BRANCH PROTOZOA: THE ONE-CELLED ANIMALS
Of this group the structure and life-history of the Amœba (Amœba sp.) and the Slipper Animalcule (Paramœcium sp.) have already been treated in Chapter VI. Another example is the
BELL ANIMALCULE (Vorticella sp.)
Technical Note.—Specimens of Vorticella may usually be found in the same water with Amœba and Paramœcium. The individuals live together in colonies, a single colony appearing to the naked eye as a tiny whitish mould-like tuft or spot on the surface of some leaf or stem or root in the water. Touch such a spot with a needle, and if it is a Vorticellid colony it will contract instantly. Bring bits of leaves, stems, etc., bearing Vorticellid colonies into the laboratory and keep in a small stagnant-water aquarium (a battery-jar of pond-water will do).
Examine a colony of Vorticella in a watch-glass of water or in a drop of water on a glass slide under the microscope. Note the stemmed bell-shaped bodies which compose the colony. Each bell and stem together form an individual Vorticella (fig. 8.) How are the members of the colony fastened together? Tap the slide and note the sudden contraction of the animals; also the details of contraction in the case of an individual. Watch the colony expand; note the details of this movement in the case of an individual.
Make drawings showing the colony expanded and contracted.
With higher power examine a single individual. Note the thickened, bent-out, upper margin of the bell. This margin is called the peristome. With what is it fringed? The free end of the bell is nearly filled by a central disk, the epistome, with arched upper surface and a circlet of cilia. Between the epistome and peristome is a groove, the mouth or vestibule, which leads into the body. Study the internal structure of the transparent, bell-shaped body. Note the differentiation of the protoplasm comprising the body into an inner transparent colorless endosarc containing various dark-colored granules, vacuoles, oil-drops, etc., and an outer uniformly granular ectosarc not containing vacuoles. Is the stalk formed of ectosarc or endosarc or of both? Note the curved nucleus lying in the endosarc. (This may be difficult to distinguish in some specimens.) Note the numerous large circular granules, the food vacuoles. Note the contractile vesicle, larger and clearer than the food vacuoles. Note the thin cuticle lining the whole body externally. A high magnification will show fine transverse ridges or rows of dots on the cuticle.
Make a drawing showing the internal structure.
Observe a living specimen carefully for some time to determine all of its movements. Note the contraction and extension of the stalk, the movements of the cilia of peristome and epistome, the flowing or streaming of the fluid endosarc (indicated by the movements of the food vacuoles), the behavior of the contractile vesicle.
Make notes and drawings explaining these motions.
Specimens of Vorticella may perhaps be found dividing, or two bell-shaped bodies may be found on a single stem, one of the bodies being sometimes smaller than the other. These two bodies have been produced by the longitudinal division or fission of a single body. In this process a cleft first appears at the distal end of the bell-shaped body, and gradually deepens until the original body is divided quite in two. The stalk divides for a very short distance. One of the new bell-shaped bodies develops a circlet of cilia near the stalked end. After a while it breaks away and swims about by means of this basal circlet of cilia. Later it settles down, becomes attached by its basal end, loses its basal cilia and develops a stalk.
"Conjugation occurs sometimes, but it is unlike the conjugation of Paramœcium in two important points: Firstly, the conjugation is between two dissimilar forms; an ordinary large-stalked form, and a much smaller free-swimming form which has originated by repeated division of a large form. Secondly, the union of the two is a complete and permanent fusion, the smaller being absorbed into the larger. This permanent fusion of a small active cell with a relatively large fixed cell, followed by division of the fused mass, presents a striking analogy to the process of sexual reproduction occurring in higher animals."
OTHER PROTOZOA
Besides the Amœba, Paramœcium, and Vorticella there are thousands of other Protozoa. Most of them live in water, but a few live in damp sand or moss, and some live inside the bodies of other animals as parasites. Of those which live in water some are marine, while others are found only in fresh-water streams and lakes.
Fig. 9.—Sun animalcule, a fresh-water protozoan with a siliceous skeleton, and long thread-like protoplasmic prolongations. (From life.)
