Part 18

The egg of the Star-fish, when first formed, is a transparent, spherical body, enclosing the germinative vesicle and dot. (See Fig. 155.) As soon as these disappear, the segmentation of the yolk begins; it divides first into two portions (see Fig. 156), then into four, then into eight, and so on; but when there are no more than eight bodies of segmentation (see Fig. 157), they already show a disposition to arrange themselves in a hollow sphere, enclosing a space within, and by the time the segmentation is completed, they form a continuous spherical shell. At this time the egg, or, as we will henceforth call it, the embryo, escapes and swims freely about. (See Fig. 158.) The wall next begins to thin out on one side, while on the opposite side, which by comparison becomes somewhat bulging, a depression is formed (m a, Fig. 159), gradually elongating into a loop hanging down within the little animal, and forming a digestive cavity. (d, Fig. 160.) At this stage it much resembles a young Actinia. The loop spreads somewhat at its upper extremity, and at its lower end is an opening, which at this period of the animal's life serves a double purpose, that of mouth and anus also, for at this opening it both takes in and rejects its food. We shall see that before long a true mouth is formed, after which this first aperture takes its place opposite the mouth, retaining only the function of the anus. Presently from the upper bulging extremity of the digestive cavity, two lappets, or little pouches, project (w w' Fig. 161); they shortly become completely separated from it, and form two distinct hollow cavities (w w', Fig. 162). Here begins the true history of the young Star-fish, for these two cavities will develop into two water-tubes, on one of which the back of the Star-fish, that is, its upper surface, covered with spines, will be developed, while on the other, the lower surface, with the suckers and tentacles, will arise. At a very early stage one of these water-tubes (w', Fig. 163) connects with a smaller tube opening outwards, which is hereafter to be the madreporic body (b, Fig. 163). Almost until the end of its growth, these two surfaces, as we shall see, remain separate, and form an open angle with one another; it is only toward the end of the development that they unite, enclosing between them the internal organs, which have been built up in the mean while.

At about the same time with the development of these two pouches, so important in the animal's future history, the digestive cavity becomes slightly curved, bending its upper end sideways till it meets the outer wall, and forms a junction with it (m, Fig. 164). At this point, when the juncture takes place, an aperture is presently formed, which is the true mouth. The digestive sac, which has thus far served as the only internal cavity, now contracts at certain distances, and forms three distinct, though connected cavities, as in Fig. 163; viz. the oesophagus leading directly from the mouth (m) to the second cavity or stomach (d), which opens in its turn into the third cavity, the alimentary canal. Meanwhile the water-tubes have been elongating till they now surround the digestive cavity, extending on the other side of it beyond the mouth, where they unite, thus forming a Y-shaped tube, narrowing at one extremity, and dividing into two branches toward the other end. (Fig. 165.)

On the surface where the mouth is formed, and very near it on either side, two small arcs arise, as v in Fig. 162; these are cords consisting entirely of vibratile cilia. They are the locomotive organs of the young embryo, and they gradually extend until they respectively enclose nearly the whole of the upper and lower half of the body, forming two large shields or plastrons. (Figs. 165, 166.) The corners of these shields project, slightly at first (Fig. 165), but elongating more and more until a number of arms are formed, stretching in various directions (Figs. 166, 167), and, by their constant upward and downward play, moving the embryo about in the water.

