Popular Science Monthly/Volume 33/September 1888/The Growth of Jelly-Fishes I

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THE

POPULAR SCIENCE

MONTHLY.

 

SEPTEMBER, 1888.


 

THE GROWTH OF JELLY-FISHES.

A CHAPTER IN THE NEW ZOÖLOGY.

By Professor W. K. BROOKS,

OF JOHNS HOPKINS UNIVERSITY.

I.

ON any landlocked and sheltered sea-beach, where the waves ripple up on to the sand without breaking, hundreds of small spiral sea-shells may usually be found in the shallows dancing up and down the sand at the water's edge, following the crest of each little wave as it flows up and spreads out over the beach, and turning to run back with it as it falls; keeping always just within the water, and exhibiting restless activity and agility, quite unlike the sluggish habit of the snails which normally inhabit the shells.

If the loiterer by the waves should be inquisitive enough to be attracted by them, and should search for the meaning of the unusual liveliness of the snails, he would find that each shell is inhabited by a hermit-crab, that, after devouring the true owner of the house, has thrust his own body into it, and carries it about, as a defense against his many enemies, among whom his pugnacious and cannibal brothers and sisters are perhaps the worst.

So much the most superficial observer may discover for him-self; but if, with a naturalist's sharp sight and thirst for knowledge, he examine more closely, he will find that about one in a dozen of the shells is coated, upon the surface which is uppermost as the crab carries it, by a white crust of a mossy substance which is not found upon the empty shells which lie on the bottom, nor upon the shells of living snails. If, impressed by this odd fact, he detach a little of the moss and examine it under a microscope, in a watch-glass filled with sea-water, he will find that it is a most remarkable and interesting community of minute animals, a polymorphic hydroid colony, well worth most exhaustive investigation. Several species and genera of hydroids are found upon the shells of hermit-crabs, but they are usually pretty much alike in general organization; and our Fig. 1, which is a highly magnified drawing of a small portion of a colony of the hydroid larvæ of Dysmorphosa, will serve to represent their character.

The crust on the surface of the shell consists of a network of tubes, cemented to the shell and to each other in a mass from

PSM V33 D596 Colony hydroid larvae of dysmorphosa magnified.jpg
Fig. 1.

which the bodies of the individual hydras protrude, somewhat as the stem of a tree rises erect from the creeping root, although the community is more like a thicket of suckers than a single tree, for all its members spring from one system of roots, and, although they may be numbered by hundreds or even thousands, form one continuous organism.

The stomach of each member of the colony is directly continuous with the hollow roots, and, through these, with the body of every other member, and any food which is captured and digested by one, serves to nourish all, since it circulates everywhere through the roots, as water flows through the mains to all the houses in a city.

The whole is the result of multiplication by buds, and all the members are derived from one, which hatched from an egg, and, fastening itself to a shell, founded a new colony. A new bud may grow out anywhere, from the roots, and as the current of food which is always sweeping by provides it with ample nourishment, it grows quickly, and the repetition of the process of budding brings about a rapid increase in the size of the community.

The existence of a mechanism for propelling food to all its members facilitates the division of labor, or polymorphism, which is the most remarkable characteristic of these hydroid communities. In human history the growth of agriculture has supplied the first need of all men, abundant food, by the labor of a few, and has thus rendered division of labor possible, and has permitted many persons to train and qualify themselves for many pursuits which do not contribute to the food-supply. The existence, among the hydroids, of a mechanism for feeding them independently of their own efforts, has permitted the same sort of specialization to grow up, and even to become more perfect in some respects than it is among mankind.

The welfare of any species requires that the individuals shall be supplied with food, protected from accidents and enemies, and enabled to reproduce the species, and while many parasitic animals, and the young of many others, are supplied with food without exertion, the conditions of their life do not usually permit much specialization, and this does not, as a rule, occur unless the individuals of the species form communities. The social ants and bees are divided into castes, and the existence of hydroid colonies, which are structurally united into compound organisms, presents the conditions which are most favorable for specialization among the members of the community. We accordingly find among them the most remarkable examples of division of labor, accompanied by structural specialization or polymorphism.