Form of body.—The Protozoa all agree in having the body composed for its whole lifetime of a single cell,[7] but they differ much in shape and appearance. Some of them are of the general shape and character of Amœba, sending out and retracting blunt, finger-like pseudopodia, the body-mass itself having no fixed form or outline but constantly changing. Others have the body of definite form, spherical, elliptical, or flattened, enclosed by a thin cuticle, and having a definite number of fine thread-like or hair-like protoplasmic prolongations called flagella or cilia. Many of the familiar Protozoa of the fresh-water ponds always have two whiplash-like flagella projecting from one end of the body. By means of the lashing of these flagella in the water the tiny creature swims about. Others have many hundreds of fine short cilia scattered, sometimes in regular rows, over the body-surface. The Protozoan swims by the vibration of these cilia in the water.
Fig. 10.—Stentor sp.; a protozoan
which may be fixed, like Vorticella,
or free-swimming, at will, and
which has the nucleus in the shape
of a string or chain of bead-like
bodies. The figure shows a single
individual as it appeared when fixed,
with elongate, stalked body, and as
it appeared when swimming about
with contracted body. (From life.)
There is no stagnant pool, no water standing exposed in watering-trough or barrel which does not contain thousands of individuals of the one-celled animals. And in any such stagnant water there may always be found several or many different kinds or species. A drop of this water examined with the compound microscope will prove to be a tiny world (all an ocean) with most of its animals and plants one-celled in structure. A few many-celled animals will be found in it preying on the one-celled ones. There are sudden and violent deaths here, and births (by fission of the parent) and active locomotion and food-getting and growth and all of the businesses and functions of life which we are accustomed to see in the more familiar world of larger animals.
Marine Protozoa.—One usually thinks of the ocean as the home of the whales and the seals and the sea-lions, and of the countless fishes, the cod, and the herring, and the mackerel. Those who have been on the seashore will recall the sea-urchins and starfishes and the sea-anemones which live in the tide-pools. On the beach there are the innumerable shells, too, each representing an animal which has lived in the ocean. But more abundant than all of these, and in one way more important than all, are the myriads of the marine Protozoa.
Although the water at the surface of the ocean appears clear and on superficial examination seems to contain no animals, yet in certain parts of the ocean (especially in the southern seas) a microscopical examination of this water shows it to be swarming with Protozoa. And not only is the water just at the surface inhabited by one-celled animals, but they can be found in all the water from the surface to a great depth below it. In a pint of this ocean-water there may be millions of these minute animals. In the oceans of the world the number of them is inconceivable. And it is necessary that these Protozoa exist in such great numbers, for they and the marine one-celled plants (Protophyta) supply directly or indirectly the food for all the other animals of the ocean.
Among all these ocean Protozoa none are more interesting than those belonging to the two orders Foraminifera (fig. 11) and Radiolaria. The many kinds belonging to these orders secrete a tiny shell (of lime in the Foraminifera, of silica in the Radiolaria) which encloses most of the one-celled body. These minute shells present a great variety of shape and pattern, many being of the most exquisite symmetry and beauty. The shells are perforated by many small holes through which project long, delicate, protoplasmic pseudopodia. These fine pseudopodia often interlace and fuse when they touch each other, thus forming a sort of protoplasmic network outside of the shell. In some cases there is a complete layer of protoplasm—part of the body protoplasm of the Protozoan—surrounding the cell externally.
Fig. 11.—Rosalina varians, a marine protozoan (Foraminifera) with calcareous shell. (After Schultze.)
When these tiny animals die their hard shells sink to the bottom of the ocean, and accumulate slowly, in inconceivable numbers, until they form a thick bed on the ocean floor. Large areas of the bottom of the Atlantic Ocean are covered with this slimy ooze, called Foraminifera ooze or Radiolaria ooze, depending on the kinds of animals which have formed it. Nor is it only in present times that there has been a forming of such beds by the marine Protozoa. All over the world there are thick rock strata composed almost exclusively of the fossil shells of these simplest animals. The chalk-beds and cliffs of England, and of France, Greece, Spain, and America, were made by Foraminifera. Where now is land were once oceans the bottoms of which have been gradually lifted above the water's surface. Similarly the rock called Tripoli found in Sicily and the Barbadoes earth from the island of Barbadoes are composed of the shells of ancient Radiolaria.
