Popular Science Monthly/Volume 8/November 1875/Hydroids
SOME of the most exquisite forms of organic Nature are to be found in that shadowy border-land which unites rather than divides the animal and vegetable worlds. It is hard to believe, even when looking with careful scrutiny at certain forms of animal life, at
the corals, for instance, the sponges, and the hydroids, that an existence which so closely resembles vegetation should be essentially animal. Each of these families of the great invertebrate kingdom has been bandied back and forth from the botanist to the zoölogist, and each has finally found its place in the animal world.
No purely empirical knowledge is sufficient to determine, among the lower forms of life, to which kingdom they should be referred. It is only by studying facts in their relations, by patiently observing the life-history, and by ascertaining the modes of nutrition and reproduction of each form, that its true place in the organic world has been determined.
It was, for many years, thought that, beyond the depth of 300 fathoms, organic life ceased to exist in the ocean. Forbes reached this zero of life in the Ægean Sea, and the fact ascertained for the Mediterranean was inferred for all other seas. The transmutation of inorganic into organic matter is only performed by vegetables, and then only under the controlling power of light. The distinction made by naturalists between the lowest forms of animal and vegetable life lies just here: vegetables convert the inorganic elements of earth, air, and water, into organized matter; animals rearrange this organized matter into animal tissue. It is well known, as no light penetrates the profounder oceanic depths, that no vegetation can exist there; an absence of animal life was therefore inferred. Certain exceptions to this definition of vegetable life, as being exhaustive, are found in the Fungi, which germinate and grow in darkness, and it is believed are nourished in great measure by organic matter, as well as in the curious carnivorous plants, which have of late attracted so much attention. This, however, does not invalidate the truth that all nutriment, in order to be fit for the maintenance of animal life, must pass, at least once, through the transmutation effected only by vegetation.
The non-existence of life below 300 fathoms, in all the oceans of our globe, was strongly supported by Forbes's investigations in the Mediterranean. The abyssal depths of the sea were thus determined by logic to be the universal empire over which reigned darkness, desolation,
and death. No investigations were made as to the facts of the case. Logic and a hasty generalization from inadequate knowledge were made, once again in the history of science, to do duty for the more laborious method of patient observation. Commerce at last gave the impulse to deep-sea exploration, which had before been lacking. The commercial world demanded a more speedy mode of communication from continent to continent, and the response came in the form of the submarine telegraph. Thousands of soundings were made to determine the best position in the ocean's bed for its successful laying, and thousands, again, to secure the broken end after the first failure. These soundings and grapplings brought from the sea-depths unmistakable proof that life in many varied and exquisite forms existed there, far away from light and vegetation, under an enormous pressure of superincumbent waters; and logic retired discomfited.
The fact that the Ægean Sea is empty of life in its greatest depths is due to local causes. The humblest life, in the farthest recesses of the ocean's bed, is, in some of its essential features, but a sluggish copy of the higher types on land. Food and air are alike necessary
to both. The circulation of currents throughout the open seas bears nutriment and oxygen to the lowly forms of animal life which lie far below the level penetrated by light, or capable of supporting vegetation. In the Mediterranean such currents are obstructed by the high rocky wall which runs under the straits of Gibraltar, from Spain to Africa. The lowest point in this wall is 10,000 feet above some portions of the bed of the Mediterranean. The currents in this sea are therefore superficial, as well as the life sustained by them.
Chemical analysis proves that the water of the open seas contains both organic matter in solution and oxygen; and that this same water, after having passed through the bodies of these lower forms of animal life, is deprived of both its organic elements and its oxygen. The theoretic difficulty which had determined the problem of life in the depths of the sea was thus removed; for, given this lowest form of animal existence, the higher are always possible.
The same awful cycle of life, death, decomposition, and life again, which is again and again repeated among the higher organisms, is found working itself out as inexorably in the oceanic depths. The elements which are appropriated from the mighty reservoir of the ocean for the maintenance of the life, are restored to it again by the death, of each organic being.
