Seaside Studies in Natural History Part 1

Seaside Studies in Natural History.

by Elizabeth Cabot Cary Aga.s.siz and Alexander Aga.s.siz.

PREFACE.

This volume is published with the hope of supplying a want often expressed for some seaside book of a popular character, describing the marine animals common to our sh.o.r.es. There are many English books of this kind; but they relate chiefly to the animals of Great Britain, and can only have a general bearing on those of our own coast, which are for the most part specifically different from their European relatives. While keeping this object in view, an attempt has also been made to present the facts in such a connection, with reference to principles of science and to cla.s.sification, as will give it in some sort the character of a manual of Natural History, in the hope of making it useful not only to the general reader, but also to teachers and to persons desirous of obtaining a more intimate knowledge of the subjects discussed in it. With this purpose, although nearly all the ill.u.s.trations are taken from among the most common inhabitants of our bay, a few have been added from other localities in order to fill out this little sketch of Radiates, and render it, as far as is possible within such limits, a complete picture of the type.

A few words of explanation are necessary with reference to the joint authorship of the book. The drawings and the investigations, where they are not referred to other observers, have been made by MR. A.

AGa.s.sIZ, the ill.u.s.trations having been taken, with very few exceptions, from nature, in order to represent the animals, as far as possible, in their natural att.i.tudes; and the text has been written by MRS. L. AGa.s.sIZ, with the a.s.sistance of MR. A. AGa.s.sIZ's notes and explanations.

CAMBRIDGE, May, 1865.

MARINE ANIMALS OF Ma.s.sACHUSETTS BAY.

ON RADIATES IN GENERAL.

It is perhaps not strange that the Radiates, a type of animals whose home is in the sea, many of whom are so diminutive in size, and so light and evanescent in substance, that they are hardly to be distinguished from the element in which they live, should have been among the last to attract the attention of naturalists. Neither is it surprising to those who know something of the history of these animals, that when the investigation of their structure was once begun, when some insight was gained into their complex life, their a.s.sociation in fixed or floating communities, their wonderful processes of development uniting the most dissimilar individuals in one and the same cycle of growth, their study should have become one of the most fascinating pursuits of modern science, and have engaged the attention of some of the most original investigators during the last half century. It is true that from the earliest days of Natural History, the more conspicuous and easily accessible of these animals attracted notice and found their way into the scientific works of the time. Even Aristotle describes some of them under the names of Acalephae and Knidae, and later observers have added something, here and there, to our knowledge on the subject; but it is only within the last fifty years that their complicated history has been unravelled, and the facts concerning them presented in their true connection.

Among the earlier writers on this subject we are most indebted to Rondelet, in the sixteenth century, who includes some account of the Radiates, in his work on the marine animals of the Mediterranean. His position as Professor in the University at Montpelier gave him an admirable opportunity, of which he availed himself to the utmost, for carrying out his investigations in this direction. Seba and Klein, two naturalists in the North of Europe, also published at about this time numerous ill.u.s.trations of marine animals, including Radiates. But in all these works we find only drawings and descriptions of the animals, without any attempt to cla.s.sify them according to common structural features. In 1776, O. F. Muller, in a work on the marine and terrestrial faunae of Denmark, gave some admirable figures of Radiates, several of which are identical with those found on our own coast.

Cavolini also in his investigations on the lower marine animals of the Mediterranean, and Ellis in his work upon those of the British coast, did much during the latter half of the past century to enlarge our knowledge of them.

It was Cuvier, however, who first gave coherence and precision to all previous investigations upon this subject, by showing that these animals are united on a common plan of structure expressively designated by him under the name Radiata. Although, from a mistaken appreciation of their affinities, he a.s.sociated some animals with them which do not belong to the type, and have since, upon a more intimate knowledge of their structure, been removed to their true positions; yet the principle introduced by him into their cla.s.sification, as well as into that of the other types of the animal kingdom, has been all important to science.

It was in the early part of this century that the French began to a.s.sociate scientific objects with their government expeditions.