At this stage of the growth of the embryo, we have what seems quite a complicated structure, and might be taken for a complete animal; this is after all but the prelude to its true Star-fish existence. While these various appendages of the embryo have been forming, changes of another kind have taken place; on one of the two water-tubes above mentioned (w'), at the end nearest the digestive cavity, a number of lobes are formed (t, Fig. 166); this is the first appearance of the tentacles. In the same region of the opposite water-tube (w) a number of little limestone rods arise, which eventually unite to form a continuous network; this is the beginning of the back of the Star-fish (r, Fig. 166), from which the spines will presently project. When this process is complete, the whole embryo, with the exception of the part where the young Star-fish is placed, grows opaque; it fades, as it were, begins to shrink and contract, and presently drops to the bottom, where it attaches itself by means of short arms (f f', Fig. 166), covered with warts, which act as suckers, and are placed just above the mouth. As soon as the Star-fish has thus secured itself, it begins to resorb the whole external structure described above; the water-tubes, the plastrons, and the complicated system of arms connected with them, disappear within the little Star-fish; it swallows up, so to speak, the first stage of its own existence; it devours its own larva, which now becomes part and parcel of the new animal. Next the two surfaces, the back and lower surface, on which the arms are now marked out, while the tentacles, suckers, and spines have already assumed a certain prominence, approach each other. At this time, however, the arms are not in one plane; both the back and the lower surface are curved in a kind of spiral; they begin to flatten; the arms spread out on one level,--and now the two surfaces draw together, meeting at the circumference, and enclosing between them the internal organs, which, as we have seen, are already formed and surrounded by walls of their own, before the two walls of the body, close thus over them. Fig. 168 represents the upper surface of the Star-fish just before this junction takes place. The complicated structure of the Brachiolaria, as the larva of the Star-fish has been called, hitherto so essential to the life of the animal, by which it has been supported, moved about in the water, and provided with food during its immature condition, has made a final contribution to its further development by the process of resorption described above, and has wholly disappeared within the Star-fish. At this stage the rays are only just marked out, as five lobes around the margin; Fig. 169 represents the lower surface at the same moment, with the open mouth (m), around which the tentacles (t) are just beginning to appear; while Fig. 170 shows us the animal at a more advanced stage, after the two surfaces have united. It has now somewhat the outline of a Maltese cross, the five arms being more distinctly marked out, while the tentacles have already attained a considerable length (Fig. 171), and the dorsal plates have become quite distinct. Fig. 172 represents the same animal, at the same age, in profile. This period, in which we have compared the form of the Star-fish to that of a Maltese cross, is one of long duration; two or three years must elapse before the arms will elongate sufficiently to give it a star-shaped form, and before the pedicellariæ make their appearance, and it is only then that it can be at once recognized as the young of our common Star-fish. Even then, after it has assumed its ultimate outline, it lacks some features of the adult, having only two rows of tentacles, whereas the full-grown Star-fish has four.

Sea-urchins.

This extraordinary process of development which we have analyzed thus at length in the history of the Star-fish, but which is equally true of all Echinoderms, has been hitherto described (so far as it was known) under the name of the plutean stages of growth. In these early stages the young, or the so called larvæ of Echinoderms, have received the name of Pluteus on account of their ever-changing forms. Let us look for a moment at the plutean stages of the Sea-urchin, as they differ in some points from those of the Star-fish. In the Pluteus of our common Sea-urchins (see Fig. 176), the arms are supported by a framework of solid limestone rods, which do not exist in that of the Star-fish, and which give to the larva of the Sea-urchin a remarkable rigidity. They are formed very early, as may be seen in Fig. 173, representing the little Sea-urchin before any arms are discernible, though the limestone rods are quite distinct. Figs. 173, 174, 175, may be compared with Figs. 160, 162, 165, of the young Star-fish, where it will be seen that the general outline is very similar, though, on account of the limestone rods, the Pluteus of the Sea-urchin seems somewhat more complicated. In Fig. 176 the young Sea-urchin has so far encroached upon the Pluteus that it forms the essential part of the body, the arms and rods appearing as mere appendages. Fig. 177 shows the same animal when we looked down upon it in its natural attitude; the Sea-urchin is carried downward, and the arms stretch in every direction around it. In Fig. 178 the Plutens is already in process of absorption; in Fig. 179 it has wholly disappeared; in Figs. 180 and 181 we have different stages of the little Sea-urchin, with its spines and suckers of a large size and in full activity. The appearance of the Sea-urchin, as soon as this larva or Pluteus is completely absorbed, is much more like that of the adult than is the Star-fish at the same stages, in which, as we have seen, there is a transition period of considerable duration.

