CHAPTER V

FLOWERS OF THE SEA

JELLYFISHES, NAMESAKES OF THE FABLED MEDUSA

The type and simplest form of that great division of aquatic, and almost exclusively marine, animals constituting the phylum Coelenterata, is the polyp. It consists of a soft-skinned body, typically cup-shaped, containing a baglike digestive cavity, or primitive stomach, open at the top, and surrounded by the soft mesenchyme. The open upper end is the mouth, which is usually encircled by few or many tentacles--hollow outgrowths from the wall of the tubular gullet. Currents of water are drawn in by waving cilia at one end of the slitlike mouth, and pass out as waste at the other side; they bring food and oxygen from which nourishment is absorbed by the cells of the wall of the stomach (endoderm). Certain outgrowths within the mesenchyme act as feeble muscles for lengthening and shortening the body and tentacles; but there are no blood vessels or excretory organs.

Most polyps are fixed on some support, but in many the young pass through a free, swimming stage before settling down for life. All coelenterates, and these only, are provided with "stinging cells," the nature and importance of which will be explained presently.

The simplest class is that of the hydroids (Hydroida), the type of which is the fresh-water hydra, so-called because, like the Hydra of ancient myth, when it is cut to pieces each part will grow into a new animal. It lives in ponds and pools of stagnant water, and is so small that a magnifying glass is necessary to study it, especially in the case of the green one of our two common American species--the other is brown. Indeed, similar hydroids of salt water are often taken and dried by unscientific collectors under the impression that they are feathery seaweeds. It is stalklike in shape, has long tentacles which always turn toward the greatest light, influenced like certain plants by heliotropism, and feeds on minute crustaceans and other minute organisms. Sometimes hydras are so abundant as to form a velvety surface in warm pools. The sexes are combined in the same individual, and the embryo forms within the body, then protrudes as a bud, which finally breaks away and after a time sinks, attaches itself at the base to some support, and grows into a perfect hydra. When quiescent or alarmed the tentacles are withdrawn, and the whole animal shrinks into a little lump.

Such is the general natural history of the group; but the oceanic hydroids have developed a vast variety of forms, and, with increased breadth of life, have added many interesting features and habits. Many of them are single, rooted in mud, or upon seaweeds, rocks or shellfish both dead and alive, and look like flowers of lovely tints; and they reproduce by putting forth separate reproductive parts, called "zooids," of various kinds. Others are in colonies that spread by extensions of the base from which arise other hydroids until a bunch of them are growing side by side; but these groups consist of hydroids differentiated into separate functions, for some devote themselves to capturing food which nourishes all, through the common base, while others produce the buds and eggs by which the colony is increased.

The most remarkable of these processes of reproduction is that which is represented by the jellyfishes so abundant in all seas, and so beautiful either when seen floating along just at the surface of the summer sea, or when at night they glow with phosphorescence like silvery, greenish rockets in the dark waves. Sometimes they occur in enormous "schools"--as we say of fish--all of one kind, filling the water thickly as far as one can see, and now and then in late summer are cast on the beach in long windrows. They range in size from a pinhead to ten or twelve feet in diameter. So big a Cyanea would probably weigh fifty pounds, but after a thorough drying would yield only a few ounces of semisolid matter, 99 per cent of the creature being water absorbed in its spongy tissues. Some are egg-shaped, others like a bell with a long clapper, but the ordinary form is that of an open umbrella, usually fringed about the edge with tentacles, sometimes short and fine, sometimes few and long, again a crowded circle of long snaky appendages. These elastic hanging tentacles are the means by which the medusa (as such a jellyfish is appropriately termed in science) captures its food, which consists not only of the minute things swarming in the plankton, but of other coelenterates, small crustacea, fishes, anything in fact that it can entangle in its sticky net and sting to death. Every one of the filmy tentacles is thickly studded with microscopic cells (cnidocells) covered by a mere film, and having a spinelike trigger projecting from it. If this trigger is touched, or the film broken, out springs the coiled thread dart which is barbed and carries into the wound it makes a poison that benumbs. Thousands of these microscopic darts may prick the skin of a captive, and paralyze its strength--as it does that of a man who gets caught naked in the trailing net of one of the great northern medusæ. Being thus captured, the prey is drawn up to the mouth, which opens in the center of the under side of the umbrella float.