Scarcely any important voyage was undertaken to foreign countries by the French navy which did not include its corps of naturalists, under the patronage of government. Among the most beautiful figures we have of Radiates, are those made by Savigny, one of the French naturalists who accompanied Napoleon to Egypt; and from this time the lower marine animals began to be extensively collected and studied in their living condition. Henceforth the number of investigators in the field became more numerous, and it may not be amiss to give here a slight account of the more prominent among them.

Darwin's fascinating book, published after his voyage to the Pacific, and giving an account of the Coral islands, the many memoirs of Milne Edwards and Haime, and the great works of Quoy and Gaimard, and of Dana, are the chief authorities upon Polyps. In the study of the European Acalephs we have a long list of names high in the annals of science. Eschscholtz, Peron and Lesueur, Quoy and Gaimard, Lesson, Mertens, and Huxley, have all added largely to our information respecting these animals, their various voyages having enabled them to extend their investigations over a wide field. No less valuable have been the memoirs of Kolliker, Leuckart, Gegenbaur, Vogt, and Haeckel, who in their frequent excursions to the coasts of Italy and France have made a special study of the Acalephs, and whose descriptions have all the vividness and freshness which nothing but familiarity with the living specimens can give. Besides these, we have the admirable works of Von Siebold, of Ehrenberg, the great interpreter of the microscopic world, of Steenstrup, Dujardin, Dalyell, Forbes, Allman, and Sars. Of these, the four latter were fortunate in having their home on the sea-sh.o.r.e within reach of the objects of their study, so that they could watch them in their living condition, and follow all their changes. The charming books of Forbes, who knew so well how to popularize his instructions, and present scientific results under the most attractive form, are well known to English readers. But a word on the investigations of Sars may not be superfluous.

Born near the coast of Norway, and in early life a.s.sociated with the Church, his pa.s.sion for Natural History led him to employ all his spare time in the study of the marine animals immediately about him, and his first papers on this subject attracted so much attention, that he was offered the place of Professor at Christiania, and henceforth devoted himself exclusively to scientific pursuits, and especially to the investigation of the Acalephs. He gave us the key to the almost fabulous transformations of these animals, and opened a new path in science by showing the singular phenomenon of the so-called "alternate generations," in which the different phases of the same life may be so distinct and seemingly so disconnected that, until we find the relation between them, we seem to have several animals where we have but one.

To the works above mentioned, we may add the third and fourth volumes of Professor Aga.s.siz's Contributions to the Natural History of the United States, which are entirely devoted to the American Acalephs.

The most important works and memoirs concerning the Echinoderms are those by Klein, Link, Johannes Muller, Jager, Desmoulins, Troschel, Sars, Savigny, Forbes, Aga.s.siz, and Lutken, but excepting those of Forbes and Sars, few of these observations are made upon the living specimens. It may be well to mention here, for the benefit of those who care to know something more of the literature of this subject in our own country, a number of memoirs on the Radiates of our coasts, published by the various scientific societies of the United States, and to be found in their annals. Such are the papers of Gould, Aga.s.siz, Leidy, Stimpson, Ayres, McCrady, Clark, A. Aga.s.siz, and Verrill.

One additional word as to the manner in which the subjects included in the following descriptions are arranged. We have seen that Cuvier recognized the unity of plan in the structure of the whole type of Radiates. All these animals have their parts disposed around a common central axis, and diverging from it toward the periphery. The idea of bilateral symmetry, or the arrangement of parts on either side of a longitudinal axis, on which all the higher animals are built, does not enter into their structure, except in a very subordinate manner, hardly to be perceived by any but the professional naturalist. This radiate structure being then common to the whole type, the animals composing it appear under three distinct structural expressions of the general plan, and according to these differences are divided into three cla.s.ses,--Polyps, Acalephs, and Echinoderms. With these few preliminary remarks we may now take up in turn these different groups, beginning with the lowest, or the Polyps.[1]

[Footnote 1: It is to be regretted that on account of the meagre representations of Polyps on our coast, where the coral reefs, which include the most interesting features of Polyp life, are entirely wanting, our account of these animals is necessarily deficient in variety of material. When we reach the Acalephs or Jelly-Fishes, in which the fauna of our sh.o.r.es is especially rich, we shall not have the same apology for dulness; and it will be our own fault if our readers are not attracted by the many graceful forms to which we shall then introduce them.]