A young dysmorphosa colony consists of a creeping root, which carries a number of hydras, all of them like a in the figure. They are the eating and digesting members of the society. Each of them has a long tubular body, almost completely filled by a capacious stomach, which opens to the exterior, at the free end of the body, through a mouth which is mounted upon a short, flexible proboscis, and is surrounded by a circlet or crown of long, elastic tentacles, radiating out in all directions around the mouth, and fringed by a poisoning apparatus of microscopic darts, which kill all the small animals which venture within the sweep of the tentacles. The food that is thus captured is conveyed to the mouth, and is swallowed and digested.

As the colony grows and the feeding members become numerous enough to store up a stock of nutriment and to bear the burden of a few non-productive parasites, hydras like b in the figure are produced. They are the fighting members, and have neither mouths nor stomachs, but each consists of an enormously elongated body, which ends in a battery of poison-darts, which is comparable to a circlet of undeveloped tentacles. The entire body of one of these fighting hydras is practically equivalent to a single enormous tentacle, although comparative anatomy shows clearly that it is not a tentacle, but that it corresponds to the whole body of a feeding hydra, tentacles and all, rather than to a single tentacle; that it is actually a hydra which has, during the evolution of the species, lost its mouth and stomach, and its power to capture and swallow food, and has become specialized for defense. These long, slender, outstretched bodies project far beyond the other members of the colony, and their poison-batteries wave in all directions over the heads of the feeding hydras. The shock of contact with them is either fatal or violent enough to paralyze any intruder, or to cause it to beat a hasty retreat.

As the community gains in numbers and strength, buds of a third sort are produced from the root, and become the reproductive hydras or blastostyles, which are shown at c in the figure. They are much like the feeding hydras in shape and in general structure, but the tentacles remain rudimentary throughout their life; they have no mouths, and their capacious stomachs do not open to the outer world, although their walls vigorously assimilate the food which flows into them through the roots.

As soon as the blastostyle is fully grown, a circlet of buds grows out from its body, just below its rudimentary tentacles. These buds soon acquire an organization which is very different from that of any of the forms which have been described, and, developing organs of locomotion, are ultimately detached from the blastostyles, and are set free to begin their independent life as solitary, swimming jelly-fish, like those which are shown at d in the figure.

The active jelly-fish is as different from all the members of the hydroid colony as a butterfly is from a caterpillar. When fully grown it is vastly larger than a hydra, and it has a well developed swimming apparatus, which is under the control of a nervous system, which again is brought into relation with the external world by means of special sense-organs. It is a gelatinous bell, from the inner surface of which the pendent stomach hangs down like the bell-clapper, while the long, graceful, thread-like tentacles are attached at regular intervals around the opening of

PSM V33 D599 Liriope tetraphylla.jpg
Fig. 2.Liriope scutigera, slightly magnified, drawn from Nature by W. K. Brooks. (The small figure in the left-hand lower corner is the planula of Turritopsis, greatly magnified; and the one in the right-hand corner, the root and the first bud of the Turritopsis hydroid.

the bell. The locomotor muscles are so distributed over the inner surface of the bell that their contraction squirts out the water in a jet which propels the animal in the opposite direction; they are then relaxed, and the elasticity of the gelatinous substance of the wall of the bell causes it to expand and to draw in another supply of water which is discharged by the next muscular pulsation. The tentacles are so elastic and hair-like that they are held by the resistance of the water, and are drawn out behind the animal into fine glassy threads which are thrown into graceful undulations at each pulsation as it swims through the water, and, when it comes to rest and sinks slowly toward the bottom, they form a web or net which is almost invisible, but far more dangerous than any spider's web, for every thread is covered with the terrible poison-darts.

Great as the difference is between the sedentary hydra and the swimming jelly-fish, comparative anatomy shows that they are modifications of the same type, and that the jelly-fish, like the blastostyle, the defensive hydra and the root, is a specialized feeding hydra.

