The Zoologist/4th series, vol 1 (1897)/Issue 672/Earthworm Studies II. Oviposition and Embryology

by Hilderic Friend

 

EARTHWORM STUDIES.

By the Rev. Hilderic Friend,

Author of 'Flowers and Flower-Lore.'

 

II. Oviposition and Embryology.

 

Our egg is not to be found in any of the famous collections on which oologists have spent fortunes, and for the acquisition of which museums have set apart large sums. Though no one ever lavished upon it such amounts as have been paid for an egg of the Great Auk or the extinct Dodo, it is practically as seldom seen, and as little known, as any of the rare eggs which collectors covet so ardently and prize so highly.

We usually think of eggs as consisting of a yolk surrounded by albumen, and enclosed in a hard shell made of lime or some form of calcium. The eggs of birds and fowls are our types, but the egg of the Earthworm has no chalk-like shell. Most eggs, together with their shells, are formed within the body of the egg-bearing animal, but this egg differs from the majority in this respect. It is true that the egg itself is formed as usual in the ovary, and passed through a tube known as the oviduct, but the shell or case is fabricated by the animal externally, and is slipped over the egg as it passes out of the oviduct and is about to be deposited. Eggs are very commonly laid in a nest, more or less elaborately constructed, and it is a rare thing for only one egg to be laid during the season by each individual. It has been correctly surmised that the number of eggs laid by a bird or other animal bears a close relation to the exigencies and dangers which the young will be likely to encounter. Hence a Pigeon lays only a single pair of eggs for each brood, while the Thrush deposits some half-dozen in its nest; and Partridges, Pheasants, Tits, and other birds lay from a dozen to a score. Then we find that Herrings and other fish lay enormous quantities of eggs as compared with many fresh-water species.

The egg of the Earthworm is never deposited in a well-formed nest. As a rule each specimen is found at a greater or lesser distance from its neighbour. As it is not laid in the open air, on the branches of trees (as the eggs of many insects are), or on the surface of the soil, like the eggs of the Ostrich or Peewit, but in damp places under the bark of trees, under stones by streams and ponds, or deep down in the moist soil, special provision has to be made for its development amid such peculiar surroundings. Or perhaps it would be more correct to say, that as the conditions differ so does the provision for meeting them.

If the eggs of a bird or fowl be varnished so as to exclude the air, or if they are enclosed in vessels, or buried in soil at a considerable depth, the young will never be hatched; yet here is an egg which can only be hatched when it is kept moist and cool, and one which may be buried at a depth of some inches, or even feet, in earth or under water, and yet retain its vitality.

The egg of the Earthworm is seldom more than a quarter of an inch in length, and, as it is usually oval, the shortest diameter is only about half that length. It was long ago pointed out that eggs almost invariably remain during the hatching period the same size as they were when first extruded, but here is a curious exception to the rule. We should look with amazement on a Pigeon's egg which increased in size till it became as large as a hen's egg during the time when the mother bird was sitting upon it, but this is exactly what happens in the egg before us during the hatching period. It both lengthens and widens, and we shall have to enquire how this is possible.

The naturalist is already well aware of the fact that when an animal regularly lays a large quantity of eggs of minute dimensions, the offspring is almost invariably unlike its parent, and has to undergo sundry transformations, changes, and developments before arriving at any degree of perfect resemblance to the adult form. Conversely, as in the case of birds, when a few relatively large eggs are laid the young usually emerges with a strong resemblance to its progenitor. The reason is obvious. A good deal of material is needed within the egg in order that a perfectly developed brood may emerge, and when the parent is compelled, through the struggle for existence, to launch a bevy of young on the sea of life, it cannot possibly fill the pockets of each (to speak metaphorically) with the almighty dollar, or provide that its offspring shall be started in life as are the progeny of individuals whose dangers are fewer and whose resources are more abundant. If we apply this argument to the egg in question we may conclude that its enemies are comparatively few. The colour of the horny egg-capsule is usually either a delicate olive-green or a light brown, and well harmonizes with its surroundings. The egg is, relatively to the size of the parent, large, and the number deposited comparatively small; while the young on emergence are found to be an exact copy of the original, an almost perfect reproduction of the parent.

It seems somewhat curious that an egg possessing so many peculiarities should have been almost absolutely ignored by scientific men and naturalists generally, the more so as it is easily obtained and readily examined. So far as I have been able to find during a long and extensive period of study, Swammerdam, who wrote the ' Book of Nature,' is almost the only observer who has devoted any attention to the systematic study of these eggs; but the result of his researches was such that he writes:—"Among all the eggs of insects, of which I have various species in my collection, I know none worthy of greater attention."

The case which contains the fluid matter out of which the future worm is to be evolved is of a horny, not of a calcareous, substance. It reminds us of the egg-capsules of the dog-fish, found everywhere on the sea-coast. Here we have a hint too of the aquatic origin of Earthworms. Chemically it corresponds almost exactly with our nails, and with the hoofs and horns of animals. It is cuticular in origin, that is, the skin, and not the blood, the spleen, or any other internal organ or substance, is the agent in its formation.