GENERAL SKETCH OF THE POLYPS.

Before describing the different kinds of Polyps living on our immediate coast, we will say a few words of Polyps in general and of the mode in which the structural plan common to all Radiates is adapted to this particular cla.s.s. In all Polyps the body consists of a sac divided by vertical part.i.tions (Fig. 1.) into distinct cavities or chambers. These part.i.tions are not, however, all formed at once, but are usually limited to six at first, multiplying indefinitely with the growth of the animal in some kinds, while in others they never increase beyond a certain definite number. In the axis of the sac, thus divided, hangs a smaller one, forming the digestive cavity, and supported for its whole length by the six primary part.i.tions. The other part.i.tions, though they extend more or less inward in proportion to their age, do not unite with the digestive sac, but leave a free s.p.a.ce in the centre between their inner edge and the outer wall of the digestive sac. The genital organs are placed on the inner edges of the part.i.tions, thus hanging as it were at the door of the chambers, so that when hatched, the eggs naturally drop into the main cavity of the body, whence they pa.s.s into the second smaller sac through an opening in its bottom or digestive cavity, and thence out through the mouth into the water. In the lower Polyps, as in our common Actinia for instance, these organs occur on all the radiating part.i.tions, while among the higher ones, the Halcyonoids for example, they are found only on a limited number. This limitation in the repet.i.tion of identical parts is always found to be connected with structural superiority.

[Ill.u.s.tration: Fig. 1. Transverse section of an Actinia.

(_Aga.s.siz_.)]

The upper margin of the body is fringed by hollow tentacles, each of which opens into one of the chambers. All parts of the animal thus communicate with each other, whatever is introduced at the mouth circulating through the whole structure, pa.s.sing first into the digestive cavity, thence through the opening in the bottom into the main chambered cavity, where it enters freely into all the chambers, and from the chambers into the tentacles. The rejected portions of the food, after the process of digestion is completed, return by the same road and are thrown out at the mouth.

These general features exist in all Polyps, and whether they lead an independent life as the Actinia, or are combined in communities, like most of the corals and the Halcyonoids; whether the tentacles are many or few; whether the part.i.tions extend to a greater or less height in the body; whether they contain limestone deposit, as in the corals, or remain soft throughout life as the sea-anemone,--the above description applies to them all, while the minor differences, either in the tentacles or in the form, size, color, and texture of the body, are simply modifications of this structure, introducing an infinite variety into the cla.s.s, and breaking it up into the lesser groups designated as orders, families, genera, and species. Let us now look at some of the divisions thus established.

The cla.s.s of Polyps is divided into three orders,--the Halcyonoids, the Madreporians, and the Actinoids. Of the lowest among these orders, the Actinoid Polyps, our Actinia or sea-anemone is a good example.

They remain soft through life, having a great number of part.i.tions and consequently a great number of tentacles, since there is a tentacle corresponding to every chamber. Indeed, in this order the multiplication of tentacles and part.i.tions is indefinite, increasing during the whole life of the animal with its growth; while we shall see that in some of the higher orders the constancy and limitation in the number of these parts is an indication of superiority, being accompanied by a more marked individualization of the different functions.

Next come the Madreporians, of which our Astrangia, to be described hereafter, may be cited as an example. In this group, although the number of tentacles still continues to be large, they are nevertheless more limited than in the Actinoids; but their characteristic feature is the deposition of limestone walls in the centre of the chambers formed by the soft part.i.tions, so that all the soft part.i.tions alternate with hard ones. The tentacles, always corresponding to the cavity of the chambers, may be therefore said to ride this second set of part.i.tions arising just in the centre of the chambers.

