known as navigation and Eastern travel extended, and true unicorns’ horns, the horns of the rhinoceros, richly carved and made into drinking cups, not at all like the narwhal’s tusk, were brought to Europe from India. One was sent to Charles II. by “the Great Sophy,” and handed over to the Royal Society by the King for experiment. These horns were asserted to be the most powerful antidote or destroyer of poison, and a test for the presence of poison in drink. There was no truth whatever in the assertion, as the Royal Society at once showed. Yet they were valued at enormous prices, and pieces were sold for their weight in gold. A German traveller in the time of Queen Elizabeth saw one which was kept among the Queen’s jewels at Windsor, and was valued, according to this writer, at £10,000.

Credulity, fancy, and hasty judgment are accountable for the belief in mythical and legendary monsters. Yet they have great interest for the scientific study of the growth of human thought and of the relationships of the races of mankind. They are often presented to us in beautiful stories, carvings, or pictures, having a childlike sincerity and a concealed symbolism which give to the wondrous creatures charm and human value.

XIV

OYSTERS

Oysters are delicate morsels—still appreciated by that class of the population which nevertheless shudders at the thought of eating the high-flavoured “whilk” or the gristly “periwinkle,” and neglects the admirable mussel, so rightly valued by our French friends. There are a number of interesting facts about the nature and life-history of oysters, and the different kinds of them—a knowledge of which does not diminish, but, on the contrary, rather adds to the pleasure with which one swallows the shell-fish. I remember the time when “natives” were sold in London at sixpence the score. When I was a schoolboy at St. Paul’s they were no more than sixpence a dozen at the best shops in Cheapside. That inevitable form of British enterprise which is known as “monopoly,” many years since laid hold of the oyster business, and rapidly raised the price of the best natives to eight times what it had been, while the typhoid “scare” came subsequently as a sort of poetical justice, and threatened to ruin the oyster monopolists. As a matter of fact, there is no difficulty in freeing oysters from any possible contamination by the typhoid germ. They have only to be kept for ten days or a fortnight in large tanks of sea-water of unquestionable purity—after removal from the fattening grounds (tanks or waterways), and they rid themselves of any possible infection. It is the interest of the oyster merchant to make sure that this treatment is strictly enforced. It is a noteworthy fact that the anciently established habit of drenching an oyster with vinegar before eating it is precisely the best treatment, except cooking them, which could have been adopted in order to destroy the vitality of typhoid germs—although the existence of such germs was unknown when the practice arose, and vinegar or lemon-juice was taken with uncooked oysters as a matter of taste, not as a safeguard.

The oyster is sometimes grandiloquently styled “the succulent mollusc”—and it is classed together with other bivalve shells and true “shell-bearing” shell-fish, such as whelks and snails (not lobsters and crabs, which are Crustacea), in a great division of animals known to naturalists as the Mollusca. This word is only a Latin form of the name _Malakia_, which was given to the cuttle-fishes by that wonderful man Aristotle, the Greek—and means “soft creatures.” A bivalve, or two-shelled mollusc, like the oyster, may be compared to an oblong notebook. The hard covers correspond to the two shells and the back to a horny piece by which the two shells are united, forming the hinge. If you place a piece of indiarubber (a thickish bit) between the covers of the notebook so that it lies near the back, and then try to shut the book, you find that it requires some pressure to do so; when you leave off pressing them the covers gape. The horny hinge-piece or ligament of the shells of the oyster and other bivalves acts in this way. The shells are only kept closed by a strong muscle, which runs across from shell to shell (Figs. 28 and 30_m_). When the oyster is at rest or when it is dead the muscle does not act, and the elastic hinge-piece or ligament causes the shells to gape. The animal within the shells may be compared to the leaves of the notebook. Suppose there are twenty-six leaves, then the outermost leaf on each side corresponds to the two soft living membraneous flaps which secrete the two shells or covers of the oyster and lie closely on them (_a_, _b_, Figs. 28 and 30); the next two on each side (rather shortened leaves, folded in from below) are the flat gills or “gill-plates” of the mollusc (_g^1_ to _g^4_ in Fig. 28); whilst we must suppose the twenty middle leaves to be “pulped” and fused together to represent the body of the shell-fish.