Ophiurans.

Fig. 183 represents an Ophiuran undergoing the same process of growth, at a period when the larva is most fully developed, and before it begins to fail. By the limestone rods which support the arms, the Pluteus of the Ophiuran, here represented, resembles that of the Sea-urchin more than that of the Star-Fish, while by the character of the water-tubes and by its internal organization it is more closely allied to the latter. It differs from both, however, in the immense length of two of the arms; these arms being the last signs of its plutean condition to disappear; when the young Ophiuran has absorbed almost the whole Pluteus, it still goes wandering about with these two immense appendages, which finally share the fate of all the rest. Fig. 182 represents an Ophiuran at the moment when the process of resorption is nearly completed, though the arms of the Pluteus, greatly diminished, are still to be seen protruding from the surface of the animal.

This mode of development, though common to all Echinoderms, appears under very different conditions in some of them. There are certain Star-fishes, Ophiurans, and Holothurians, passing through their development under what is known as the sedentary process. The eggs are not laid, as in the cases described above, but are carried in a sort of pouch over the mouth of the parent animal, where they remain till they attain a stage corresponding to that of Fig. 168 of the Star-fish, and having much the same cross-shaped outline, when they escape from the pouch (as the young Ophiopholis, Fig. 184), and swim about for the first time as free animals. Fig. 185 represents a cluster of young Star-fishes of the sedentary kind at about this period. But while this mode of growth seems at first sight so different, we shall find, if we look a little closer, that it is essentially the same, and that, though the circumstances under which the development takes place are changed, the process does not differ. The little Star-fish or Ophiuran, in the pouch, becomes surrounded by the same plutean structure as those which are laid in the egg; it is only more contracted to suit the narrower space in which they have to move; and the water-tubes on which the upper and lower surfaces of the body arise, the shields, spreading out into arms at the corners, exist, fully developed or rudimentary, in the one as much as in the other, and when no longer necessary to its external existence they are resorbed in the same way in both cases. This singular process of development has no parallel in the animal kingdom, although the growth of the young Echinoderm on the Brachiolaria may at first sight remind us of the budding of the little Medusa on the Hydroid stock, or even of the passage of the insect larva into the chrysalis. But in both these instances, the different phases of the development are entirely distinct; the Hydroid stock is permanent, continuing to live and grow and perform its share in the cycle of existence to which it belongs, after the Medusa has parted from it to lead a separate life, or if the latter remains attached to the parent stock, after it has entered upon its own proper functions. The life of the caterpillar, chrysalis and butterfly, is also distinct and definitely marked; the moment when the animal passes from one into the other cannot be mistaken, although the different phases are carried on successively and not simultaneously, as in the case of the Acalephs. But in the Echinoderms, on the contrary, though the aspect of the Brachiolaria, or plutean stage, is so different from that of the adult form, that no one would suppose them to belong to the same animal, yet these two stages of growth pass so gradually into one another, that one cannot say when the life of the larva ceases, and that of the Echinoderm begins.

The bearing of embryology upon classification is becoming every day more important, rendering the processes of development among animals one of the most interesting and instructive studies to which the naturalist can devote himself, in the present state of his science. The accuracy of this test, not only as explaining the relations between animals now living, but as giving the clew to their connection with those of past times, cannot but astonish any one who makes it the basis of his investigations. The comparison of embryo forms with fossil types is of course difficult, and must in many instances be incomplete, for while, in the one case, death and decay have often half destroyed the specimen, in the other, life has scarcely stamped itself in legible characters on the new being. Yet, whenever such comparisons have been successfully carried out, the result is always the same; the present representatives of the fossil types recall in their embryonic condition the ancient forms, and often explain their true position in the animal kingdom. One of the most remarkable examples of this in the type we are now considering, is that of the Comatula already mentioned. Its condition in the earlier stages of growth, when it is provided with a stem, at once shows its relation to the old stemmed Crinoids, the earliest representatives of the class of Echinoderms.