It is thus evident that the Protozoa is an ancient group of animals. As a matter of fact zoologists are certain that it is the most ancient of all animal groups. All of the animals of the ocean depend upon the marine Protozoa and the marine Protophyta, one-celled plants, for food. Either they feed on them directly, or prey on animals which in turn prey on these simplest organisms. A well-known zoologist has said: "The food-supply of marine animals consists of a few species of microscopic organisms which are inexhaustible and the only source of food for all the inhabitants of the ocean. The supply is primeval as well as inexhaustible, and all the life of the ocean has gradually taken shape in direct dependence on it." The marine Protozoa are the only animals which live independently; they alone can live or could have lived in earlier ages without depending on other animals. They must therefore be the oldest of marine animals. By oldest is meant that their kind appeared earliest in the history of the world, and as it is certain that ocean life is older than terrestrial life—that is, that the first animals lived in the ocean—it is obvious that the marine Protozoa are the most ancient of all animal groups.
As already learned in the examination of examples of one-celled animals, it is evident that life may be successfully maintained without a complex body composed of many organs performing their functions in a specialized way. The marine Protozoa illustrate this fact admirably. Despite their lack of special organs and their primitive way of performing the life-processes, that they live successfully is shown by their existence in such extraordinary numbers. They outnumber all other animals. The conditions of life in the surface-waters of the ocean are easy and constant, and a simple structure and simple method of performing the necessary life-processes are wholly adequate for successful life under these conditions.
CHAPTER XVI
BRANCH PORIFERA: THE SPONGES
THE FRESH-WATER SPONGE (Spongilla sp.)
Technical Note.—Fresh-water sponges may perhaps not be readily found in the neighborhood of the school, but they occur over most of the United States, and careful searching will usually result in the finding of specimens. They are compact, solid-looking masses, sometimes lobed, resting on and attached to rocks, logs, timbers, etc., in clear water in creeks, ponds, or bayous. They are creamy, yellowish-brown or even greenish in color and resemble some cushion-like plant far more than any of the familiar animal forms. They can be distinguished from plants, however, by the fact that there are no leaves in the mass, nor long thread-like fibres such as compose the masses of pond algæ (pond scum). When touched with the fingers a gritty feeling is noticeable, due to the presence of many small stiff spicules. Sponges should be removed entire from the substance they are attached to, and may be taken alive to the laboratory. They die soon, however, and should be put into alcohol before decay begins.
Note the form of the sponge mass. Is it lobed or branched? Examine the surface for openings. These are of two sizes; the larger are osteoles or exhalant openings, while the smaller and more numerous are pores or inhalant openings. The sponge-flesh is called sarcode. Examine a bit of sarcode under the microscope; note the spicules. Have these spicules a regular arrangement? Of what are they composed?
Draw the entire sponge, showing shape and openings; draw some of the spicules.
Embedded in the body-substance, especially near the base, note (if present) numerous small, yellowish, sub-spherical or disk-like bodies, the gemmules. These are reproductive bodies. Each gemmule is a sort of internal bud. It is composed of an interior group of protoplasmic cells, enclosed by a crust thickly covered with spicules. In winter the sponge dies down and the gemmules are set free in the water. In spring the protoplasmic contents issue through an aperture in the crust, called the micropyle or foraminal opening, and develop and grow into a new sponge.
For a good account of the fresh-water sponge, see Pott's "Fresh-water Sponges."
A CALCAREOUS OCEAN-SPONGE (Grantia sp.) (fig. 7, D, E, F.)
Technical Note.—For inland schools, specimens preserved in alcohol or formalin must be used. They may be obtained from dealers in naturalists' supplies (see p. 453). Specimens of some species of this genus can be obtained at almost any point on the Atlantic or Pacific coasts of this country.
Examine the external structure of a specimen. Note the elongate, sub-cylindrical form, the attached base, the free end. Note the large exhalant opening, osteole or osculum, at the free end; the numerous small inhalant openings elsewhere on the surface (best seen in dried specimens). Note the spicules covering the surface of the body, and the longer ones surrounding the osculum. Cut the sponge in two longitudinally and note the simple cylindrical body-cavity, the gastric cavity or cloaca. Note the thickness of the body-wall; note the tubes running through the body-wall from cloaca to external surface. Through these tubes water laden with food enters the gastric cavity, where the food is digested, the water and undigested particles passing out through the osculum. Crush a bit of dried sponge, or boil a bit of soft sponge in caustic potash and mount on a glass slide. Examine under a microscope and note the abundance of spicules and the variety in their form. Two kinds may always be found, and sometimes three. These spicules are composed of carbonate of lime and can be dissolved by pouring on to them a drop of hydrochloric acid.