The oyster’s gill-plates, commonly called “the beard,” are covered on the surface by microscopic hairs of a very remarkable kind (Fig. 29). They are soft, living protoplasm, and are continually “lashing,” bending forwards and straightening again at the rate of some three or four hundred strokes to the minute. They all work rhythmically together, and produce a strong current in the water, which bathes the surface of the oyster when the shells are open. Such microscopic vibrating hairs are very common in aquatic animals, and are called “cilia.” The current which they produce causes oxygen-holding water to flow from without over the gills, and so aerate the blood of the oyster, and also carries into the chamber protected by the shells excessively minute particles, chiefly microscopic plants, which are driven on to the small, open mouth of the oyster, placed far up on its body. These microscopic food-particles are wafted down the oyster’s throat by similar vibrating hairs into the stomach and intestine. An oyster has no other means of taking food, and almost without cessation, as the oyster lies on the sea bottom with its muscle relaxed and its shell “gaping,” the nourishing stream is kept going. If poisonous matter, bad water, or some violent disturbance make themselves apparent, the shell-muscle acts, and the oyster tightly closes his shell. Such things make themselves “apparent” to the oyster, for it has a nervous system, and though it has no eyes (the nearly allied “scallop” has a number of eyes) it has a delicate sense of smell and touch, and also what is usually considered to be an organ of hearing.

The oyster has also a heart and blood-vessels (Fig. 30) and blood; in some few bivalves and snails the blood is red like our own. The beating of the heart may be seen by careful examination of a freshly opened specimen. The oyster has also a “liver,” or digestive gland, and a kidney and a soft, branched, tubular structure embedded in the body, within which the egg-cells and sperm-cells grow by means of which the oyster propagates itself in the summer. Our north European oyster produces in the same individual both egg-cells, and the male fertilising sperm-cells or spermatozoa. The eggs are just visible to the unaided eye (Fig. 31), and as many as a million are produced in the warm breeding season by a single ripe oyster. About a fortnight after the eggs have been shed, the same tubular chambers in the oyster’s body which produced the eggs by growth from their inner walls, produce the spermatozoa, so that they are too late to fertilise the eggs of the same oyster. They pass out of the oyster into the sea water, and are carried within the shelter of the shells, and so on to the surface of the protected bodies of other neighbouring oysters by the currents created by the “ciliated” gill-plates of these neighbours.

The sperm particles or spermatozoa (Fig. 32) are produced by millions, and form a cloud finer than dust in the sea water. They are carried within the shells of both egg-producing and sperm-producing oysters, and are driven along into the openings of the tubular reproductive sacs, and into those sacs in the case of those oysters which are at the time producing eggs. There they fertilise the eggs. The minute eggs begin to develop whilst still within the parent’s body, and continue to do so whilst remaining within the shelter of the shell, adhering to the gill-plates (Fig. 33). In a day or two each fertilised egg has developed into a very minute creature, provided with a tiny circlet of cilia or vibratile hairs, the movements of which cause it to swim (Fig. 33F). The parent oyster is now said to be “white-sick.” In the course of a couple of days the young oyster still within its parent’s shell becomes dark in colour, and has formed on its surface a pair of symmetrical shells, not like those of an adult oyster, but convex (Fig. 34) like those of a clam or a cockle. The head region, with its circlet of vibrating cilia, can be projected between the open shells or withdrawn between them when the shells are shut. The mother oyster, laden with these little dark specks, is now said to be “black-sick.”

In the course of a week or so the brood of dark young oysters escapes by thousands from the parent’s shell into the surrounding water. They swim by their circlet of vibrating hairs, or “velum,” as it is called, towards the surface, and are carried far and wide by the tides. They are active, transparent little “dots,” very unlike their parent (Fig. 34). The next thing that happens to them—after a few days, perhaps weeks—is that owing to the increasing weight of their shells, they sink to the bottom. More than half perish by dropping thus on to bad ground; a vast number have already been eaten by young fishes and shrimps. Those which are lucky enough to fall on to something hard—stones, rocks, old oyster-shells, or the shells of living oysters—become cemented to those hard substances by the new shelly substance formed by the growing edge of the lowermost of their little shells, which now spread out, lose their cockle-like shape as they grow, and become, the one (the left by which it is fixed) large, deep, and bossed, the other flat. The little oysters are only one-fortieth of an inch in diameter when first they become fixed, but they grow rapidly, feeding in the same way as their parents. Vast numbers are eaten by other animals. In some localities in two years, in others in three years, they have grown to a couple of inches in length, and now produce in the summer breeding season a certain quantity of eggs and sperm to start new generations. The oyster continues to grow, and at five to seven years of age is in full vigour and maturity; at ten years of age it produces few eggs, or sperm-cells; and in the course of another year or so, under natural conditions, dies.