The bed of the ocean, from the tiny lakelets left by the retiring tide to the greatest depths ever reached by trawl and dredge, is found to be teeming with exquisite forms of life. Delicate plant-like forms are found clinging to rocks and shells, or spreading themselves over the broad fronds of the algae. Every peculiarity of vegetation is mimicked; graceful stems rising from tangled roots send out branches which bear raceme-like clusters of buds, and delicate bells whose beauty no words can describe.
A hundred and fifty years ago nothing was known of these beautiful hydroids. The first investigation deserving the name was made by Abraham Trembley. This man was born in Geneva in the year 1700. While residing at the Hague, as tutor to the sons of Count de Bentinek, he made a series of remarkable observations upon the fresh-water hydra. The results of his observations were published first by Réaumur in 1742, and two years later by himself. In 1727 Peysonnel had paved the way for Trembley by proving the animality of the corals. Jussieu visited the coasts of Normandy to investigate the coral question, after Peysonnel's publication of his views, and there conclusively demonstrated the animality of Tubularla indivisa, one of the loveliest of the hydroid family. The hydroids are among the coral-makers. The vast beds of millepores found about the Pacific islands and the West Indies are the work of an animal allied to coryne, one of the Tubularians. The chitinous investment of the Sertularians also forms membranous coral of considerable size and great beauty. It was some time, however, after the discoveries of Peysonnel, Jussieu, and Trembley, before the great authorities of the day, Réaumur and Linnæus, gave in their adhesion to the animal theory, and stamped it as correct. Since that day some of the world's greatest naturalists have made the study of the Hydroidœ their life-work, and have not felt it an unworthy occupation to be the annalists of this humble family.
The nomenclature of the hydroids is still so unsettled that we will avoid as much as possible the use of scientific terms in describing the different portions of the colonies and their respective functions, for it is here that naturalists differ, not in the names of the varieties.
The hydroids measure from a few lines in height to several feet. Dana mentions an East Indian species which grows to the height of three feet; while Semper describes a gigantic Plumularian, which forms submarine forests extending over great areas of sea-bottom, and growing as high as six feet. The stems, he says, sometimes measure an inch in diameter at the base. Tubularia indivisa grows to the height of about ten inches.
The Hydroidœ are divided into four families: Tubularinœ (Figs. 3, 4, 5, 6), Campanularinœ (Fig. 10), Sertularinœ (Figs. 1, 7, 8, 9), and Hydrinœ (Fig. 12).
Every hydroid, however greatly the species may differ in external form, has a certain structural plan to which it adheres in all its modifications. The general type (Fig. 2.) may be simply described as an animal sac whose walls are composed of an inner and outer membrane. The outer wall corresponds to the skin, the inner to the lining of the stomach, in higher organisms. The simple elongated sac is not only the simplest form of hydroid, but is generally the earliest phase in the development of the more complicated forms.
The sac (Fig. 2) sends off branching processes, e e, and cœcal protuberances, d, throughout the extent of which the inner and outer membrane is continuous. Sometimes large numbers of these stems proceed from a basal net-work, the connection between every part of the animal colony being kept open through this basal reticulation, and the continuity of the two membranes being maintained intact. The basal portion, with the stems, branches, and the flower-and-fruit-like clusters, of this curious organism form the hydrasoma, as it is called by both Huxley and Allman.
The simple, sac-like form of the hydroid is the lowest term in a series which consists of an almost infinite number of terms. We find in this family the same orderly sequence which marks organic Nature everywhere. While the ideal type is adhered to, and a morphological unity may be proved, yet there is an orderly and beautiful gradation, in which each form becomes more complicated than the form which precedes it.
The clusters of buds (Fig. 4), and closed or open flowers (Fig. 3), are really individual zoöids, bound into an organic unity by a basal reticulation. With a single exception, every hydroid, at some period of its existence, lives this social life, being united with a number of other individuals into a plant-like group, and is really only one of an assemblage of zoöids possessing a common circulatory and nutritive system, the individuals of which are in organic union with each other.