In some species of Dysmorphosa the jelly-fish which is set free from the blastostyle is the last stage in the long series, and it quickly acquires reproductive organs, lays its egg or discharges its spermatozoa as the case may be, and dies; but in other species PSM V33 D600 Planula of liriope scutigera.jpgFig. 3.—Planula of Liriope scutigera, highly magnified, drawn from Nature by W. K. Brooks: a, surface layer of cells, which is shown in section on the right half; b, gelatinous substance; c, inner layer of cells, shown in section in the lower right-hand quadrant; d, the central cavity; a', the point where the month is to be formed. it no sooner begins its own independent life than it produces buds which are ultimately set free, as jelly-fish like the parent, each of which soon becomes a mature male or female. The eggs are thrown out into the water, where they are fertilized by union with the male cells, and each egg then begins the process of development, which is to result in the founding of a new hydroid colony. The life of the jelly-fish is very short, and simply serves to multiply the species, and to scatter the eggs far and wide along the shore of the ocean, and thus to secure the wide distribution of the hydroids. The egg hatches, however, neither into a jelly-fish like the parent, nor into a hydra, but into a minute microscopic animal of extremely simple structure, which is known as a planula. Fig. 2, a, which is a highly magnified drawing of the planula of another species, will serve to show what it is like. It has no mouth nor tentacles, and its pear-shaped body is covered with cilia, by means of which it swims slowly through the water for a short time, but, unless its slight locomotor power soon brings it into contact with some solid body upon the bed of the ocean, it dies. With the discovery of a solid resting-place its purpose is accomplished: it loses its cilia, and, cementing its body fast, it elongates and becomes converted into a root, from which the bud which is to form the first feeding hydra soon arises, and, acquiring a mouth and tentacles, begins to accumulate food and to provide for the growth of a new colony. In some other species the planula becomes a feeding hydra instead of a root, and the history of the various hydroids shows clearly that the root is directly comparable with a hydra and is a member of the community which, like the others, is specialized for a particular purpose.

On a sea-beach there are few hard solid bodies except the shells of mollusks, and these are therefore the only available resting-places for the planulæ, but a colony which is founded upon the shell of a living mollusk has no chance of prosperity, for the mollusk is sure to soon plow its way under the sand or into the mud, and a delicate hydroid can not survive such rough usage, nor is the case of a planula which finds an empty shell any better, for the first storm will either bury it under the sand, or toss it high and dry above low-tide mark, or sweep it off into some deep channel to be buried under the mud and sediment.

Everything is favorable to the new colony which is started on a shell which a hermit-crab has selected for a house, and the chances are that it will grow and prosper and soon become a vigorous, flourishing settlement; for the crab does not creep like a snail, but trots around on the tips of his claws with the shell held well up above the sand, and he is far too intelligent and wide awake to permit himself to be stranded on the beach, or swept away into muddy quicksands. As the gentle waves ebb and flow on the shore he follows them back and forth, keeping close to the edge, where the food which is washed out of the sand is most abundant and the aeration of the water most perfect. As long as the sea is calm he may be trusted to carry his load of hydroids into the places which are most favorable for them, and as soon as a storm approaches he trots off with his charge to a safe shelter in deeper water and waits until it has passed. A colony which is founded on his shell is sure to flourish and increase, for this location affords all the elements of prosperity, and, while small colonies are often found in other places, the most vigorous and largest ones are, as a rule, found only in this peculiar habitat.

In the following diagram I have attempted to exhibit at one view all the phases in the remarkable life-history of Dysmorphosa. The sign of equality between two stages indicates that the one on the left becomes transformed into the one on the right, without multiplication and without loss of identity; the sign x indicates asexual multiplication by buds, and the sign < sexual reproduction by fertilized eggs:


I. Dysmorphosa.
Egg=Planula=Root ×
Feeding hydra × Feeding hydra,
Blastostyle ×
Fighting hydra,
Medusa × Medusa < eggs.
Medusa < eggs.
Medusa × Medusa < eggs.
Medusa < eggs.
×
Feeding hydra × Feeding hydra,
Blastostyle ×
Fighting hydra,
Medusa × Medusa < eggs.
Medusa < eggs.
Medusa × Medusa < eggs.
Medusa < eggs.
×
Feeding hydra × Feeding hydra,
Blastostyle ×
Fighting hydra,
Medusa × Medusa < eggs.
Medusa < eggs.
Medusa × Medusa < eggs.
Medusa < eggs.
Feeding hydra × Feeding hydra,
Blastostyle ×
Fighting hydra,
Medusa × Medusa < eggs.
Medusa < eggs.
Medusa × Medusa < eggs.
Medusa < eggs.
×
Feeding hydra × Feeding hydra,
Blastostyle ×
Fighting hydra,
Medusa × Medusa < eggs.
Medusa < eggs.
Medusa × Medusa < eggs.
Medusa < eggs.
×
Feeding hydra × Feeding hydra,
Blastostyle ×
Fighting hydra,
Medusa × Medusa < eggs.
Medusa < eggs.
Medusa × Medusa < eggs.
Medusa < eggs.