Enormous as is the output of young by a single oyster—amounting to something like a million a year in probably four or five successive years—yet it must be remembered that on the whole, taking all the various oyster-beds into account, some of which increase whilst others dwindle or actually die out altogether, there is no increase in the oyster population of the seas. Taking them all round, five million young oysters start life in order that one may finally come to maturity, so many and varied and incessant are the dangers, the predatory enemies, the destructive effects of cold currents, bad ground, and other chances of life and death on to which the swarming swimming young are launched.

The above brief history applies to the North Sea or Channel oyster, which is also found (with other species) in the Mediterranean. The American and the Portuguese oyster differ from it in being of distinct sexes, and in the fact that the eggs are discharged into the sea by the females, and are there fertilised by the sperm discharged by the male oysters, instead of in the parent’s body.

Other “molluscs,” such as snails and whelks, enclose their fertilised eggs, when they lay them, in egg-shells. Some snails enclose a single egg in a shell which is filled up with clear liquid—corresponding to the “white” of a bird’s egg—in which the egg floats and develops. The eggs of the common snail are not bigger than a hemp-seed, but some Indian snails lay eggs as big as those of a robin, with a hard, calcareous shell, and the young snail has quite a large coiled shell of its own before it escapes from the egg-shell. So that it looks, when one of the big snail’s eggs is broken, as though a snail had managed to get inside a bird’s egg without making a hole in it! The whelks and their kind lay many eggs in one shell or capsule, and the sea-slugs produce a sort of firm jelly, in cords like vermicelli, the jelly enclosing hundreds of little sacs filled with liquid, in which the true germs or fertilised egg-cells float. These are all methods for protecting the young in their earliest condition. One of our pond-snails—the _Paludina_—keeps her eggs, whilst they develop, inside the dilated end of the tube which leads from the egg-producing organ or ovary to the exterior. The young snails nearest the opening to the exterior are the furthest advanced in development, and are as big as a dried pea. All stages, from the minute germ just fertilised to well-formed young, may be found in these snails, and the whole course of their development and gradual change and growth can be minutely studied with the microscope in one specimen.

Similar devices for protecting the young in their earliest helpless stages of growth from the egg-shell are found in all classes of animals. What is very curious is the fact that, of two closely allied animals, one species will recklessly lay its eggs and leave them, whilst another has special arrangements for retaining in the parent’s body the eggs as they develop, and so preserving them from danger. Such parents are called “viviparous.” Of course, in all viviparous animals, as well as in those which lay their eggs in hard shells, the fertilisation of the egg must be effected within the maternal body. Amongst our common fishes there is the viviparous blenny, often found in pools at low tide on the seashore. All the other British fresh-water and marine fishes lay their eggs and abandon them, excepting some sharks, dog-fish, and skates, which are viviparous; others of the shark and skate tribe lay eggs of large size encased in hard, horny shells. Every one knows that frogs and toads lay their eggs, but there are some kinds in which the eggs remain inside the mother’s body during the development of the young, which only escape into the world as well-formed little frogs. All the hairy, warm-blooded quadrupeds known as “the mammals” are viviparous, except the duck-mole and the spiny ant-eater of Australia. These extraordinary little “beasts” lay eggs like those of a bird.

The most ingenious devices for the protection of the young are (as perhaps those who believe in the superior intelligence of the male would expect) put into practice by the male parent. Thus, there is a large fish in tropical rivers which takes the eggs laid by the female into his capacious mouth, and swims about with them for three or four weeks, giving them the advantage of a current of water which runs through his mouth to his gills. When the young hatch they swim out of their fond father’s mouth. The male of pipe-fishes and of the little “sea-horse” receives the eggs laid by the female into a pouch excavated along his ventral surface. There the young hatch, and are guarded by the nursing father. On the other hand, some fathers impartially eat their own young, as well as those of other parents, and the mother has a hard job to protect her offspring. A female octopus (the poulp or eight-armed cuttle-fish) sits over her eggs in a nest built of pebbles at the bottom of the sea (or of an aquarium tank in the instance studied by me many years ago at Naples), and squirts a stream of pure sea-water over them. She resents the approach of a fish or a crab or a landing-net with splendid fury and recklessness of attack. Often the males of fishes, frogs, and birds guard the eggs, or guard the nest where the female is occupied in caring for the eggs or the young.