These coincidences are still more striking among living animals, where they can be more readily and fully traced, and often give us a key to their relative standing, which our knowledge of their anatomical structure fails to furnish. This is perhaps nowhere more distinctly seen than in the type of Radiates, where the Acalephs in their first stages of growth, that is, in their Hydroid condition, remind us of the adult forms among Polyps, showing the structural rank of the Acalephs to be the highest, since they pass beyond a stage which is permanent with the Polyps; while the adult forms of the Acalephs have in their turn a certain resemblance to the embryonic phases of the class next above them, the Echinoderms. Within the limits of the classes, the same correspondence exists as between the different orders; the embryonic forms of the higher Polyps recall the adult forms of the lower ones, and the same is true of the Acalephs as far as these phenomena have been followed and compared among them. In the class of Echinoderms the comparison has been carried out to a considerable extent, their classification has hitherto been based chiefly upon the ambulacral system, so characteristic of the class, but so unequally developed in the different orders. This places the Holothurians, in which the ambulacral system has its greatest development, at the head of the class; next to them come the Sea-urchins or Echinoids; then the Star-fishes; then the Ophiurans and Crinoids, in which the ambulacral system is reduced to a minimum. Another basis for classification in this type, which gives the same result, is the indication of a bilateral symmetry in some of the orders. In the Holothurians, for instance, there is a decided tendency toward the establishment of a posterior and anterior extremity, of a right and left, an upper and lower side of the body. In the Sea-urchins, in many of which the mouth is out of centre, placed nearer one side than the other, this tendency is still apparent, while in the three lower groups, the Star-fishes, Ophiurans, and Crinoids, it is almost entirely lost, in the equal division of identical parts radiating from a common centre. A comparison of the embryonic and adult forms in these orders, confirms entirely this classification based upon structural features. The Star-fishes, in their earlier stages, resemble the mature Ophiurans, while the Crinoids, the lowest group of all, retain throughout their whole existence many features characteristic of the embryonic conditions of the higher Echinoderms. In this principle of classification, already so fertile in results, we may hope to find, in some instances, the solution of many perplexing points respecting the structural rank of animals, the confirmation of classifications already established; in others, an insight into the true relations of groups which have hitherto been divided upon purely arbitrary grounds.

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DISTRIBUTION OF LIFE IN THE OCEAN.

We have seen that while our bay is rich in certain species, it is wholly deficient or but scantily supplied with others, and that the character of the animals inhabiting its waters is more or less directly connected with general physical conditions. Such an area, limited though it be, gives us some insight into the laws which, in their wider application, control the distribution of marine life along the shores of the most extensive continents. The coast of Massachusetts, taken as a whole, is like that of New England generally, a rocky coast; yet it has its sandy and muddy beaches, and though it lies for a great part open to the sea, it has nevertheless its sheltered harbors, its quiet bays and snug recesses.

A comparison of these limited localities with far more extensive reaches of shore, where similar physical conditions prevail, shows that they reproduce, in fainter and less various characters of course, in proportion to their narrower boundaries, but still with a certain fidelity, the same combinations of animal and vegetable life. In other words, a sandy beach, however small, gives us some idea of the nature of the animals we may look for on any sandy coast, as, for instance, clams of various kinds, razor-shells, quahogs, snails, &c., creatures who can penetrate the sand, drag themselves through it or over it, leaving their winding trails as they go, and to whom the conditions prevailing in such spots are genial. So the narrowest mud flat on the sea-shore or muddy beach will give us the same dead and inanimate aspect which characterizes a more extensive coast of like character, where the gases always generated in mud are deadly to many kinds of animals. The beings who find a home in such localities are of closely allied species, chiefly a variety of worms, who burrow their way into the mud, and seem to court the miasma so fatal to other creatures. The same is true of any stony beach or rocky shore not more than a quarter of a mile in length; it gives us an idea of the animal population on any similar coast of greater extent.