The zoöids springing from one common base are of two kinds, and perform for the community two special offices. The grape-like clusters contain the generative elements, both ova and spermatozoa, while the flowers provide for the nutrition of the whole colony. These zoöids, which each investigator names according to his peculiar theory of scientific nomenclature, we will call nutritive and generative buds; the nutritive buds being destined for the preservation of the colony, the generative for the perpetuation of the species. The attached extremity of the animal in the fixed, or its equivalent in the free, species is called the proximal end, and the opposite extremity, which bears the two forms of buds, the distal end of the hydroid. The terms upper and lower cannot be used, because some varieties grow erect, while others grow in an inverted position.
The nutritive buds consist of an open digestive sac (Fig. 2); around the mouth is a series or several series of tubular offsets, ranged radially about the stem. The shape of these blossom-like zoöids varies in the different species. In some varieties they are unprotected, while in others the tentacles may be withdrawn into a horny, cup-shaped sheath. The number of tentacles varies with the different species. The plates of Tubularia indivisa and Hydra vulgaris show the tentacles expanded. The other plates give, in the magnified portions, only the chitinous sheath, into which the polyp has withdrawn itself.
In the Plumularians, a branch of the Sertularian group, curious little cups of the horny sheath are developed. Unlike the cups which contain the living flower, these extensions are filled with the sarcode, or soft, gelatinous flesh of the animal. This sarcode, or protoplasmic flesh, acts like the flesh of the rhizopods and amœbæ; long filamentary processes are extended, just as the rhizopods improvise legs or arms when they need them, till sometimes the horny sheath is invested in this living gossamer. The function of these cups is not known. Allman considers them as special zoöids, whose morphological differentiation from the other zoöids is carried to an extreme. Hincks believes them to be a lower form of life, in organic union with the higher zoöids of the hydroid colony.
The horny sheath, which is described by earlier writers as an excretion, is by Allman considered to be rather the result of metamorphosis of tissue. In many varieties the stem and branches of the creature are slender, horny, and pipe-shaped, and the chitinous sheath is jointed at regular intervals, the joint being a mere break in the continuity of the chitine, not a movable hinge; while the living pulp within forms a continuous body, and is invested by its sheath as the pith of a plant is invested by its stalk.
The generative buds are cæcal offshoots from the body, the reproductive elements always developing between the inner and outer membrane (see Fig. 2, d). They sometimes, after development, free themselves from the parent stem, and lead a roving life as medusæ. In some cases the nutritive bud has its alimentary function suppressed, and, though not itself sexual, it is henceforth destined to produce sexual buds, either directly or through the medium of a non-sexual bud.
There is, it may almost be said, no differentiation of organs among the hydroids. In the adult form they possess no organs of sense, and have no circulatory, respiratory, nor nervous systems. All the functions of life are performed without the intervention of special organs. Voluntary motion takes place without muscles, sensibility is present without nerves, respiration is performed without lungs, and digestion goes on without a true stomach. The sea-water which flows within and about the creature bears to it the oxygen necessary to the maintenance of vital combustion, as well as the small living creatures and comminuted organic matter which form its food. Like the Fig. 7.—Sertularina cupressina.
sea-anemones, the hydroids reject such portions of their food as they do not assimilate through the mouth. In the fresh-water hydra an orifice has been observed at the lower extremity of the stomach. This, however, does not correspond to the alimentary canal of higher organisms; it is the analogue, in the simple hydra, of the ramifying cavity which permits a free circulation throughout the compound group.
A circulation has been observed in the varieties which possess a horny sheath, which is, however, very different in some respects from the circulation of the blood in higher organisms. The somatic fluid, as it is called, is loaded with granules which, upon microscopic examination, prove to be composed of disintegrated elements of food, of solid colored matter secreted by the walls of the somatic cavity, of cells detached from the living tissue of the animal, and of particles of effete matter. The fluid seems to be more nearly akin to cliyme, or chyle, than it is to blood. There is perpetual motion in the somatic fluid; the flow will sometimes be steady for a while, and then a sudden reversal will take place in the direction of the current, before it has reached an extremity. The gastric cavity is traversed by the stream, as well as all portions of the hydrasoma. In some species the cause of the flow has revealed itself under the microscope. The cavities through which the current moves are seen to be clothed with cilia—tiny lashes whose rhythmic motion forever propels the fluid; this ciliary action is no doubt greatly aided by the contractility of the walls. In many species the cilia, if there be any, are too minute for detection; but it is a fair provisional inference that where the somatic flow is observed the like cause may account for the like effect.