Some of the sponges may have buds growing out from them near the base. These buds are young sponges developed asexually. If allowed to develop fully the buds would have detached themselves from the parent and each would have become a new sponge.
Make drawings showing the form of a whole sponge; the appearance of the inner face of the sponge bisected longitudinally; the shape of the spicules.
A COMMERCIAL SPONGE
Technical Note.—For the study of the skeleton of an ocean-sponge with more complex body buy several common small bath-sponges without large holes running entirely through them. The teacher should have also a few specimens of small marine sponges preserved in alcohol or formalin. Such specimens should be part of the laboratory equipment (see account of laboratory equipment, p. 450), and can be readily and cheaply obtained from dealers in naturalists' supplies.
The bath-sponge or slate-sponge consists simply of the hard parts or skeleton of a sponge animal. In life all of the skeleton is enclosed or covered by a soft, tough mass composed of layers of cells. Note the many openings on the surface of the sponge. Crush a bit of the skeleton and examine it under the microscope. Note that it is composed of fine fibres of a tough, horny substance called spongin, instead of tiny distinct calcareous spicules.
OTHER SPONGES
The sponges are fixed, plant-like aquatic animals. The members of a single family live in fresh water, being found in lakes, rivers, and canals in all parts of the world. All the other sponges, and there are several thousand species known, live in the ocean. They are to be found at all depths, some in shallow water near the shore and others in deeper water, even to the deepest depths yet explored. They are found in all seas, though especially abundantly in the Atlantic Ocean and Mediterranean Sea.
Fig. 12.—The skeleton of a
"glass" sponge (skeleton
composed of siliceous spicules)
from Japan. (From specimen.)
Form and size.—The shape of the simplest sponges is that of a tiny vase or nearly cylindrical cup, hollow and attached at its base. At the free end there is a large opening. But there is a great deal of variety in the form and size of different sponges. There is, indeed, much variation in the shape and general character of different individuals of the same species. Unlike most other animals, sponges are fixed, and the character of the surface to which a sponge is attached has much influence upon its shape. If this surface is rough and uneven the sponge may follow in its growth the sinuosities of the surface and so become uneven and distorted in shape. At best, only a few kinds of sponges have any very even and symmetrical shape. Most of them are very unsymmetrical and grow more like a low compact bushy plant than like the animals we are familiar with. The smallest sponges are only 1 mm. (1/25 in.) high, while the largest may be over a meter (39 in.) in height. In color living sponges may be red, purple, orange, gray, and sometimes blue. Most sponges have the whole body of one color.
Skeleton.—A very few sponges have no skeleton at all. The others have a skeleton or hard parts composed of interwoven fibres of the tough, horny substance called spongin, or of hosts of fine needles or spicules of silica or of carbonate of lime. The siliceous skeletons of some of the so-called glass-sponges (fig. 12) are very beautiful. The lime and siliceous sponge spicules exhibit a great variety of outline, some being anchor-shaped, some cross-shaped, and some resembling tiny spears or javelins.
Structure of body.—The skeleton of a sponge whether composed of interlacing fibres or of short spicules is always invisible from the outside when the sponge is alive. It is embedded in, or clothed by, the soft, fleshy part of the body. The soft part of the sponge is composed simply of two layers of cells, one constituting the external surface of the body, and the other lining the interior cavities and canals of the body. Between these two cell-layers there is a mass of soft gelatinous substance all through which protoplasm ramifies, and in which are embedded numerous scattered cells. There are, as seen in the case of Spongilla and Grantia, no systems of organs such as characterize the higher animals. No heart, lungs, alimentary canal, nervous system, organs of locomotion, eyes, ears, or other organs of special sense; the sponge has none of these. It is simply an aggregate of cells, arranged in two layers, and supported usually by a skeleton of horny fibres or calcareous or siliceous spicules. Its body is usually shapeless, unsymmetrical and without front or back, right or left. It is not to be wondered at that sponges were for a long time believed to be plants.
Feeding habits.—The sponges feed on minute bits of animal or plant substance and on the microscopic unicellular plants or animals which float in the water which bathes their bodies. The water entering the sponge-body through the various openings of the surface is moved along by the waving or lashing of the flagella of the cells which line the canals, and these currents of water bear with them the tiny organisms which are taken up by these same cells and digested. The incoming currents of water meet in the central cavity or cavities of the body and pass out through the large opening called the osculum at the free end of the vase-like body, or if the body is branched, through the large openings at the tips of these branches.