There are various species of oysters common in all parts of the world which are eaten as delicacies. Primeval (Neolithic) man ate oysters (the common sort) in Denmark in enormous quantity—great heaps of the discarded oyster-shells are found, buried among which are discovered stone axe-heads and bits of rude pottery. In the West Indies travellers relate that the oysters “climb” the trees which overhang the water of quiet creeks and inlets of the sea. The fact is that the branches of the mangrove trees dip into the water, and the young oyster “spat” attaches itself to the immersed twigs. After a year or two, the tree grows vigorously, and raises its branches up in its growth, so that the oysters are carried far up above the sea waves. Of course they die under these conditions, but their position suggests the explanation that the oysters have climbed up the trees. Ship barnacles fix themselves, similarly, to the twigs of willow trees in the quiet sea lochs of the West of Scotland, and this led 500 years ago to the belief that the catkins of the willow tree ripen into barnacles. Since it was also held that the little animal of the barnacle hatches out of its shell as a young goose—the so-called “barnacle goose”—the marvellous story was believed that these geese are actually budded from willow trees. I believe that the supposed relationship of the goose and the ship’s barnacle arose solely from the accidental similarity of the names of the two animals—the “bernack” goose and the sea “barnacle” being names of independent origin. The names were different originally in sound and signification, but were corrupted by fisher-folk into one and the same word. Hence a fantastic fable took its growth.

In Paris you may test and compare several local varieties of the common oyster in a celebrated oyster-shop. There are Courseilles, Cancales, Marennes, Ostend, Zeeland, Arcachon, English natives, Côtes Rouges (red banks), and Black Rocks. And you can eat sea-urchins there, too, if you wish. They have not, however, got the celebrated oysters from the Lake Fusaro, near Naples. This was the ancient _Acherusia palus_, and in the neighbouring Lake Avernus and the Lucrine lake oysters were cultivated by the ancient Romans, the young oysters being made to affix themselves at “the fall of the spat” to wooden “stands” or frames, which were then placed in the lake (a salt-water lake), where they had abundant minute vegetable food and grew large and fat. The same cultivation, with the same shape of “stands,” is carried on at the present day in the Lake Fusaro. My friend, Mr. Günther, of Magdalen College, Oxford, has published pictures of Roman tiles from this neighbourhood showing the oysters adhering in rows to the wooden frames. These tiles were apparently sold to holiday visitors in the time of the Roman emperors as a memento of a happy day spent at the Lucrine lake, just as a sugar basin or a mug is now sold at our seaside resorts with the inscription, “A present from Margate,” or Southport, or Blackpool, and the picture of a shrimp above it.

The care of the breeding oyster and the plans adopted by the owners of oyster-beds for catching the “spat,” or young oysters, when they fall to the bottom, by placing movable tiles or frames for them to fix themselves to, form an important part of the craft of the oyster-man. It is a difficult business, and is variously carried out in England, France, Holland, and America. The young oysters, when they have fixed themselves, are carried on the movable tiles or frames from one region to another for the purpose of encouraging their growth and avoiding a variety of dangers to their life and health (sometimes from the Bay of Biscay to the mouth of the Thames!). They are often—but not always—finally fed up in sea-ponds or inlets, which are peculiar in containing an enormous number of those very minute microscopic plants, with beautifully shaped siliceous shells, which are known as diatoms. These are so abundant in such ponds as to form a sort of powder or cloud near the bottom, and the oysters draw them, day and night, by their gill-currents into their mouths, digest them, and grow fine and fat. The district of Marennes, on the west coast of France, is celebrated for having sea-ponds or tanks in which a wonderful diatom of a bright blue colour abounds; so abundant are they that the cloud produced by them in the pools is of a deep cobalt-blue. When oysters are placed in these tanks to fatten, their gills or beards become rich blue-green in colour. They lose the colour after ten days, when removed to ordinary tanks. These are the celebrated green oysters or “Marennes vertes” of French restaurants. The colouring matter of the little diatoms—swallowed by the million and digested—is taken up by the blood of the oyster from its stomach, and is excreted by certain corpuscles on the surface of the gills—as I showed some twenty-five years ago—just as red madder is deposited in the bones of a pig fed upon madder, and as the feathers of the canary take up the colour of cayenne pepper when it is mixed with the canary’s food. It used to be thought that the green colour of the green oyster is due to copper—and that opinion was supported by the curious fact that the blood of all oysters and other molluscs, and also of lobsters, scorpions, and king-crabs, does really contain a minute quantity of copper, just as our blood contains iron! It was also supported by the fact that occasionally a fraudulent fishmonger, when asked to supply green oysters, has been convicted of colouring the beards of ordinary oysters with green copper salt, so as to imitate the real article! The real history of the green-bearded oysters is now quite certain, and any one interested in the matter should look at the coloured pictures of the beautiful little blue-coloured _Navicula ostrearia_—the diatom on which this oyster feeds, published by me in the _Quarterly Journal of Microscopical Science_ in 1885.