Everyone knows that the finger-nails are most easily trimmed after the hands have been washed in warm water. The reason is plain. Horny substances absorb moisture, and swell in proportion to the amount taken up, at the same time becoming soft and pliable. It may here be observed that several species of Earthworm, besides the semiaquatic Allurus, go through the process of oviposition under water. I have not seen this fact recorded by other observers, but have often myself discovered worms submerged on the margins of Derwentwater and elsewhere at this period. Slight chemical changes produce greater or lesser degrees of hardness in the substance. Hence horny substances are not all alike hard, and the horny capsule of the worm is tolerably elastic; so that when kept in a moist condition it can be slightly expanded by the internal pressure exerted by the growing worm. But how can the worm grow? The chick can become no larger than the shell-surrounded yolk and albumen will permit, but when the young worm is hatched it will very probably be an inch in length. Let us see how this contingency is provided for. The following illustration will help to make the matter plain. If we took a small tube of gelatine, and placed within its cavity a tiny globule, we could secure the contents of the tube by drawing the two ends to a point. If now the globule could expand on the application of moisture, it must either burst its case, cause it to expand in the direction of its shortest diameter, or force open the ends of the tube. Now the egg-capsule of the worm can expand slightly, but not to a sufficient extent to allow the worm to reach full dimensions. Consequently the embryo gradually forces open the sealed extremities of the case, and thus paves we way for its ultimate escape, at the same time that it loosens its swaddling bands, and develops little by little into a perfect worm. While the beak of the embryo bird develops and hardens within the shell sufficiently to enable it to peck its way out of the calcareous covering, the worm has no such tool for Opening its prison-house, and so these other means must be provided for its escape.

It may occur to some observant reader that a condition analogous to this is found in the case of the dung-flies' eggs, which are deposited with their horn-like projections upwards. In both instances, if the eggs are removed from their moist lodging-place, they shrivel and become lifeless. Worms again are not quite alone in the possession of the power to extend the egg-case during incubation. Huber long ago observed the same fact in relation to the eggs of ants, and those of certain sawflies can similarly expand to meet the requirements of the growing grub within.

The question now arises—How does the worm lay its eggs? Although many careful observations have been made for the purpose of deciding this question, I believe I had the good fortune to be the first to observe and record the actual process. If the different books which have been written on the subject of Annelids be examined, it will be found that they either pass over this question in silence, or give a very vague and unsatisfactory account of the process.^[1] Some time ago, however, a pleasant surprise was granted me. I was trimming up my flower-beds ready for Christmas. The soil was inhabited by a large number of Earthworms of various species. When I first began the study of these creatures only about ten British species were known. I have now raised the number to a quarter of a hundred. As I was examining the different species on the day in question, I presently detected a happy pair in the very act of manufacturing their cocoon. It was the first time in all my experience as a naturalist that such a treat had fallen to my lot, but I have since repeated the observation more than once on other species of Annelids. The process is as follows:—

When two worms are about to form an egg-case it is necessary that they should work in unison. One worm cannot do the work alone, though each worm is at the same time both male and female or hermaphrodite. A pair therefore approach each other from opposite directions, each having its head towards the other's tail. Near the middle of each adult worm is a swollen portion called the girdle or clitellum. This peculiar organ yields the horny substance of which the egg-case is formed, but it is at first soft and pliable, hardening after exposure to the air and cold. When the worms are ready for the process of oviposition the chitine is formed into a girdle around their two bodies, so that for the time being they are tied together. When the case is complete the necessary contents are poured into it from the two animals, after which they withdraw from each other backwards, and so allow the capsule or egg-case and its precious freight to slip over their heads and fall to the ground; the ends are then drawn together, and the cocoon left to its fate.

Should all the conditions be satisfactory, the egg soon begins to show signs of life. It is left to nature to hatch, and the time occupied in the process varies greatly. If an egg-case be opened after some time, a tiny embryo worm, or sometimes a pair, will be found inside, surrounded by a glutinous fluid. The young worm as it grows expands its case, and ultimately emerges—not as a caterpillar or larva, for it goes through no metamorphoses as does the butterfly or frog, the sawfly or even the fish, but as a worm; and now it has only to hasten development and become adult. The adult stage is reached when a girdle has been assumed, just as is sometimes the case among human beings.

It sometimes happens, however, that things do not go well. I have often observed that the eggs of worms are liable to be rendered abortive by the invasion of a smaller worm. It is another illustration of the amusing rhyme about the big fleas and little fleas. Into the life-history of these parasites, however, I must not now enter, as the subject requires a chapter to itself.

It would occupy too much space if I were to detail the wonderful process which goes on within the egg-case. The embryology of the worm has been fully studied, and is replete with marvels. If an egg is examined when the young embryo is almost ready to emerge, it will be possible, through the semitransparent and greatly dilated case, to watch the movements of the worm, trace the current of blood along the elaborate system of vessels, and eventually observe the emergence of the baby Annelid into the world.

For the further study of this intricate subject, I may refer the reader to Mr. Beddard's valuable Monograph, and the works which are there enumerated.

 

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1. Since this article was forwarded to the printer I have received from Dr. Ed. de Ribaucourt an extract from the 'Bulletin Scientifique,' vol. xxx. pp. 168–176, containing a "Notice Physiologique sur les Lumbricides d'Europe," in which reference is made to the act of copulation, but no allusion to the construction of the egg-case.