The same currents of water bring also oxygen for the sponge's breathing and carry away the carbonic acid gas given out by the body-cells.
As a German naturalist has said, the one necessary condition for the life of a sponge is the streaming of water through its body. All sponges have a system of canals for this water-current and all have means, in the waving flagella or cilia with which these canals are lined, for producing these currents. When a live sponge is put into a vessel of water, currents are immediately set up, and they always flow into the body through the many fine openings and out of the body through the osculum.
Development and life-history.—Although the sponge in its adult condition is permanently attached by its base to the sea-bottom or to some rock or shell, when it is first born it is an active free-swimming creature. The sponges reproduce in two ways, asexually and sexually. The asexual mode of reproduction of the fresh-water sponge by gemmules has already been described. The ocean sponges also reproduce asexually either by forming interior gemmules or external buds. In this latter method a bud forms on the outer surface of the body which increases in size and finally grows into a new sponge individual. In some species this new sponge does not become separated from the body of the mother, but remains attached to it like a branch to a tree-trunk. By the continued production of such non-separating individuals, a colony of sponges is formed which has the general appearance of a branching plant. In other species the new sponge formed by the development and growth of a bud falls off and becomes a distinct separate individual.
In the sexual mode of reproduction, male or sperm-cells and female or egg-cells are developed in the same individual. The sperm-cells are motile and swim about in the cavities and canals of the sponge-body until they find egg-cells, which they fertilize. The fertilized eggs begin to develop and pass through their first stages in the sponge-body. Finally the embryo sponge, which is usually a tiny oval or egg-shaped mass of cells, escapes from the body of the parent into the water. The young sponge has some of its outer cells provided with cilia, and by means of these it swims about. After a while it comes to rest on the ocean-floor or on some rock or shell, attaches itself, and begins to take on the form and character of the parent. It leads hereafter a fixed sedentary life.
The sponges of commerce.—The sponge-skeletons which are the "sponges" that we use all belong to a few species, not more than half a dozen. Most of the commercial sponges come from the Mediterranean Sea, though some come from the Bahama Islands, some from the Red Sea, and a few from the coasts of Greece, Asia Minor, and Africa. The commercial sponges do not live in very deep water; they are usually found not deeper than 200 feet. The living sponges are collected by divers, or are dragged up by men in boats using long-poled hooks, or dredges. "When secured they are exposed to the air for a limited time, either in the boats or on shore, and then thrown in heaps into the water again in pens or tanks built for the purpose. Decay thus takes place with great rapidity, and when fully decayed they are fished up again, and the animal matter beaten, squeezed, or washed out, leaving the cleaned skeleton ready for the market. In this condition after being dried and sorted, they are sold to the dealers, who have them trimmed, re-sorted and put up in bales or on strings ready for exportation. There are many modifications of these processes in different places, but in a general way these are the essential-steps through which the sponge passes before it is considered suitable for domestic purposes. Bleaching-powders or acids are sometimes used to lighten the color, but these unless very delicately handled injure the durability of the fibres."
Classification.—The sponges are classified according to the character of the skeleton. In one group are put all those sponges which have a skeleton of calcareous spicules, and this group is called the Calcarea. All other sponges are grouped as Non-Calcarea, the members of this group either having no skeleton at all, or having a skeleton composed of siliceous spicules or of spongin fibres. According to the absence or presence of a skeleton and the character of the skeleton when it exists the Non-Calcarea are subdivided into smaller groups.
CHAPTER XVII
BRANCH CŒLENTERATA: THE POLYPS, SEA-ANEMONES, CORALS, AND JELLYFISHES
The structure and life-history of an example of the polyps (the Fresh-water Hydra, Hydra sp.) has been studied in Chapters X and XI.
OTHER POLYPS, SEA-ANEMONES, CORALS, AND JELLYFISHES
Technical Note.—The teacher should have, if possible, several pieces of coral and a few specimens of Cœlenterates in alcohol or formalin, which will show the external character, at least, of these animals (see account of laboratory equipment, p. 450). If the school is on the coast, the pupils should be shown the sea-anemones of the tide-pools.