These correspondences are of course modified by differences in climatic conditions. The animals on a sandy beach or a rocky shore, on the coast of Great Britain, for instance, are not absolutely identical with those of a sandy beach or a rocky shore on the coast of New England, but they are more or less nearly related to them. Naturalists refer to this reiteration, all the world over, of like organic combinations under similar circumstances, when they speak of "representative species." The aggregate result is the same, though the individual forms are slightly modified. And here lies one secret of the infinite variety in nature, by which the old seems ever new, and the same thought has an eternal freshness and originality, endlessly repeated, yet never hackneyed.

In this sense our bay presents, on a miniature scale, a variety of physical and organic combinations, which may be compared to those more extensive divisions in the geographical distribution of animals and plants, called by naturalists zoölogical or botanical provinces or districts, the animal and vegetable populations of which are technically designated as their faunæ and floræ. Such organic realms, as we may call them, have long been recognized on land, and the most extensive among them are easily distinguished. No one will fail to recognize the tropical zone, with its royal dynasty of palms and all the accompanying glories of a tropical vegetation, its birds of brilliant plumage, its large Mammalia, lions, tigers, panthers, elephants, and its great rivers haunted by gigantic reptiles. Nor is the representation of vegetable and animal life less characteristic in the temperate zone, where the oak is monarch of the woods, with all his attendant court of elms, walnuts, beeches, birches, maples, and the like, where birds of more sober hues, but sweeter voices, take the place of the brilliant parrots and many-tinted humming-birds of the tropical forest; while buffaloes, bears, wolves, foxes, and deer represent the larger Mammalia. In the arctic zone, though marked by peculiar and distinctive features, vegetation has dwindled to a minimum; the birds are chiefly gulls and ducks, which go there for the breeding season in the summer, and the reindeer and polar bears are almost sole possessors of the snow and ice-fields; but this meagreness in the representation of the larger land Mammalia is amply compensated in the numbers of heavy aquatic Mammalia, the whales, walruses, seals, and porpoises of the Arctic seas.

During the last half-century, since the geographical distribution of animals and plants has become a subject of more careful investigation among naturalists, these broad zones of the earth's surface, with their characteristic populations and vegetation, have been subdivided, according to more limited and special combinations of organic forms, into narrower zoölogical and botanical areas. The application of these results to marine life is however of much more recent date, and indeed it would seem at first sight, as if the water, from its own nature, could hardly impose a barrier so impassable as the land. The localization of the marine faunæ and floræ is nevertheless as distinct as that of terrestrial animals and plants, and late investigations have done much to explain the connection of this distribution with physical conditions.

A glance at the coast of our own continent, starting from the high north and making the circuit of its shores, from Baffin's Bay to Behring's Straits, will show us to what a variety of physical influences the animals who live along its shores are subjected. On the shores of Baffin's Bay, especially on the inner coast of Greenland, where the glaciers push their way down to the very brink of the water, and annually launch their southward-bound icebergs, we shall hardly expect to find a very abundant littoral fauna. On its western shore, where the ice does not advance so far, and a greater surface of rock is exposed, the circumstances are more favorable to the development of animal life. Here abound the winged Mollusks (Pteropods), often swept down to the coast of Nova Scotia by the cold current from Baffin's Bay; the "whale feed," as the fishermen call them, because the whales devour them voraciously. Here occur also many compound Mollusks, especially a variety of Ascidians, and the highly colored stocks of Bryozoa. With them is found the Comatula of the northern waters, one of the few modern Crinoids, and beside these a number of Star-fishes, Sea-urchins, and Holothurians, not differing so essentially from those already described as to require special mention.

