Popular Science Monthly/Volume 33/October 1888/The Growth of Jelly-Fishes II


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By Prof. W. K. BROOKS,




IN the first part of this article I described the life-histories of two hydroids: one, Liriope, in which each egg gives rise to only one jelly-fish, which is solitary and free at all stages of its existence, and without any power to multiply asexually; and a second species, Dysmorphosa, in which there is no limit to the number of adults to which a single egg may give rise, and in which the life-history is a complicated alternation of generations, with a sessile polymorphic hydroid stage from which the sexual jelly-fishes are produced by budding.

I shall now briefly sketch the more prominent features in the history of the process of specialization which has gradually evolved a complicated life-cycle like that of Dysmorphosa from one as simple and direct as that of Liriope, The parasitic jelly-fishes are peculiarly instructive in this connection. The genus Cunina includes a number of species which, while young, are parasites on other jelly-fishes. The free-swimming adult of one of them (Cunocantha octonaria) is shown in Fig. 9. It is quite

PSM V33 D760 Jelly fish cunocantha octonaria.jpg

Fig. 9.—Side view of Cunocantha octonaria, slightly magnified, drawn from Nature by W. K. Brooks.

common upon the coast of Virginia and North and South Carolina. The adult is not a parasite, but as soon as the larvæ hatch from the eggs they make their way into the bell of another jelly-fish, and live there as parasites until they complete their development and assume the adult form. The jelly-fish which affords a home for these parasites is shown at k in Fig. 15. It is known as Turritopsis.

The hydra which hatches from the egg of the Cunina is free, like the hydra-larva of Liriope. It has a short globular body, and PSM V33 D761 Hatched hydra of the cunina octonaria.jpgFig. 10.—The Hydra, which hatches from the egg of Cunina octonaria, drawn from nature by W. K. Brooks: o, body; p, month; f, tentacles. an enormously elongated proboscis, at the tip of which the mouth is situated (Fig. 10). It has four short tentacles which are turned backward away from the mouth, and are terminated by round knobs, which are used for clinging to the body of the Turritopsis, for as the parasitic larva sucks its food out of the stomach of its host, it does not need to use its tentacles for capturing living animals. As soon as it finds its way into the bell of a Turritopsis it fastens itself securely by its tentacles to its inner surface in the angle at the base of the stomach, where it is in no danger of being swept away by the current which the Turritopsis produces while swimming, and, once securely fastened, it bends down its long proboscis, passes it up through the mouth of the Turritopsis into its stomach, and sucks out the digested food.

Turritopsis is shown at k in Fig. 15; and Fig. 11, which I have copied from McCrady, the discoverer of this remarkable case of parasitism, shows the outline of the inner surface of the bell, and of the stomach of Turritopsis, with three of the parasitic Cunina larvæ in place, fastened by their tentacles, and with their mouths inserted into the stomach of their host.

Thus protected by the bell, and supplied with abundant food, which it neither captures nor digests, but sucks, all ready for assimilation, into its own stomach, the larva has a very "soft thing," and is naturally in no hurry to complete its development or to seek its fortune in the open water. It grows rapidly, acquires more tentacles, and, as its stomach grows larger, and it becomes able to suck in and to assimilate more food than it needs for its own growth, it gives rise to buds, which become parasitic hydras like itself, and remain attached to it and share all its advantages. The budding continues until a complicated colony of long proboscides, bodies, and tentacles is formed. A young colony of these larvæ is shown in Fig. 12, and an older one in Fig. 13.

PSM V33 D762 Turritopsis with parasitic cunian larvae.jpg

Fig. 11.—Outline of a Turritopsis, with parasitic Cunian larvæ, copied from McCrady.

The hydra larva of the Liriope is only a short transitional stage in the youth of the adult animal, but in Cunina the larval life has become vastly more important; and this is clearly due to the fact that it has found a home which is extremely favorable to it as a larva, an environment where all its wants are supplied, and where it enjoys so many advantages that the speedy acquisition of the wandering life and high organization of the adult is no longer desirable.

To all ordinary animals the period of infancy is full of danger. Young animals are encompassed on every side by peril from enemies, diseases, and accidents, and the prospect of long life increases enormously as childhood passes and maturity approaches.

Short infancy and rapid development are therefore, in ordinary cases, the conditions which are most favorable for the perpetuation of the species and the welfare of the individual: but this does not hold good of Cunina. The hydra stage has therefore been prolonged, and the larva has acquired the power to produce other larvæ to share its advantages. After a time, however, a flange or collar grows out from the body of each hydra, among the bases of the tentacles, as shown at e in Fig. 13; and, folding down toward the mouth, gives rise to a swim-bell and bell-cavity. The larva is then set free, and it escapes into the water as a young jelly-fish (Fig. 14), with an enormous proboscis (d), a relic of its parasitic

PSM V33 D763 Larvae of cunina and hydras transforming into medusae.jpg

Fig. 12.—A. colony of three young parasitic larvæ of Cnnina.
Fig. 13.—An older colony, consisting of six Hydras, some of which have begun to become transformed into Medusæ.

life, and a small bell (e), which, however, grows very rapidly, so that the animal soon assumes the adult form, shown in Fig. 9.