The animals which are included in the branch Cœlenterata are, at least in living condition, unfamiliar to most of us. Like the sponges, they are almost all inhabitants of the ocean; a few, like Hydra, live in fresh water. Like the sponges, too, most of the members of this branch are fixed, and in their general appearance suggest a plant rather than an animal. The name zoophytes, or plant-animals, which is often applied to these animals is based on this superficial resemblance. But many of the Cœlenterates lead an active free-swimming life. This is true of the jellyfishes which float or swim about on or near the surface of the ocean. Many of the zoophytes spend part of their life in an active free-swimming condition before settling down, becoming attached and thereafter remaining fixed. In localities near the seashore many animals belonging to this great group can be readily found and observed. The beautiful sea-anemones with their slowly-waving tentacles, the fine many-branched truly plant-like hydroids with their hosts of little buds, and the soft colorless masses of jelly, the jellyfishes, which are cast up on to the beaches by the waves are all animals belonging to the branch Cœlenterata.
General form and organization of body.—The general or typical plan of body-structure for the Cœlenterata, these animals which come next to the sponges in degree of complexity, can best be understood by imagining the typical cylindrical or vase-like body of the simple sponges to be modified in the following way: The middle one of the three layers of the body-wall not to be composed of scattered cells in a gelatinous matrix, but to be simply a thin non-cellular membrane; the body-wall not to be pierced by fine openings or pores, but connected with the outside only by the single large opening at the free end, and this opening to be surrounded by a circlet of arm-like processes or tentacles, which are continuations of the body-wall and similarly composed. Such a body-structure, which we saw well shown by Hydra, is the fundamental one for all polyps, sea-anemones, corals, and jellyfishes. The variety in shape of the body and the superficial modifications of this type-plan are many and striking, but after all the type-plan is recognizable throughout the whole of this great group of animals.
The two chief body-shapes represented in the branch are those of the polyps on the one hand, and the jellyfishes or medusæ on the other. The polyp-shape is that of a tube with a basal end blind or closed, attached to some firm object in the water and with the free end with an opening, the mouth-opening. At this mouth-end there is a circlet of movable, very contractile tentacles. The mouth may open directly into the interior of the body, which interior may be called the digestive cavity, or it may lead into a simple short tube produced by the invagination or bending in of the body-wall, which may be looked on as the simplest kind of œsophagus. This œsophageal tube opens into the body-cavity or digestive cavity. This cavity may be incompletely divided by longitudinal partitions which project from the sides into the cavity.
The jellyfish or medusoid body-form corresponds in general to an umbrella or bell. Around the edge of this umbrella are disposed numerous threads or tentacles (corresponding to the circlet of tentacles in the polyp). The mouth-opening is at the end of a longer or shorter projection which hangs down from the middle of the under side of the umbrella. The interior body-cavity or digestive cavity extends out into the umbrella-shaped part of the body, usually in the condition of canals radiating from the centre and a connecting canal running around the margin of the umbrella.
Structure.—Although the Cœlenterata show little indication of the complex composition of the body out of organs, as it exists among the higher animals, yet they do show an unmistakable advance on the simple, almost organless body of the sponges. This is chiefly shown by the differentiation among the cells which compose the body. In the polyps and jellyfishes some of the cells are specialized to be unmistakable muscle-cells, some to be nerve-cells and fibres, and so on. A very simple nervous system consisting of small groups of nerve-cells connected by nerve-fibres exists. Some very simple special sense-organs may occur. The digestive system, although in the simpler Cœlenterates consisting merely of the cylindrical body-cavity enclosed by the body-wall and opening by the single hole at the free end of the body, in some is rather complex and is composed of different parts. Those Cœlenterates which are not fixed but lead an active, free-swimming life, viz., the jellyfishes or medusæ, are the most highly organized.
The tentacles which surround the mouth-opening and serve to grasp food and carry it into the mouth, and the stinging or lasso threads with which these tentacles are provided are special organs possessed by most of these animals.
Skeleton.—Like the sponges, some of the Cœlenterata possess a hard skeleton. This skeleton is always composed of calcium carbonate and is called coral. Those polyps which form such a skeleton are called the corals. Coral will be described in connection with the account of the coral-polyps.