Along the shore of Labrador and Newfoundland, the coast is wholly rocky, and especially about Newfoundland it is deeply indented with bays. Here there is ample opportunity for the growth of certain kinds of animals in sheltered nooks. The number of species is, however, much greater along the shores of Maine, Nova Scotia, and New Brunswick than in Labrador, owing no doubt to the milder climate. The beautiful shore of Maine, with its countless islands, and broken, picturesque outline, is very rich in species. Parts of this coast are remarkable for a variety of naked Mollusks, as well as for the great numbers of bright-colored Actiniæ, and also for the more brilliant kinds of Holothurians, the Cuvieria, and the like. The latter are especially abundant in the Bay of Fundy, and here also occurs the only Northern representative on our coast of the Sea-fans or Gorgoniæ, so common on the shores of Florida.

Farther south, from Cape Cod to Cape Hatteras, the character of the coast changes; it becomes more sandy, and though here and there the aspect is varied by a rocky promontory or a stony beach, yet the general character is flat and sandy. With this new character of the shore, the fauna is also greatly modified, and it is worthy of remark, that while thus far the representative species have reflected the character of animals to the north of them, they now begin to represent rather those of the Carolina shores. South of Cape Cod come in a kind of Scallop and Periwinkle, very different from the larger Scallops found on the coast of Maine and the British Provinces; our Sea-urchin is replaced by the Echinocidaris, with its few long spines, and an entirely new set of Crustacea and Worms make their appearance on this more sandy bottom. And here we must not forget that not only is the aspect of the animal life changed, as we pass from a rock-bound to a sandy coast, but that of the vegetation also. The various many-tinted sea-weeds of the rocky shore disappear almost entirely, and their place is but poorly supplied by the long eel-grass, which is almost the only marine plant to be found in such a locality. Beside its more sandy character, the coast from Cape Cod to Cape Hatteras is affected by the large amount of fresh water poured into the sea along its whole line, greatly modifying the character of the shore animals. The Hudson, the Delaware, the Susquehanna, the Potomac, the James, the Roanoke, and the large estuaries connected with some of these rivers, give a very peculiar character to the shore, and bring down, not only a vast supply of fresh water, but also a large quantity of detritus of all sorts from the land. Under these circumstances life would be impossible for many of the animals which live farther north. The only locality on the North Atlantic shore, where the conditions are somewhat similar, is at the mouth of the St. Lawrence, that great drainage-bed through which the Canadian lakes empty their superfluous waters into the Gulf of St. Lawrence.

The whole coast of the Carolinas, from Cape Hatteras to Florida, is a sandy beach; but though in this respect it resembles that immediately to the north of it, it differs greatly in other features. Comparatively little fresh water is poured into the ocean along this shore, and its more southerly range, instead of being protected by sand-spits like Pamlico and Albemarle Sounds, or broken by estuaries and inlets like the coast of Virginia, lies broadly open to the sea. On its extensive beaches we have the large Pholas, burrowing deep below the surface, and the Cerianthus, those long, cylindrical Actiniæ, enclosed in sheaths, with their bright crowns of gayly-colored tentacles; the free colonies of Halcyonoids abound also on this coast, and a new set of Sea-urchins (Spatangoids and Clypeastroids) make their appearance.