XV

MATERNAL CARE AND MOLLUSCS

The American and Portuguese species of oysters, which are called respectively _Ostrea virginiana_ and _Ostrea angulata_, as opposed to the common oyster, which is known as _Ostrea edulis_, are not hermaphrodite like the latter, but have distinct males and females. Moreover, the young are not fertilised within the parent’s body, nor do they pass their earliest stages of growth within the parent’s shell adhering to the “beard,” or gills, as in the common oyster. The eggs (Fig. 31) are, on the contrary, discharged by the females into the sea, and at the same time the males discharge a cloud of microscopic sperm filaments, or spermatozoa (Fig. 32), which dart about in the water and fertilise the eggs. That is a more prodigal and less certain process than that pursued by the common oyster. The American and Portuguese oyster have to pay for it. The female produces in one season not a million eggs, as does the common oyster, but nine millions. And out of every fifty million so produced (in some five or six years) only a single male and a single female individual, taking the whole oyster population of these species into consideration, survive to maturity.

This enormous excess of egg-production in order to ensure the survival of a single pair to replace their parents is a very frequent thing in aquatic animals. But there are many devices by which the necessity for such lavish scattering of a new brood is avoided. The common oyster is already a step in advance of the American in this matter, since it protects its young in the very earliest stages within the shelter of its shell. A further advance in this direction is found in the fresh-water mussels (not to be confused with the very different sea-mussels, since they are bitter and tough, and quite inedible, though used as bait in sea-fishing). The pond-mussel (_Anodon_) and the river-mussel (_Unio_) are of distinct sexes, and the gills of the female become swollen up at the breeding season so as to form two large bags, into which the eggs are laid by her, as many as 500,000 in number. They are fertilised by the sperm filaments discharged by the males, which are carried into the female’s shell by currents produced by the vibrating hairs on the gills, as in the common oyster. But the young remain much longer in the mussel’s gills than do the young oysters in those of their parent’s. Late in the season you find the bag-like gills of the female pond- and river-mussels full of extraordinary little creatures one-thirtieth of an inch long, each provided with a pair of triangular shells. They are discharged into the water, and swim very actively by rapidly opening and shutting the little shells (Fig. 35). The common scallop (_Pecten_, or Pilgrim’s shell) swims every now and then in the same way as do these young mussels, and so do some other bivalves. The young fresh-water mussels produce a long, sticky thread, which trails from the shell (Fig. 35 _by_). Very few have the good chance to get further on in life than this stage, for all depends on their stumbling across fish—a stickleback, or a perch, or a pike—as they blindly snap their shells and wobble through the water. The lucky triangular mite whose sticky thread happens to touch a fish’s body becomes immediately fastened by it to the fish and then grips the skin with its snapping shells, the edges of which are provided with a few long, sharp teeth. The fish probably is quite unaware of the lodgment of the young mussel on its skin, but there it remains, and gets buried for a time in the soft tissues of the fish, becoming thus actually a parasite for some two or three months during the winter season. It nourishes itself on the juices of the fish, and grows to the size of a pin’s head, whilst it is carried away from its birthplace by the peregrinations of its host, the fish. Its shell now ceases to be triangular, and becomes like that of its parents. Eventually the young mussel drops off the fish and rests on the muddy bottom of pond or river, where it remains for many years, growing vastly in size, and barely moving during its long life from the spot where it fell.

A beautiful little bivalve common in weedy streams in England is known as _Cyclas_ (it has no English name); it has a pair of shells shaped like those of a cockle, but smooth, and only half as big as one’s little finger-nail. The nursing of the young in the gill-sacs is carried to a much further point by _Cyclas_ than by the pond- or river-mussel. Before they are ejected by the parent they are quite large—like their parent in appearance, and half as big as a hemp-seed. Necessarily there are not many produced in a season—there is not room for more than twenty or thirty young in the gill-sacs.

XVI

THE HEART’S BEAT

The beat of the heart is one of those great and elemental features of man’s life which, in spite of our familiarity with it and its momentary recurrence, never loses its quality of mystery and isolation. The ceaseless accompaniment to our lives which the heart is always beating, like the inexorable stroke of an unseen pendulum, fills even the stoutest and bravest at times with a sense of awe. It seems now and then as though an independent living thing were in our breasts, and when it quickens and struggles, as it were, with its work, or languishes and hesitates in its efforts we have a sense of helpless domination by an existence—a living thing—over whose vagaries we have no control.