The third and highest order of Polyps is that of the Halcyonoids. Here the part.i.tions are reduced to eight; the tentacles, according to the invariable rule, agree in number with the chambers, but have a far more highly complicated structure than in the lower Polyps. Some of these Halcyonoids deposit limestone particles in their frame. But the tendency to solidify is not limited to definite points, as in the Madreporians. It may take place anywhere, the rigidity of the whole structure increasing of course in proportion to the acc.u.mulation of limestone. There are many kinds, in which the axis always remains soft or cartilaginous, while others, as the so-called sea-fans for instance, well known among the corals for their beauty of form and color, are stiff and hard throughout. Whatever their character in this respect, however, they are always compound, living in communities, and never found as separate individuals after their early stages of growth. Some of those with soft axis lead a wandering life, enjoying as much freedom of movement as if they had an individual existence, shooting through the water like the Pennatulae, well known on the California coast, or working their way through the sand like the Renilla, common on the sandy sh.o.r.es of our Southern States.

ACTINOIDS.

_Actinia, or Sea-Anemone_. (_Metridium marginatum_ EDW.)

Nothing can be more unprepossessing than a sea-anemone when contracted. A mere lump of brown or whitish jelly, it lies like a lifeless thing on the rock to which it clings, and it is difficult to believe that it has an elaborate and exceedingly delicate internal organization, or will ever expand into such grace and beauty as really to deserve the name of the flower after which it has been called.

Figs. 2, 3, 4, and 5, show this animal in its various stages of expansion and contraction. Fig. 2 represents it with all its external appendages folded in, and the whole body flattened; in Fig. 3, the tentacles begin to steal out, and the body rises slightly; in Fig. 4, the body has nearly gained its full height, and the tentacles, though by no means fully spread, yet form a delicate wreath around the mouth; while in Fig. 5, drawn in life size, the whole summit of the body seems crowned with soft, plumy fringes. We would say for the benefit of collectors that these animals are by no means difficult to find, and thrive well in confinement, though it will not do to keep them in a small aquarium with other specimens, because they soon render the water foul and unfit for their companions. They should therefore be kept in a separate gla.s.s jar or bowl, and under such circ.u.mstances will live for a long time with comparatively little care.

[Ill.u.s.tration: Figs. 2, 3, 4. Actinia in different degrees of expansion. (_Aga.s.siz_.)]

[Ill.u.s.tration: Fig. 5. The same Actinia (Metridium marginatum) fully expanded; natural size.]

They may be found in any small pools about the rocks which are flooded by the tide at high water. Their favorite haunts, however, where they occur in greatest quant.i.ty are more difficult to reach; but the curious in such matters will be well rewarded, even at the risk of wet feet and a slippery scramble over rocks covered with damp sea-weed, by a glimpse into their more crowded abodes. Such a grotto is to be found on the rocks of East Point at Nahant. It can only be reached at low tide, and then one is obliged to creep on hands and knees to its entrance, in order to see through its entire length; but its whole interior is studded with these animals, and as they are of various hues, pink, brown, orange, purple, or pure white, the effect is like that of brightly colored mosaics set in the roof and walls. When the sun strikes through from the opposite extremity of this grotto, which is open at both ends, lighting up its living mosaic work, and showing the play of the soft fringes wherever the animals are open, it would be difficult to find any artificial grotto to compare with it in beauty. There is another of the same kind on Saunders's Ledge, formed by a large boulder resting on two rocky ledges, leaving a little cave beneath, lined in the same way with variously colored sea-anemones, so closely studded over its walls that the surface of the rock is completely hidden. They are, however, to be found in larger or smaller cl.u.s.ters, or scattered singly in any rocky fissures, overhung by sea-weed, and accessible to the tide at high water.