Farther south, along the Florida coast, a new element comes in, that of the coral reefs, enclosing shallow channels near the shore, and thus providing sheltered harbors on their leeward side, while on their seaward side they slope steeply to the ocean. Beside this, the reef itself affords a home for a great variety of creatures, who bore their way into it and live in its recesses, as some insects live in the bark of trees. Perhaps a more favorable combination of circumstances for the development of marine life does not exist anywhere than about the coral reefs of Florida, and certainly nowhere is there a more rich and varied littoral fauna, especially on their western shore within the Gulf of Mexico. Here swims the Portuguese Man-of-War, borne gayly along on the surface of the water by its brilliant float, here the blue Velella sets its oblique sail to the wind, and hosts of the lighter and more brightly tinted corals fringe the shore with a many-colored shrubbery. In these waters are also found the blue and yellow Angel-fish, the Parrot-fish (Scarus), and the strange Porcupine-fish (Diodon). Vegetable life is comparatively scanty in these tropical waters, where there are scarcely any sea-weeds, except the corallines or limestone Algæ of the reefs. The shore of the Gulf of Mexico, as a whole, has much the same character as that of the Carolinas, until we reach the point where the mountains and plateau of Mexico come down to the coast. From this point to the Isthmus of Panama the coast is again rocky.

Crossing the Isthmus and following the Pacific shore of the continent northward, we find a sandy open shore alternating with rocky beaches as far north as Acapulco. Along this coast there is to be found a great variety of corals, especially Sea-fans, growing on the rocks, but no reef. The Pocillopora, an Acalephian coral, the Pacific representative of the Millepore of Florida, is especially abundant. On the peninsula of Lower California we come again upon a rocky coast, with steep bluffs, extending into the sea. Within the Gulf of California are found, on its sandy coast, peculiar kinds of Sea-urchins, Spatangoids, and Clypeastroids, which occur nowhere else on this coast. From Cape St. Lucas up to the Straits of Fuca, with the exception of the large estuary forming the Bay of San Francisco, there are scarcely a couple of harbors of any consequence. The whole shore is most inhospitable, and the violent northwest winds in summer, and the southeast winds in winter, render it still more bleak and difficult of approach. In consequence of these conditions, the fauna is scanty along a great part of the shore; the best spots for collecting are the beaches, near the head of the peninsula, opposite the islands of Santa Barbara and San Diego, and that within the harbor of San Francisco. On the former, large Craw-fishes abound (Palinurus), akin to those of Florida, though specifically different from them. In the latter, the great amount of fresh water prevents the fauna from being exclusively marine; this harbor is, nevertheless, the great centre of the viviparous fishes, and contains also a large variety of peculiarly shaped Sculpins.

Farther north, between the Straits of Fuca and the island of Sitka, the shore resembles that of Maine, with its many islands, bays, and inlets; a succession of long, narrow islands forms a barrier along the coast, enclosing the shore waters, so as almost to make them into an inland sea. But little fresh water empties upon this part of the coast, and here, where the salt water is little modified by any deposit from the land, but where the violence of the ocean is broken by this barrier of islands, there is a full development of marine life. The shores of the Gulf of Georgia, and those of Vancouver's Island, seem to be especially the home of the Star-fishes. The fauna of this locality has been but little investigated, and yet the number of species of Star-fishes known from there is greater than from any other region; many of them are of colossal size, measuring some four feet in diameter. This coast seems also very favorable for the development of Hydroids, in consequence of which its waters swarm with a variety of Jelly-fishes. The Pennatula, that pretty compound Halcyonoid, with its feather-like sprays, is another characteristic type of this fauna. Beyond this, from Sitka to Behring's Straits, the same rocky coast prevails as in Labrador and Greenland. In Behring's Straits we return again to the forests of beautiful compound Mollusks, or rather to a variety of "representative species," resembling the Bryozoa and Ascidians so abundant in Baffin's Bay. The depth of the water, however, is much less here than on the corresponding Atlantic coast, where, south of Greenland, along the shore of Labrador, the water is very deep, while in Behring's Straits the depth is not greater than from one hundred to one hundred and twenty fathoms. The respective faunæ of these two shores are also affected by the difference of temperature, the cold current from Baffin's Bay sweeping down upon the coast of Labrador, while, through Behring's Straits, the warm current from the Pacific pours into the Arctic Ocean.