At intervals around the margin of the umbrella are small organs by which, it is believed, the creature maintains a sense of balance and direction, and perhaps of temperature or light, or both; for many medusæ sink out of sight by day and come to the surface at night; and when the sea is rough they descend to quiet depths. Thus they have the power not only to move ahead by the alternate contraction and dilatation of the disk, but to so alter their specific gravity as to sink or rise at will. They thus show the rudiments of both a muscular and a nervous system.

Very interesting, and often of great beauty, are the free-swimming, colonial, hydroid polyps called siphonophores. On a long stem or string are arranged, at the top, a bulb filled with gas or air, as a float, then a series of swimming bells whose pulsations carry the colony about, beneath which are various polyps and tentaclelike appendages, some to gather food, whose digested products circulate through the whole colony, others performing reproductive functions. The variety of form is considerable; and one of the most peculiar, and the only siphonophore familiar to most persons, is the exquisite Portuguese man-of-war, whose prismatically tinted bulb, as big as one's fist, is commonly met with in the Gulf Stream in the North Atlantic, and often is seen in great flocks in the tropics, bobbing on the surface of the waves in calm weather. Beneath that bulb trails a long tuft of tentacles and zooids, performing various functions, and so foreshadowing the division of labor that in the higher animals is effected by the different limbs and organs.

SEA ANEMONES, CORALS, AND SEA FANS

Sea anemones are simply large polyps of more complicated structure than the hydroid polyps. Instead of a simple, baglike, enteric cavity, the slitlike mouth admits food into a flattened gullet which leads to an enlarged digestive cavity. The gullet does not hang free, but is joined to the outer wall of the body by a series of radiating partitions, between which shorter ones extend from the inner surface of the ectoderm; and below the gullet the stomach wall extends in lobes between these partitions, through which holes permit the nutritive juices to circulate throughout the whole body. The whole upper surface of the polyp is covered by short tentacles arranged in circles. A current of water, induced by waving cilia, is constantly flowing in at one corner of the mouth and out at the other, supplying the animal with oxygen and a certain amount of minute food, and carrying off waste; but the anemones capture by means of their tentacles small fishes, mollusks and everything that can be caught and swallowed. As some anemones exceed a foot in diameter, large and powerful prey may sometimes be taken. It is interesting to note that anemones distinguish very quickly between what is good to eat and what is not. Most of them are sitting near shore on rocks or in tide pools, or are clinging to the larger seaweeds or clustered on the supports of wharves where the waves and tidal currents are continually washing about them, often with much violence, and dashing against them strands of weed or the small wreckage always floating in such a place. None of this is seized, or at least is not swallowed; but whether we are to conclude that this choice is made by intelligence, or only by chemical perception is a matter for study. When harm threatens, or when they crave rest, they withdraw all their gorgeous tentacles, infold them within their mouth, and shrink down into roundish gray lumps that attract neither the eye nor the appetite of any marauder.

The coral polyps differ from anemones only in details of structure that we need not consider, except to note the striking difference that here the base and the radiating partitions instead of being membranous secrete a firm skeleton either of lime or of the horny material termed chitin. The flesh overflows the walls, folding down from the top, so that the skeleton becomes really internal, although naked at the broad base. Some of the tropical stony corals are like big anemones, several inches across; and it is only when they infold all their richly colored tentacles and become a dull and shapeless lump that their stony cup is revealed. These are solitary, and form loosely lying corals, like that called the "mushroom." New ones are produced by the parent throwing off buds which for a time remain attached by a stalk, but finally fall off and settle down to grow--a process that may go on for a score of years. In the case of the huge coral masses called madrepores the buds remain attached to the parent. If they spread out naturally, W. Saville Kent explains, they build up by accumulation the large rounded masses known as "brain" corals and "star" corals, which are most numerous on coastline reefs, or form the base of the outer barrier reef. On the other hand, where the budding is terminal, or oblique, branching, treelike growths result in "staghorn" and similar forms.

The coral animals do not alone construct the reefs. Stony hydroids (millepores), shells of all sorts of mollusks, limy sea mosses (Bryozoa), animalcules and diatoms and various algæ stiffened or cased with lime or flint, and blown sand, contribute to build them up, especially when they near the surface of the sea.