The description of Polyp structure given above includes all the general features of the sea-anemone; but for the better explanation of the figures, it may not be amiss to recapitulate them here in their special application. The body of the sea-anemone may be described as a circular, gelatinous bag, the bottom of which is flat and slightly spreading around the margin. (Fig. 2.) The upper edge of this bag turns in so as to form a sac within a sac. (Fig. 6.) This inner sac, _s_, is the stomach or digestive cavity, forming a simple open s.p.a.ce in the centre of the body, with an aperture in the bottom, _b_, through which the food pa.s.ses into the larger sac, in which it is enclosed. But this outer and larger sac or main cavity of the body is not, like the inner one, a simple open s.p.a.ce. It is, on the contrary, divided by vertical part.i.tions into a number of distinct chambers, converging from the periphery to the centre. These part.i.tions do not all advance so far as actually to join the wall of the digestive cavity hanging in the centre of the body, but most of them stop a little short of it, leaving thus a small, open s.p.a.ce between the chambers and the inner sac. (Fig. 1.) The eggs hang on the inner edge of the part.i.tions; when mature they drop into the main cavity, enter the inner digestive cavity through its lower opening, and are pa.s.sed out through the mouth.

[Ill.u.s.tration: Fig. 6. Vertical section of an Actinia, showing a primary _(g)_ and a secondary part.i.tion of _g'_; _o_ mouth, _t_ tentacles, _s_ stomach, _f f_ reproductive organs, _b_ main cavity, _c_ openings in part.i.tions, _a_ lower floor, or foot.]

The embryo bears no resemblance to the mature animal. It is a little planula, semi-transparent, oblong, entirely covered with vibratile cilia, by means of which it swims freely about in the water till it establishes itself on some rocky surface, the end by which it becomes attached spreading slightly and fitting itself to the inequalities of the rock so as to form a secure basis. The upper end then becomes depressed toward the centre, that depression deepening more and more till it forms the inner sac, or in other words the digestive cavity described above. The open mouth of this inner sac, which may, however, be closed at will, since the whole substance of the body is exceedingly contractile, is the oral opening or so-called mouth of the animal. We have seen how the main cavity becomes divided by radiating part.i.tions into numerous chambers; but while these internal changes are going on, corresponding external appendages are forming in the shape of the tentacles, which add so much to the beauty of the animal, and play so important a part in its history. The tentacles, at first only few in number, are in fact so many extensions of the inner chambers, gradually narrowing upward till they form these delicate hollow feelers which make a soft downy fringe all around the mouth.

(Fig. 7.) They do not start abruptly from the summit, but the upper margin of the body itself thins out to form more or less extensive lobes, through which the part.i.tions and chambers continue their course, and along the edge of which the tentacles arise.

[Ill.u.s.tration: Fig. 7. View from above of an Actinia with all its tentacles expanded; _o_ mouth, _b_ crescent-shaped folds at extremity of mouth, _a a_ folds round mouth, _t t t_ tentacles.]

[Ill.u.s.tration: Figs. 8, 9. Young Actiniae in different stages of growth.]

The eggs are not always laid in the condition of the simple planula described above. They may, on the contrary, be dropped from the parent in different stages of development, sometimes even after the tentacles have begun to form, as in Figs. 8, 9. Neither is it by means of eggs alone that these animals reproduce themselves; they may also multiply by a process of self-division. The disk of an Actinia may contract along its centre till the circular outline is changed to that of a figure 8, this constriction deepening gradually till the two halves of the 8 separate, and we have an Actinia with two mouths, each surrounded by an independent set of tentacles. Presently this separation descends vertically till the body is finally divided from summit to base, and we have two Actiniae where there was originally but one. Another and a far more common mode of reproduction among these animals is that of budding like corals. A slight swelling arises on the side of the body or at its base; it enlarges gradually, a digestive cavity is formed within it, tentacles arise around its summit, and it finally drops off from the parent and leads an independent existence. As a number of these buds are frequently formed at once, such an Actinia, surrounded by its little family, still attached to the parent, may appear for a time like a compound stock, though their normal mode of existence is individual and distinct.