The egg hatches into a planula, which becomes attached and is converted into a root, from which feeding hydras bud; from the roots of these feeding hydras, other feeding hydras, and, after a time, defensive hydras and blastostyles or reproductive hydras, are budded in very great numbers, and, while the diagram correctly represents the complexity of the colony, it conveys no conception of its size or of the number of its members. Each blastostyle produces a considerable number of buds, which are ultimately set free as swimming jelly-fish or Medusæ, and each medusa multiplies by budding, and thus gives rise to a second generation of Medusæ, which probably repeat the process in their turn, so that a very great and practically unlimited number of sexual egg-producing adults results from a single egg.

What a contrast between the direct and simple history of ordinary animals, where each adult is the total progeny of the egg, and such a life-history as this, where the egg not only produces an unlimited number of sexual adults able to bud off others like themselves, but also gives rise to an innumerable number of larvæ which never become sexually mature nor assume the adult form.

Those who are familiar with the subject know how much paper and ink have been wasted in discussing the individuality of hydroids, but we need not enter into into this dead issue, for, beyond question, each feeding hydra, each defensive hydra, each blastostyle, and each jelly-fish is an individual in the same sense that a horse or a dog is one; and the most remarkable peculiarity of Dysmorphosa is the enormously great number of descendants from each egg. Another peculiarity must also be noted. The life-history is not a simple process of growth, nor a metamorphosis, like that which occurs among insects.

The caterpillar, which hatches from the butterfly's egg, is perhaps as unlike the butterfly as the hydra is unlike the jelly-fish, PSM V33 D603 Larva of liriope.jpgFig. 4, a section, and Fig. 5, a surface view of the larva of Liriope, to show the formation of the mouth, e, and the stomach, d. but it never loses its identity, and the individual which hatches from the egg is the same one which passes through all the caterpillar molts, becomes a chrysalis, and finally escapes as a perfect butterfly, just as the chick which hatches from a hen's egg is the individual which finally becomes a hen and lays eggs in her turn. The growth of the butterfly is accompanied by great and sudden changes from one stage of development to another, but it is simply a process of growth and development, while the life of Dysmorphosa is quite different. The planula, which hatches from the egg, becomes metamorphosed into a root, just as the caterpillar becomes changed into a chrysalis; but here the resemblance stops, for the root goes no further, and it may still remain a root after numbers of jelly-fishes have grown up, laid their eggs, founded new colonies, and died. The feeding hydras and defensive hydras never grow up into jelly-fish, but, as long as they live, continue to perform their proper parts in the colony, and this is equally true of the blastostyles, for these do not become jelly-fish; they simply produce jelly-fish buds, and each one may persist as a blastostyle and continue the process of budding long after the younger buds have completed their history.

In all these particulars the life of Dysmorphosa is a great departure from the normal life-history of animals, for, as a rule, each embryo which hatches from an egg is destined to become an adult animal, and only one.

The simpler aspects of the phenomena of life are older or more primitive than their more complex manifestations, and all analogy forces us to believe that Dysmorphosa is the descendant of some remote ancestor whose life-history was as simple and direct as that of a bird or a mammal or a frog, a butterfly, a snail, a crab, or a star-fish, and that originally each egg became converted into an adult by growth and metamorphosis.

If this is true, how has its complexity arisen? What were the stages in the gradual acquisition of the life-history which is shown in our diagram? What forces have produced the change, and what is its significance or advantage?

Not very long ago such questions were held to be unanswerable and meaningless, but at the present day we are all familiar with the process of reading the past history of life by the study of comparative anatomy and embryology, and are ready to accept the evidence of the series of living hydroids which show us the character of the changes through which the ancestors of Dysmorphosa have passed, as they have gradually acquired the structure which is exhibited by their living descendants.

I shall now briefly describe a few American species of jelly-fish, which exhibit successive stages in the process of complication, PSM V33 D604 Hydra stage of the liriope larva.jpgFig. 6.—Hydra stage of the larva of Liriope; d, stomach; c, mouth; f, tentacles. and serve to show that the remote ancestor of Dysmorphosa must have been a jelly-fish, which passed, during its development, through a transitory larval hydra stage, which was only a step in the process of growth of the embryo into an adult. In this form each egg produced one animal; the adult life was all important, and the hydra stage was passed as quickly as possible; and during the history of the species this has gradually become a more and more important part of the whole life, until finally the adult jelly-fish has become comparatively unimportant and simply serves to secure the distribution of the species, while the larvae have acquired the power to bud and to build up colonies, the members of which have become specialized in various directions, by division of labor, for the benefit of the whole.