The distribution of reef-building corals is interesting. At present they are limited to about 35 degrees each side of the equator, but are irregularly distributed, owing mainly to differences of temperature in the water, which must not be colder than 68 degrees F. Hence they exist farther away from the equator in the path of warm ocean currents. The Gulf Stream accounts for the coral islands along the coast of Florida and in the Bermudas, which is their farthest point on the American coast; and the warmth of the water accounts for their extensive presence along the eastern coasts of Australia and Africa, when few exist on the western sides of these continents; similarly the western coasts of South and Central America are nearly free of coral banks. Other causes of limitation exist. For example, the noticeable absence of coral growth along the coast of South America is largely, if not altogether, owing to the fresh water and silt brought down by the great rivers there--both prejudicial to coral life.

Coral colonies increase and ultimately form banks wherever warm, pure sea water is constantly present, and not more than about 125 feet deep. Here, spreading and continually rising on the skeletons of dead generations, they form a long line close to the land called a "fringing" reef; and outside of this, beyond a space swept by the currents, may arise a second, still more flourishing bank, termed "barrier" reef. The great barrier reef that extends for 1,200 miles along the eastern coast of Australia--a vast chain of banks and islands--is an amazing example of what these minute animals can accomplish, given time; and geology can point to still more stupendous results of their work in the early history of the globe.

Very characteristic, in the great coral-growing region of the South Sea archipelago, is the ring-shaped island or "atoll," which incloses a quiet lagoon, usually with an open entrance. The reason for such a form has excited much discussion, one explanation being that its origin was about a small island that slowly subsided, the coral keeping pace in rising as the island sank, until finally the land disappeared; another that the circular reef arose from a submerged elevation, and when it came near the surface ceased to grow except on its outer border because it ceased to get suitable water and food, until after a time the central part died out, leaving a ring. Both explanations may be true of different situations.

When a reef comes near to the surface the branching coral is knocked to pieces by the waves, and there are added to this breakage shells and bones, calcareous seaweeds, and what not; and all this is ground into sand by the surf, washed high on the top of the ridge and manured by dead plants and animals, and by the droppings of birds, until finally a soil forms beyond the reach of the tides. Then, if it is in the far southern seas, a drifting coconut may lodge there and be rolled high enough to be left to strike its roots into the sand and begin the grove that by and by will make the islet attractive to men. The thick husk of the coconut resists harm from sea water, near which this palm prefers to grow in just such a sandy, shelly soil as the uprising reef affords. The nuts that so often fall into the surf or are carried out by rivers make long voyages without losing their vitality. Here, again, the situation of most coral islets in the course of currents is advantageous, for thus not only these nuts but other useful seeds and colonizing elements drift directly to their doors, as it were. Birds, wandering widely over the waters, espy the bit of land, and aid by their visits to increase its fertility and often add to its flora. Reefs near shore, especially in Florida and southward, become jungles of mangroves, which not only spring from floating seeds but send down from their branches sprouts that become rooted in the mud and spread the growth interminably. Such a "mangrove key" soon attracts an extensive population of plants and animals and speedily becomes a considerable island.

A great variety of corals, however, are not reef builders, and some species secrete little if any lime; these solitary relatives are found scattered all over the oceans, in deep water as well as shallow, wherever the bottom is suitable, and an immense amount of interesting information about them is to be found in books devoted to this beautiful group of animals.

The class includes two or three other orders of coral--polyps that grow in a solitary way or in groups, forming those elegant objects called sea fans, sea pens, and so forth, which can be referred to only briefly. One of these is the order Alcyonaria, in which some are soft-bodied, others are strengthened by a network of spicules. A very beautiful one is the "sea pen," which takes the shape of an ostrich plume; another is the strange mass of parallel tubes called organ-pipe coral; and some of them are very large, the great tree coral of the eastern Atlantic depths being sometimes as tall as a man, while it looks like a sturdy, leafless tree. As in all the others, however, it is covered by a living fleshy coat of protoplasmic substance studded with polyps whose gay colors and waving tentacles give it the appearance of being clothed with minute sessile blossoms. The best known of this group, probably, is the red coral of commerce, which is the scarlet, ivorylike interior stem of a branching alcyonarian colony. This coral has from the earliest time been cut into cameos by lapidaries, as well as used for making necklaces and other toilet ornaments.