The heart of man is no special endowment of the human race, nor even of the higher animals. As I mentioned a few pages back, the oyster and other shell-fish have a heart which keeps time and beats the seconds for their uneventful lives, as does that of man for his more varied career. Not only the molluscs, but the insects, the spiders, the crabs, lobsters, and shrimps, and even the worms, have each a rhythmically beating heart. In all of them the significance of this heart and its beat are the same—it is driving the nourishing, oxygen-carrying blood through the great vessels (arteries), which branch from it like a tree into the living tissues of the body, whence it returns by other vessels (the veins) back to the heart.

In man and the warm-blooded quadrupeds, in birds, reptiles, and fishes, the blood is of a splendid red colour, and the transparent vessels can be easily traced in their graceful ramifications and intricate networks, in consequence of the red blood showing through their walls. The red colour is due to a peculiar body, which can be easily separated from the blood as crystals. It has the special duty of carrying oxygen gas dissolved and attached to it; and of giving up that essential element to cause slow burning or oxydation in all parts of the body whilst taking up fresh supplies of oxygen on its passage through the lungs or the gills. In many of the lower animals (for instance, the oyster) the blood is devoid of this red crystalline substance (which, by the bye, is called hæmoglobin), and accordingly we cannot easily catch sight either of the heart or the blood-vessels (see, however, Fig. 30). But in shell-fish the blood has a very pale blue tint, and this colour is due to a substance like hæmoglobin, which also can be crystallised, and is the oxygen-carrier. Some sea-worms have a green substance of a similar nature dissolved in their blood, and one can trace their blood-vessels as a beautiful green network. A good many worms, for instance the common earth-worm and the leeches (a discovery made by Cuvier, and referred to by him on his deathbed), and many sea-worms have deep-red-coloured blood, due to the presence of the same crystalline substance which we find in man’s blood. And even a snail, common in the ponds at Hampstead and such places—the flat coiled snail known as _Planorbis_—has blood of a fine crimson colour, due to the presence of the same red oxygen-carrier, as an exception to the colourless or pale-blue blood found in most shell-fish. Perhaps if oysters, too, had red blood, there would be a prejudice against eating them in the uncooked condition.

The heart is essentially an enlargement of the great stem or main blood-vessel which, like the trunk of a tree, has branching roots at one end of it and ordinary branches at the other. The trunk branches, and roots of the “heart-tree” are, of course, hollow blood-holding tubes, not solid fibrous structures, as are the woody branches and trunk of a vegetable tree. Further, the finest rootlets and the finest terminal branches in the case of the heart-tree are connected to one another by the network of very fine branches or by great blood-holding cavities, which occupy all parts of the body of an animal. The enlarged part of the trunk of the tree-like system of blood-vessels—the heart—has powerful muscles forming its walls, the fibres disposed so as to surround the contained chamber. When these muscular fibres contract, they squeeze the walls of the chamber together and drive the blood out of it into the forward branches, called “arteries.” It is prevented from going backwards into the hinder branches called “veins” (which we compare to the roots of a tree) by flaps which are so set on the inside of the great vessel at the entrance to those branches that the flaps are made to move out across the space by the backward current, and thus prevent any backward flow, whilst a forward current merely presses them flat against the wall of the vessel, and thus no obstruction to a forward flow is presented. These flaps are called the valves of the heart. The consequence of this arrangement is that whilst blood flows freely into the heart from the veins or hinder (root-like) set of vessels, it is driven by the muscular contraction of the heart—only in one direction—namely, forwards into the arteries. This movement in one direction is helped in some elongated hearts by the contraction of the wall of the heart beginning behind and spreading quickly forward like a wave. The heart of the common earth-worm and of small transparent worms with red blood like it, which are common in the mud of ponds and rivers and can be easily watched with the microscope so that one can see through their glass-like skin what is going on inside them, shows very beautifully this wave of contraction. The heart in these worms is a long contractile vessel which runs the whole length of the body along the back. You can watch the red blood flowing into it through the veins in each ring or segment of the worm’s body—slowly swelling it out—so that it looks like a long red cord. Then, suddenly, there is a movement like a flash in its rapidity, passing from behind forwards! The walls of the red cord-like heart contract so as to drive the blood forward into the arteries, which also are present in every ring of the worm’s body. At the same time you can see the valves, which hang at the entrance of the veins to the heart, swing with a sudden “chuck” and close those vessels against the driven blood. The red cord becomes colourless progressively from behind forwards, owing to the squeezing out of the blood, and by the time the movement has reached the head of the worm, the hinder part of the cord-like heart is beginning slowly to dilate again with the influx of red blood from the veins.