The exquisite colors of the hydroids, which rival the tints of our loveliest flowers, are due to the colored granules secreted by the animal and discharged into the somatic fluid. A charm is added to these flowers of the sea by the flashing opalescent gleams of color which shine out from their crystalline walls. Even the exquisite representations of Allman, in his monograph on "The Tubularian Hydroids," fail to give an idea of the beauty of form and color to be found in the real object. The Hydra viridis is so called from its brilliant green color. This green is said by Allman to be of the nature of chlorophyll, and to possess the power, like the chlorophyll of plants, of decomposing carbonic acid, assimilating the carbon, and yielding up the oxygen. If this be true (and there is no reason to doubt it, Allman being one of the highest authorities), it only furnishes, in this form of animal life, one more curious resemblance to vegetation, and denies one more tradition of its animality.
The most singular facts in connection with hydroid life lie in the variety of its modes of reproduction. It would almost seem as though every form of reproduction known in Nature had been mutely prophesied in the primeval world when the fossil hydroid and trilobite lived side by side in the Silurian seas.
They are generated, like plants, by buds and by artificial sections; like plants, they are able, from a small fragment, to produce the whole organism. They, however, go farther than most plants in this power of reproducing lost parts; for a small fragment taken from any portion will suffice for the production of a new individual; a single tentacle will produce a flower and stem, and finally a whole colony. A transverse section of the stem will produce a flower at the distal end, and a continuation of the stem, with the process by which it attaches itself, at the proximal end of the section. Just so far it shows orientation—that the stem has a distal and proximal end. There is no sign of bilaterality in most species, and in others the indication is so slight that it is hardly worthy of the name. This development of the flower always at the distal, and of the stem always at the proximal, extremity of the section, shows conclusively that the stem grows both ways, and that in every segment there exists a neutral plane midway between both ends.
Besides these plant-like modes of reproduction, hydroids are generated, like the actiniæ, by spontaneous fission, a development of one individual into two or more by a natural vertical cleavage.
They multiply by ova, by ovules, by independent ciliated embryos, like the lower invertebrates, the reptiles, and birds. Some varieties possess a sort of marsupial pouch, in which the undeveloped young
are retained till they attain maturity; and, like the mammals, in some cases, the individual quits the parent after attaining perfect development. Added to all these modes of reproduction, in which the analogy must not be pressed too closely to those of higher organisms, they possess two very curious modes of their own; one given by Allman in his monograph, the other by Carpenter in the latest edition of "The Microscope, and its Revelations." The Tubularian and Campanularian hydroids, Allman tells us, develop upon their stems bell-shaped medusæ (Figs. 4, 5, 11), which free themselves and swim the adjacent waters. All free-swimming medusæ have not yet been traced to hydroid stems; but, as all which have been carefully studied through their life-history are found to originate there, it is supposed to be true of the others.
The most remarkable fact in regard to these medusæ is, that the immature form shows a higher type, a greater differentiation of organs, than the parent hydroid. The medusa possesses, in common with the parent, a digestive cavity and enidæ; and, in addition to these, an organ at the base of each tentacle, which, if it does not unite within itself the senses of sight and hearing, at least is akin to those organs in the lower invertebrates. They certainly possess distinct bundles of muscles and nerve-ganglia, which are not found in the parent form. When the roving medusa has sown its wild-oats, and comes to settle down into a respectable family hydroid, it loses all these advantages belonging to its wandering life, and becomes in its later form identical with the parent; it returns to the privileges and traditions of its fathers.