Thus the whole coast of our continent is peopled more or less thickly with animals. But now arises a new set of inquiries; how far into the sea do these animals extend? how wide is their domain? Do they wander at will in the ocean, or are they bound by any law to keep within a certain distance of the shore? These questions would seem to be easily answered, for wherever we go on the surface of the sea, and as far as the eye can penetrate into its depths, we find it full of life; and yet a closer examination shows that all these beings have their appointed boundaries. Along the shores, animal and vegetable life seems to be distributed in certain definite combinations. Those who are familiar with rocky beaches readily recognize the different bands of color produced by the various kinds of sea-weed growing at given distances between high and low-water-mark. First comes the olive green rockweed (the Fucus), and with it are found barnacles and small Crustacea, myriads of which are to be seen hopping about in this rockweed when the tide is out. Below these are the brown crispy Rhodersperms and Melanosperms, and associated with them are Star-fishes, Crabs, and Cockles. Next in order is the Laminarian zone. Here we have the broad fronds of the Laminaria, the "devil's aprons," as the fishermen call them; in this zone is the home of the Sea-urchin, and here will be found also a few small fishes. Lastly we have the Coralline zone, so called on account of the lime deposit in the sea-weeds, giving them the rigidity of corals; among these the Lobsters make their appearance, and here are to be found also numerous clusters of Hydroids, the nurses of the Jelly-fishes.

This distribution is not casual; these belts of animal and vegetable life are sharply defined and so constantly associated, that they must be controlled by the same physical laws. The first important investigations on this subject were made by Örsted, the distinguished Danish naturalist. He undertook a complete topographical survey of the coast near which he lived, carrying his soundings to a depth of some twelve fathoms, and found that both the fauna and flora of the shore were divided, according to the depth of the water, into bands of vegetable and animal life, corresponding very nearly with those given above. His observations were, however, limited, not extending beyond the neighborhood of his home. It is to Edward Forbes, the great English naturalist, whose short life was so rich in results for science, that we owe a more complete and extensive investigation of the whole subject.

Aided by a friend, Captain McAndrew, who placed his yacht at his disposal, he made a series of observations on the British, Scandinavian, and Danish coasts, and explored also with the same object the shores of the Mediterranean. Not content with sounding the present ocean, he sunk his daring plummet in the seas of past geological ages, and by comparing the nature and position of their fossil remains with those of living marine faunæ, he measured the depths of the water along their shores. He collected a vast amount of material, and the results of his labors have formed the basis of all subsequent generalizations upon this subject. Nevertheless he arrived at some erroneous conclusions, which, had he lived, he would no doubt have been the first to correct. Dredging from low-water-mark outward, he found that, from the Laminarian and Coralline zone, the animals began gradually to decrease in number, and that, at a depth of two or three hundred fathoms, the dredge always came up nearly empty. He inferred that at a certain depth the weight of water became too great to be endured by animals, and that the ocean beyond this line, like the land beyond the line of perpetual snow, was barren of life. This result seemed the more probable on account of the immense pressure to which animals are subjected, even at a comparatively moderate depth. A column of water thirty-two feet high is equal to one atmosphere in weight; this pressure being increased to the same amount for every thirty-two feet of depth, it follows that a fish one hundred and twenty-eight feet, or some twenty fathoms below the surface, is under the pressure of almost four atmospheres plus that of the air outside. Wherever tides run high, as in the Bay of Fundy, for instance, where an animal is under the pressure of one atmosphere at low tide, and of three atmospheres at high tide, we see that marine animals are uninjured by great changes of pressure. Yet it seems natural to suppose that there is a limit to this power of resistance; and that there must exist barren areas at the bottom of the ocean, as destitute of life as the regions on the earth which are above the line of perpetual snow. No doubt pressure does influence the distribution of life in the ocean; but it would seem, from subsequent observations, that the boundaries assigned by Forbes were far too narrow, and that the structure of many marine animals enables them to live under a weight, the one hundredth part of which would be fatal to any terrestrial animal.