Development and life-history.—The polyps and jellyfishes reproduce both asexually and sexually. The asexual mode is usually that of budding. On a polyp a bud is formed by a hollow outgrowth of the body-wall. The bud grows, an opening appears at its distal end, a circlet of tentacles arises about this mouth-opening and a new polyp individual is formed. This individual may separate from the parent or it may remain attached to it. By the development of numerous buds, and the remaining attached of all of the individuals developing from these buds, a colony of polyp individuals may be formed, plant-like in appearance. The various polyp individuals of a colony may differ somewhat among themselves, and these differences are correlated with a division of labor. Thus some of the individuals may devote themselves to getting food for the colony, and these have mouth and tentacles. Others may be devoted to the production of new individuals by budding or by producing germ-cells, and may not have any mouth-opening or any food-grasping tentacles.
In case of many polyps all or some of the new individuals which arise by budding do not become polyps, but develop into medusæ or jellyfish, which separate from the fixed polyp and swim off through the water. These medusæ or jellyfish produce sperm-cells and egg-cells. The sperm-cells fertilize the egg-cells and a new individual develops from each fertilized egg. This new individual is at first an active free-swimming larva called a planula, which does not resemble either a medusa or polyp. After a while it settles down, becomes fixed and develops into a polyp. Thus a polyp may produce a medusa or jellyfish which, however, produces not a new jellyfish, but a polyp. This is called an alternation of generations, and is not an uncommon phenomenon among the lower animals. It results from such an alternation of generations that a single species of animal may have two distinct forms. This having two different forms is called dimorphism. Sometimes, indeed, a species may appear in more than two different forms; such a condition is called polymorphism.
Not all medusæ or jellyfish are produced by polyp individuals, nor do jellyfish always produce polyps and not jellyfishes. There are some jellyfishes (we might call them the true jellyfishes) which always have the jellyfish form, producing new jellyfishes either by budding or by eggs, and there are some polyps which always have the true polyp form, producing new individuals, either by budding or by eggs, always of polyp form and never of jellyfish form. That is, some species of Cœlenterata exist only in polyp form, some species exist only in jellyfish form, while some species (those having an alternation of generations) exist in both polyp and jellyfish form, these two forms appearing as alternate generations.
Classification.—The branch Cœlenterata is divided into four classes: (1) the Hydrozoa, including the fresh-water polyps, numerous marine polyps, many small jellyfishes and a few corals; (2) the Scyphozoa, including most of the large jellyfishes; (3) the Actinozoa, including the sea-anemones and most of the stony corals; (4) the Ctenophora, including certain peculiar jellyfishes.
Fig. 13.—The Portuguese Man-of-War (Physalia sp.). (From specimen from Atlantic Coast.)
The polyps, colonial jellyfishes, etc. (Hydrozoa).—To the class Hydrozoa belongs the Hydra already studied. There are a few other fresh-water polyps and they all belong to this class. The most interesting members of the class are the "colonial jellyfishes," constituting the order Siphonophora. These colonial jellyfishes are floating or swimming colonies of polypoid and medusoid individuals in which there is a marked division of labor among the individuals, accompanied by marked differences in structural character. The individuals are accordingly polymorphic, that is, appear in various forms, although all belong to the same species. Because these various individuals forming a colony have given up very largely their individuality, combining together and acting together like the organs of a complex animal, they are usually not called individuals, nor on the other hand organs, but zooids, or animal-like structures. The beautiful "Portuguese man-of-war" (fig. 13) is one of these colonial jellyfishes. It appears as a delicate bladder-like float, brilliant blue or orange in color, usually about six inches long, and bearing on its upper surface which projects above the water a raised parti-colored crest, and on its under surface a tangle of various appendages, thread-like with grape-like clusters of little bell- or pear-shaped bodies. Each of these parts is a peculiarly modified polyp- or medusa-zooid produced by budding from an original central zooid. The Portuguese man-of-war is very common in tropical oceans, and sometimes vast numbers swimming together make the surface of the ocean look like a splendid flower-garden.