What causes the muscles of the heart to contract at regular intervals? There is no doubt that the “stimulus” which excites the heart muscles to contraction is in these simpler animals merely the tension or strain produced by the presence of a sufficient quantity of blood which has flowed into the heart from the veins. The heart muscle, after its rapid contraction, rests; it has no other rest, no sleep, as have all the other parts of the body. It must rest and take refreshment after each effort. Whilst it rests the blood quietly flows in and dilates the heart’s cavity; then the rested muscular wall of the heart, gently stretched by the recovery after compression of its elastic components, nourished and oxygenated by the blood, is ready for another “stroke,” and again it contracts tightly, emptying its cavity of blood, which is driven into the arteries. So it goes on—effort and rest, effort and rest alternating without cease. Whilst it is the stroke of the heart which causes the blood to flow through the arteries into the finest network of hair-like vessels, what is it that causes the blood to flow on through the collecting veins, to reach the heart, and actually to distend that collapsed cavity after its stroke? It must be remembered that a very low pressure is enough to effect this. In the simplest arrangements of worms and such-like animals, there is probably some pressure transmitted to the blood in the veins by the heart-stroke; but the elasticity of the heart-wall and its necessary tendency to resume its dilated condition after its squeezing by its rings of muscle, is what is chiefly effective in drawing on the blood in the veins into the heart.

In man and the higher animals the whole mechanism of the heart is greatly complicated by the action of the nervous system upon it and upon the contraction or expansion of the blood vessels. In this way the rate of the beat of the heart is affected and brought into relation with the needs of the blood circulation in remote parts of the body. The beat of the heart in the human species is more rapid in children than in adults, and more rapid in women than in men, and it differs in all individuals under differing conditions. Before birth it is 140 per minute, in the first month after birth 130, and gradually diminishes to 90 at nine years of age, and at twenty-one to 70 in man and to 80 in woman. But these figures only represent a general average; there are healthy men whose pulse usually is less than 45 per minute, and there are individuals who, without being invalids, yet have the movement of the heart so liable to increase in rapidity through mental or other excitement, acting by nerves directly on the heart muscle, that the pulse often goes up to 120. In the horse and the ox the pulse or heart beat is 36 to 40 a minute; in the sheep 60 to 80; in the dog 100 to 120; in the rabbit 150; and in small creatures, like mice and moles, 200, and even more! I do not know what is the record for the elephant, but as it seems that the larger the mammal the slower the pulse, one would not expect more than 20 to 25 beats a minute in his case.

It is easy to watch the beating of the heart of a flea or other small insects—under the microscope—since the skin is sufficiently transparent. It is not usually much more rapid than in man, but in the very transparent little fresh-water shrimps which are called water-fleas (_Entomostraca_) I have seen the heart beating so rapidly that I could not count its rate. The heart in insects and shrimps and their like is remarkable for the fact that whilst it pumps out blood through arteries both in front and behind, it has no actual veins opening into it. All the veins, which in their ancestors entered the heart in a row on each side of it, have united, and their walls broken down, so that the heart lies in a sac full of venous blood from which it draws its fill, when it dilates, through a series of valve-bearing openings on its surface, openings which, in an earlier stage of development, were connected with individual veins.

The heart of the _Ascidians_ or sea-squirts, common sac-like marine creatures of most varied form, size, and colour, is perhaps the most extraordinary in the whole animal series. I have often watched it in transparent individuals of this group. It is an oblong sac with branching vessels at either end. It beats for some thirty or forty strokes so as to drive the blood forwards; it then pauses, and the onlooker is astounded to see the wave of movement changed, and the heart steadily beating the same number of strokes in the reversed direction. What were arteries become veins, and the veins become arteries. Then again there is a pause—which seems like a moment of hesitation and doubt—and the original direction of movement is resumed; then again there is a pause and a reversal, and so on, with absolute regularity. It is still a matter for investigation as to why and how this altogether exceptional alternating reversal of the heart’s action is brought about.

It is a curious fact in illustration of the essential character of the heart and its beat that “hearts” are produced in some animals by dilatation of the lymph-vessels—a system of delicate vessels, difficult to see, which take up the colourless fluid which the blood-vessels exude into the tissues and return it to the heart. The eel has a pair of these “lymph-hearts” in its tail, and the common frog has a pair near the shoulder-blades and another pair at the hips. These sacs have muscular walls, and pulsate rhythmically like the blood-heart, driving on the lymph fluid through the lymph vessels to join the blood-stream.