The life-history of this species of Cunina is given in the following diagram:

III. Cunina Octonaria.—Egg = Planula = Hydra = Medusa < eggs.
Hydra = Medusa < eggs.
Hydra = Medusa < eggs.
PSM V33 D764 Cunocantha parasitica.jpg
Fig. 14.

The egg becomes converted into a planula, this into a hydra, and this into a medusa, exactly as in the case of Liriope, except that the case is complicated by the budding of new hydras, each of them destined to become a medusa, from the body of the hydra which hatches from the egg, during its parasitic life, and before it becomes a medusa. Each Liriope-egg produces only one adult, while the number of adults which may be derived from a Cunina egg is quite large, although every individual in the series ultimately becomes an adult, and multiplies by sexual reproduction.

In another species of Cunina, Cunocantha parasitica, a new complication is introduced, for the hydra which hatches from the egg never becomes a jelly-fish, but remains a parasite as long as it lives, budding off other larvæ which grow up into adults. Its life-history is like this:

IV. Cunocantha Parasitica.—Egg = Planula = Hydra x Hydra = Medusa < eggs.
Hydra = Medusa < eggs.
Hydra = Medusa < eggs

If the hydras which are formed by budding were to remain as hydras, like the one which hatches from the egg, and were to bud off jelly-fish, we should have a life-history which is exhibited by many species, and is shown in this diagram:

Egg = Planula = Hydra x Hydra x Hydra = Medusa < eggs
Hydra = Medusa < eggs
Hydra x Hydra = Medusa < eggs
Hydra = Medusa < eggs

Turritopsis, the jelly-fish, which is infested by the Cuinina larvæ, has a life-history which is very similar to the one given in this diagram, with the addition of a slight but highly important modification.

PSM V33 D765 Turritopsis.jpg

Fig. 15.—Turritopsis.

The planula is shown in the left-hand lower corner of Fig. 2. It soon attaches itself to some solid body and becomes a root, which goes no further, but, as shown in the right-hand lower corner of Fig. 2, soon produces a bud which becomes a feeding hydra. Multiplication by budding now goes on rapidly, in such a way as to build up a branching, tree-like colony, with a feeding hydra at the tip of each twig. Two branches from one of these trees are shown in Fig. 15. Ultimately each of these hydras produces a number

PSM V33 D766 Eutima.jpg

Fig. 16.—Eutima.

of buds around the base of its body, as shown at B in the figure, and these buds ultimately become detached and grow up into the adult jelly-fish, K.

V. Turritopsis.—Egg = Planula = Root x Hydra x Medusa < eggs.
Medusa < eggs.
Hydra x Medusa < eggs.
Medusa < eggs.
Hydra x Medusa < eggs.
Medusa < eggs.

The life-history of Turritopsis is therefore like this, and the chain which connects the egg with the adult is broken three times, for the root, which is directly derived from the egg, goes no further, nor do the hydras which bud from the root become jelly-fish, and the latter form still a third set of individuals.

The larval life is long and important; the number of sexual adults produced by each egg is very great indeed, and the life-history is extremely complicated, but each one of the individuals is in the direct line of succession; for, while neither the root nor the hydras ever become converted into any higher form, the root produces hydras, and each one of these produces jelly-fish.

In the next species to be considered, a Eutima which is common on our coast (Fig. 16), another stage of complexity is introduced by the restriction of the power to bud jelly-fish to certain hydras, while others become specialized for nutrition. This specialization has come about gradually, and the various species of living hydroids exhibit all the steps in the process. In some species, as in Turritopsis, all the hydras perform both functions, and PSM V33 D767 The planula of eutima.jpgFig. 17. are alike in structure; in others, those which are placed at the tips of the branches and are best able to obtain food devote themselves to this purpose and produce no jelly-fish, while these are budded only from those hydras which are near the base of the colony. In some cases the two sets of hydras are alike in structure, but in other species the feeding hydras at the tips of the branches are very large, with capacious stomachs and long tentacles, while the reproductive hydras have small tentacles and mouths. In still other species, as in Eutima, they are true blastostyles, without mouths, and with rudimentary tentacles, and all the work of nutrition is performed by the feeding hydras.