The huge Rhizostoma, and the beautiful Chrysaora, common to the English coast, Carpenter tells us, are oceanic medusæ developed from a small hydroid stem. The embryo emerges in the form of a ciliated ovule, resembling some of the infusoria. One end contracts, forms a foot and attaches itself, the other sends out four tubular offshoots, as tentacles, and "the central cells melt down to form the cavity of the stomach." This hydra-like form multiplies in the ordinary way by budding, for an indefinite length of time. After a while, however, a change takes place, the stem shows constrictions, beginning near the distal end, till the whole stem looks like a rouleau of coins; the constrictions deepen, making the stem look like a pile of saucer-shaped bodies; the disks become serrated, and finally the tentacles which belonged to the original medusæ disappear, and new tentacles are formed upon the uppermost disk of the pile. Soon this disk begins to show a sort of convulsive struggle which results in its freeing itself, and swimming away as a medusa; each disk develops in the same way, and in turn separates itself from the parent stem. The original zoöid often returns to its hydra-life and reproduces itself by budding in the old fashion, and finally becomes "the progenitor of a new colony, every member of which may in its turn bud off a pile of medusa-disks."
The bodies thus detached have all the characteristics of the fully-developed medusæ. Each consists of an umbrella-shaped disk divided along its margin into lobes, generally eight in number, and of a stomach terminating in a probosciform mouth. As the creature grows, the spaces between the marginal lobes fill up; from its border long tentacles are developed, and a fringe of tendril-like filaments sprout forth from the margin. The young medusa eats voraciously, and grows proportionately large; the Chrysaora, which we have been describing, attaining a diameter of fifteen inches, and the Rhizostoma sometimes reaching to three feet. These medusæ are familiarly known as sea-nettles. When they have reached full development the generative organs appear in four chambers arranged round the stomach, and are contained in curious fluted membranous ribbons which hold the sperm-cells in the male, and ova in the female. The fertilized embryos repeat the same wonderful cycle just described, developing into a hydroid from which medusa-disks are budded off.
The relation which late investigations have established between the stationary hydroids, and the medusæ, forms one of the most interesting cases, yet known, of the curious phenomenon called alternate generation. In the majority of cases we find a non-sexual, plant-like form interposed between the ovum and the directly or indirectly sexual form of medusa, though this is not always the case, as direct development has been observed from ovum to medusa.
The nearest approach, in the adult form, to special organs are the digestive cavity, and the cnidæ. The stomach, however, possesses no true parietal walls, and in one form—the fresh-water hydra—the stomach will do duty for the skin, and the skin for the stomach, if necessary; they seem to be able to live very comfortably, and digest their food without difficulty when turned wrong-side outward.
The cnidæ are barbed filaments inclosed in tiny sacs, which they can shoot out at will, for their own protection, or for the capture of their prey, as the case may be. In the hydra the sac is ejected, and a central dart is projected into the body attacked. There must be a minute poison-sac in communication with the darts, as it is found that any soft-bodied victim, released from the clasp of the tentacles, is invariably dead, no matter how short the time of its imprisonment may have been. The effects of the cnidæ in the medusae are very well known, and have gained for them their popular name of sea-nettles. Many an unlucky swimmer has found himself wrapped in the long thread-like filaments of these transparent, floating bells, and been almost maddened as he found himself inextricably inclosed in what seemed an invisible sheet of living fire. A tentacle of the hydroid, when carefully pressed between two glass slides, or in a compressorium, may be seen, under the microscope, to dart out thousands of these little barbed arrows.