The simplest thing in the animal world which can claim the name of a heart—or, at any rate, be compared with that organ—is found in those microscopic animalcules which consist of only a single “cell” or corpuscle of living protoplasm. These animalcules may be compared to a single brick or unit of structure, whereas all other animals consist of thousands, or even millions, of such corpuscles or units aggregated and fitted together as are the bricks and planks of a house. In most of these uni-cellular animalcules you may observe with a high-power microscope a little spherical liquid-holding cavity, which slowly enlarges, then bursts at the surface and collapses. After a brief interval it forms again, and again bursts to the exterior. In the “bell-animalcule”—a beautiful active little creature only one-thousandth of an inch in diameter—it may be seen to form, swell, collapse, and re-form as often as twenty times in a minute (see Fig. 41). Soluble colouring matter taken in by the animalcule with food is excreted by the liquid accumulated in and ejected to the exterior by this spherical chamber. It is called the “pulsating” or “contractile” vacuole, and by its rhythmical pulsating movement of dilatation and collapse presents definite points of similarity to the alternately dilating and contracting hearts of higher animals. The entering flow of liquid here, as in the veins and heart of higher animals, is continuous. The rhythm is due, as is the rhythm of the heart, to the alternation of a brief period of activity or contraction, and a brief period of consequent exhaustion, rest, and repair on the part of living contractile substance.

XVII

SLEEP

An enterprising journalist has recently published the replies of a number of well-known men to an inquiry as to how many hours’ sleep they are in the habit of taking, and what they find to be the best remedy for sleeplessness. Such an inquiry naturally leads on to further thoughts about “Sleep.” What a mysterious, yet sweet and lovable thing it is! How strange it is that we all regularly and gladly abandon ourselves to it! How terrible is the state of those who cannot do so! And then one is led to ask, what is it? and why is it? Do all living things sleep for some part of the twenty-four hours? How does it differ from mere resting, and in what does its virtue consist?

Shakespeare has said the most beautiful words that have ever been uttered about sleep, and that because he knew what it was to seek for it in vain—

“Methought I heard a voice cry, ‘Sleep no more!
Macbeth does murder sleep,’ the innocent sleep;
Sleep, that knits up the ravell’d sleave of care,
The death of each day’s life, sore labour’s bath,
Balm of hurt minds, great Nature’s second course,
Chief nourisher in life’s feast.”

And again, when the strenuous life of the great Bolingbroke has at last overtaxed his brain, and he can no more find rest and unconsciousness at night, Shakespeare makes him say—

“How many thousand of my poorest subjects
Are at this hour asleep! O sleep, O gentle sleep,
Nature’s soft nurse, how have I frighted thee,
That thou no more wilt weigh my eyelids down,
And steep my senses in forgetfulness?
Why rather, sleep, liest thou in smoky cribs,
Upon uneasy pallets stretching thee,
And hush’d with buzzing night-flies to thy slumber,
Than in the perfum’d chambers of the great,
Under the canopies of costly state,
And lull’d with sound of sweetest melody?”

Poets have as a rule been too ready to make much of the likeness of sleep and death, whereas there is an absolute difference in their mere appearance. Sleep makes even those who are ill-favoured and coarse look beautiful, imparts to its subjects a graciousness of expression and of colour, and a gentle rhythmic movement, whilst suffusing them as it were with an “aura” of contented trustfulness. These things are far from the cold stillness of pallid death. And this depends upon the fact that in sleep, though many of the activities of the body and mind are checked, and even arrested, there are yet still present the never-ceasing pulse of the heart, the flow of the blood, the intake and output of the breath, and a certain subdued but still active tension of muscles, so that though the body and limbs are relaxed they never assume the aspect of complete mechanical collapse which we see in death. The pupils of the eyes are strongly contracted during sleep, not relaxed and expanded as are those of wide-awake people in the dark. There are some well-known works of art—both painting and sculpture—in which the dead are not truly represented, but are made to retain the resistfulness and pose of living men and women; others show true observation in presenting the startling and distinctive flaccidity of the newly dead, which is followed after a few hours by the equally characteristic rigor mortis, or stiffness of the dead. There are many fine studies of sleep by sculptors, but none which to my thinking so delicately and truthfully present its most beautiful and peculiar effects on the muscular “tone” as a work in the Luxembourg Gallery in Paris, called “Le Nid”—a baby of a year old and a little girl of three or four years, asleep side by side on the cushion of a capacious arm-chair. The pose and the details of muscular relaxation differ greatly and characteristically in the two children. One would like to see sleep at different ages and under various conditions of fatigue similarly portrayed, for there is a range and variety of expression in those who sleep, not perhaps as extensive, but as beautiful as that to be found in those who are awake.