The planula of Eutima is shown in Fig. 17. After a short swimming life, it fastens itself to some solid body, and elongating, becomes a root (Fig. 18); and a bud, m, soon grows out from it to form the first feeding hydra, which soon acquires a mouth (Fig. 19, l) and tentacles, i, and begins to capture and digest food and to accumulate a reserve of nutriment, while the root continues to throw

PSM V33 D768 Elongated root eventually feeding the hydra.jpg

Fig. 18.

out new buds, as shown in Fig. 18 at m. For a long time all the buds become feeding hydras; but at last, when the mouths are numerous enough, buds which remain mouthless are formed, and become the blastostyles or jelly-fish producers. The following diagram shows the life-history of Eutima:

VI. Eutima.—
Egg = Planula = Root x
Feeding hydra x Blastostyle x Medusa < eggs.
Feeding hydra Medusa < eggs.
Feeding hydra x Feeding hydra Medusa < eggs.
Blastostyle x Medusa < eggs.

Diagram No. 1, which was given in the beginning of this article, to illustrate the life of Dysmorphosa, shows the next stage in the process of complication, and a comparison will show that it is derivable from Diagram VI by slight changes, just as VI is derivable from V, and this from the preceding, and so on until finally we reach a simple, direct life-history, in which each egg produces one adult, which passes through a transitory larval hydra stage.

Forty years ago, a zoölogist of the old school might have believed that the life-history of Dysmorphosa has always been complex, and that of Liriope always simple; but the doctrine that all the representatives of any great group of animals owe their common characteristics to descent from a common ancestor is one of the fundamental principles of modern elementary zoölogy, and as this doctrine forms the basis rather than the aim of this article, I assume, without discussion, that the remote ancestors of Liriope and Dysmorphosa were the same, and that all the life-histories which have been described are modifications of that which was exhibited by this ancestor.

The series which has been given shows that this ancestor must have developed directly from the egg, its adult stage must have been the most important part of its life, and the hydra stage only a transitory larval condition. As, in certain lines of descent from

PSM V33 D769 Young hydroid colony of eutima.jpg

Fig. 19.—Young hydroid colony of Eutima.

this ancestor, the conditions of life became more and more favorable for the larvæ, and as successive generations of larvae became more and more adapted to these conditions, the larval life gradually increased in length and importance, and threw the adult sexual stage more and more into the background, until, in the case of Dysmorphosa, we have a colony of long-lived larvæ, which embody all that is most distinctive and characteristic of the species. and the adult jelly-fish lives only long enough to effect the wide distribution of the eggs, and the establishment of new colonies of larvæ.

Something very similar to this has occurred in a few insects. The caterpillar stage of most butterflies is simply a preparatory step directed toward an end, the production of the perfect insect; but the bagworm is a butterfly in which the larval life is most important, for, while the caterpillar lives long, the female insect never escapes from her cocoon, but after her final transformation lays her eggs within it and dies, while the male lives only long enough to find and fertilize the female, and then dies also.

In the case of the hydroids, the power of budding, a power which is almost absent in insects, enables the larval life to assume a degree of importance which it could not have if the larva remained simple, for it has rendered division of labor possible, and has produced polymorphic communities, most of the members of which are out of the line of succession. The gradual reduction of the adult life is also facilitated by the process of budding, as this effects a great increase in the number of adults which come from each egg, and thus secures the sexual reproduction of the species, notwithstanding the shortening of the life of each adult.

The shells of hermit-crabs often carry colonies of another hydroid, which is so similar to Dysmorphosa that a drawing of one will answer for the other. They are almost exactly alike, and it is only after careful examination that any difference between them is discovered; but, inconspicuous as the difference is, it is highly important, for in the second form, Hydractinia, the adult locomotor jelly-fish stage has been completely lost, and the whole life of the species has become centered in the larvæ. The blastostyles produce buds, which acquire some rudimentary traces of the organization of jelly-fish, but they never become free or complete their development. While still on the blastostyles, they produce eggs or spermatozoa, and having thus accomplished their purpose and secured the perpetuation of their race, they die. The life of Hydractinia is shown in Diagram VII:

VII. Hydractinia.
Egg = Planula = Root x
Feeding hydra x Feeding hydra Medusa < eggs.
Blastostyle x Medusa < eggs.
Defensive hydra Medusa < eggs.
Feeding hydra x Feeding hydra Medusa < eggs.
Blastostyle x Medusa < eggs.
Defensive hydra Medusa < eggs.
Feeding hydra x Feeding hydra Medusa < eggs.
Blastostyle x Medusa < eggs.
Defensive hydra Medusa < eggs.

This is by no means the end of the story, for the many species of hydroids without any jelly-fish stage present all stages in the gradual simplification of the sessile medusa buds, until at last all traces of the structure of the jelly-fish disappear, and they are degraded into simple accumulations of reproductive cells—reproductive organs—on the bodies of the hydroids.

No group of animals presents a more complete record of the process of evolution of species than the hydro-medusæ, and the comparative study of the different species gives, with a wealth of detail which is entirely beyond the scope of a short article, all the steps in the progress of modification. The minute gradations are so numerous that a long training is required to grasp them all without confusion, and to read the history which they exhibit, but those which I have selected are sufficient to illustrate the manner in which the larval life has gradually grown into prominence, and has become evolved and specialized, while the adult life has dropped more and more into the background, and has finally disappeared completely.