Chronologically, the Hydrœ (Fig. 12) come first in the group Hydroidœ for they were first carefully studied and truly classified by Trembley. His observations, though made in the earlier half of the eighteenth century, were so accurate, and his delineations so correct. that he is still quoted in the latest works as authority. The hydra is found generally in fresh water, though some few species have been discovered, in this country, in that which is somewhat brackish. It loves still or slowly-running water, and attaches itself generally to the under-side of the leaves or to the stalks of aquatic plants. Its body is extremely contractile, and consists, like the oceanic hydroids,
in its earliest stage of development, of a simple elongated sac, with an opening which answers the purpose of a mouth. Around the mouth are a series of hollow filaments which it can entirely withdraw, and it then looks like a minute tubercle. The tentacles are roughened by the clusters of thread-cells, or enidæ, already described. The threads have been observed in some instances to be, when extended, as much as eight inches long, and are shot out, it is thought, by the propulsive power of a liquid injected into the central cavity. It grows erect, horizontal, or inverted, as the case may be, and lives only upon animal food. The little creatures are extremely voracious and not over-nice. Trembley observed two hydras attack, at the same time, the opposite extremities of a worm. Each having swallowed its respective half of the worm, he watched to see the result. The worm would not yield to the force of circumstances; and break, and the problem looked a difficult one of solution. The larger hydra, however, proved itself superior to circumstances, it quietly swallowed worm, antagonist, and all; and, after having sucked out the worm, disgorged his dinnerless foe!
Trembley tried the experiment, already alluded to, of turning one inside out, and fastening it in that position. The domestic economy did not appear to be at all disturbed; the little creature eating with as much relish, and digesting with as much ease, to all appearance, as in its normal position. He inserted one hydra within the cavity of another, and fastened them with a bristle which was run through both. Returning after a short absence he found them strung, side by side, upon the bristle. He repeated the experiment and watched the manœuvres of the two. The hydra inside managed to work its way through the small aperture made in the side of its neighbor by the bristle, and soon occupied the position he had before observed, side by side with its companion on the bristle. He then turned one of them inside out, inserted it in that position, and fastened them securely together. Soon the pair, finding that there was no help for it, philosophically yielded, and united their fortunes; the inner one of the couple providing nourishment for them both. They seemed to live quite comfortably, on these very close terms of intimacy, for some time.
Hydras generate in summer by buds, which grow to maturity and are then sloughed off. These young buds often produce others before they separate from the parent stem, and they others again; so that there are sometimes twenty generations produced in a month's time. In autumn oviform gemmules are extruded, lie quiescent till spring, and are then developed. Any number of artificial sections may be made, and from each a perfect animal will be developed. Wherever a wound or cut has been made, buds sprout more quickly than from the sound tissue, and the hydras generated by artificial sections are more prolific than those generated in the ordinary way. The sprouting, as may be seen in the plate (Fig. 12), takes place from any portion of the body. The leaves, flowers, and stems, of this specimen of Hydra vulgaris, together form the hydrasoma. This specimen was selected more to illustrate the plant-like character of the organism than for its intrinsic beauty.
The geographical distribution of the Hydroidæ has not yet been determined; but, like other low forms of life, we find them spreading over vast areas of space, and extending back through uncounted ages of time. We have already spoken of their distribution in depth. A well-defined specimen was taken up in the deepest cast recorded by Wyville Thomson, in his "Depths of the Sea"—that made in the Bay of Biscay, and to a depth of nearly three miles. But, though their existence is proved at these enormous depths, they love best the rock-bound pools left by the retiring tide and the shallow water which fringes our islands and continents; and there they probably attain their greatest beauty and most perfect development.
Their distribution in time reaches back to the earliest dawnings of life upon our globe. The Graptolites of the Lower and Upper Silurian, the Hydroid Medusœ of the Jurassic, the Hydractinea of the Cretaceous, Miocene, and Pliocene, the Serturella of the Pleistocene, and the numberless forms of the present day, are the representatives of this family in geologic and historic time.
Like other humble forms of life, it shows a marvelous persistency. It has lived, almost unchanged, while great dynasties of higher organisms have one after the other risen, developed, and perished, or left only a few meagre representatives among the fauna of the present day. The fragility of their chitinous envelope and the perishable nature of their protoplasmic flesh would, of course, render it impossible that any full record of their existence should ever be found in the rocks of the primeval would, but the fragments which have, here and there, left their impress on the various geologic strata, show them to have been the contemporaries of the oldest forms of life which inhabited the Silurian seas, and to have quietly existed in the depths of those ancient waters over which the great fish and saurian dynasties lorded it through so many centuries.