One of the most graceful Medusæ of our Southern coast, from Florida to the Chesapeake Bay, is the beautiful Liriope shown, somewhat enlarged, in Fig. 2. It is not very different from Dysmorphosa in shape, but it is much larger, and a most active and elegant creature, with a bell like cut-glass, and long, waving tentacles. Its movements are so instinct with grace that an admirer of the lines and curves of Nature could desire no better or more fascinating occupation than the observation of an aquarium stocked with a few specimens of this attractive jelly-fish. The drawing is accurate, so far as mere shape goes, but no drawing can represent its jewel-like brilliancy or the elegance of its movements. Its chief interest to us, however, centers in its life-history, which is very different from that of Dysmorphosa. Its proper home is the deep water of mid-ocean, not the shallows near shore; and it has no attached stage of development, but floats or swims at all periods of its life. The eggs are thrown out into the water, and each one soon develops into the embryo which is shown, highly magnified, in Fig. 3.

This embryo, which is a planula adapted for floating instead of swimming, is a hollow sphere, the walls of which are formed of

PSM V33 D605 Side and broad view of a young liriope.jpg
Fig. 7, side view, and Fig. 8, a broad view of a young Liriope: b, swim-bell; d, stomach; e, mouth; f, larval tentacles; g, long tentacles of adult; h, short tentacles of adult; i, areas of adhesion; k, otocysts; l, radial canals.

two spherical shells, an outer one, a, which forms the surface of the body, and an inner one, c, which lines the central cavity, the two being separated from each other by a gelatinous layer, b, which serves to float the embryo in the water. The central cavity has at first no opening to the exterior, and the two shells are concentric, but they soon approach each other at a point, a', and an opening to the exterior is formed. The central space now becomes the stomach, d, in Figs. 4 and 5, and the opening becomes the month, e, in Figs. 4 and 5. Soon after the mouth is formed a circlet of tentacles is developed around it, and the larva becomes a hydra, but a hydra adapted to a floating life rather than a fixed life, as shown in Fig. G.

It is now able to capture and digest food and to lead an independent life, and it grows rapidly, although it has as yet no locomotor organs, and drifts at the mercy of the waves.

Soon the stomach becomes flattened and the mouth pushes in toward the center of the sphere, carrying with it the surface layer of cells, and thus giving rise to a concave, hollow bell-cavity opening to the exterior, as shown in Fig. 7, in which d is the flattened stomach, e the mouth, and f the cavity of the swim-ball.

As the mouth is pushed in, the tentacles are left behind and remain on the edge of the bell in the position which they occupy in the adult. The animal grows rapidly, the tentacles lengthen, and, after some slight changes which do not now concern us, the larva becomes converted into an adult, like the one shown in Fig. 2, and again reproduces its kind by fertilized eggs.

The life-history of Liriope is simple and direct. There is a metamorphosis, and the animal passes through a planula stage, a hydra stage, and a medusa stage; but its identity is never lost, and the larva is the same individual as the adult, as is shown in the following diagram:

II. Liriope.Egg Planula free Hydra Medusa < eggs.

Although most of the text-books state that the direct history of Liriope has been produced by the gradual simplification of a complicated life-history like that of Dysmorphosa, there is no evidence whatever that this is the case, and the fact that the development of Liriope resembles that of all ordinary animals is in itself an indication that its simplicity is not secondary but primitive. This view is rendered still more certain by the study of other jelly-fishes which exhibit successive steps in the process of complication.

 


 
Dr. Kael Pettersen, Director of the Tromsö Arctic Museum, has suggested that the object of polar expeditions could be obtained most easily, surely, and cheaply, by dispatching, instead of single sporadic expeditions, every year, for a period of ten or eleven years, a number of well-equipped steamers from certain suitable points toward the pole. Amid the ever-varying shifting of the polar drifts and currents, some of these vessels might possibly get through. He recommends four points of departure for such vessels: one along East Spitzbergen and Franz-Josef Land, and northward; one east of Franz-Josef Land, from the Yenisei or Obi; one by way of Franz-Josef Land, starting from the New Siberian Islands or the Lena; and one from a suitable spot in Bering Strait.