Usually the central zooid in a Siphonophore to which the other zooids are attached is not a bladder-like float, but is an upright tube of greater or less length. In the Siphonophore shown in figure 14, the compound body is composed of a long central hollow stem with hundreds or thousands of variously shaped parts, each of which is reducible to either a polyp or medusazooid, attached around it. The upper end is enlarged to form an air-filled chamber, a sac-like boat, by means of which the whole colony is kept afloat. Around the upper end of the central stem are many medusoid structures, the swimming-bells, by means of whose opening and closing the whole colony is made to swim through the water. Each swimming-bell is a modified medusa-zooid, without tentacles, without digestive or reproductive organs, but exercising the power of swimming by contracting and forcing the water out of the hollow bell just as is done by the free medusæ. Below the swimming-bells, at the lower end of the central stem, are grouped many structures presenting at first sight a confusion of variety and complexity, but on careful examination revealing themselves to be polyp- and medusa-zooids modified to form at least five kinds of particularly functioning structures. There are many flattened scale-like parts whose function is simply that of affording a passive protection, in times of danger, to the other structures. These protecting-scales are greatly modified medusa-zooids, each consisting of a simple cartilage-like gelatinous mass penetrated by a food-carrying canal. Under the broad leaves of these protecting-zooids are a number of pear-shaped bodies which have a wide octagonal mouth-opening at their free end, and possess in their interior certain digestive glands. Each one is provided with a very long flexible tentacle which bears many fine stinging-threads. The tentacle waves back and forth in the water, and on coming in contact with an enemy or with prey its poisonous stinging-threads shoot out and paralyze or wound the unfortunate animal. These pear-shaped bodies are the feeding structures, each being a modified polyp-zooid. Scattered among these dangerous structures are many somewhat similarly shaped but wholly harmless structures, the sense-structures. Each of these has a pear-shaped body but without mouth-opening, and also a long, very sensitive, tentacle-like process. The sense of feeling is highly developed in these tentacles, and they discover for the colony the presence of any strange body. These sense-structures are modified polyp-zooids. Finally there are two other kinds of structures, usually arranged in groups like bunches of grapes, which are the reproductive structures, male and female. They are modified medusa-zooids grown together and without tentacles. This whole colony, or this compound animal, floats or swims about at the surface of the ocean, and performs all of the necessary functions of life as a single animal composed of organs might. Yet the Siphonophore is more truly to be regarded as a community in which the hundreds or thousands of animals, representing five or six kinds of individuals, all of one species, are fastened together. Each individual performs the particular duties devolving upon its kind or class. Thus there are food-gathering individuals, locomotor individuals, sense individuals, and reproductive individuals. The modifications of the various kinds of individuals are more extreme than in the case of the various kinds of individuals composing a bee-community, for example, but the holding together or fusing of all into one body or corporation is a condition which makes this greater modification necessary and not unexpected. And there is no difficulty in seeing that each of these parts is really, structurally considered, a modified polyp or medusa.
Fig. 15.—A jellyfish or medusa, Gonionema vertens, eating two small fishes. (From specimen from Atlantic Coast.)
The large jellyfishes, etc. (Scyphozoa).—To the class Scyphozoa belong most of the common large jellyfishes. When one walks along the sea-beach soon after a storm one may find many shapeless masses of a clear jelly-like substance scattered here and there on the sand. These are the bodies or parts of bodies of jellyfishes which have been cast up by the waves. Exposed to the sun and wind the jelly-like mass soon dries or evaporates away to a small shrivelled mass. The body-substance of a jellyfish contains a very large proportion of water; in fact there is hardly more than 1 per cent of solid matter in it.
The jellyfishes occur in great numbers on the surface of the ocean and are familiar to sailors under the name of "sea-bulbs." Some live in the deeper waters; a few specimens have been dredged up from depths of a mile below the surface. They range in size from "umbrellas" or disks a few millimeters in diameter to disks of a diameter of two meters (2-1/6 yards). They are all carnivorous, preying on other small ocean animals which they catch by means of their tentacles provided with stinging-threads. The tentacles of some of the largest jellyfishes "reach the astonishing length of 40 meters, or about 130 feet." Many of the jellyfishes are beautifully colored, although all are nearly transparent. Almost all of them are phosphorescent, and when irritated some emit a very strong light.
The sea-anemones and corals (Actinozoa).—Almost everywhere along the seashore where there are rocks and tide-pools a host of various kinds of sea-anemones can be found. When the tide is out, exposing the dripping seaweed-covered rocks, and the little sand- or stone-floored basins are left filled with clear sea-water, the brown and green and purple "sea-flowers" may be found fixed to the rocks by the base with the mouth-opening and circlet of slowly-moving tentacles hungrily ready for food (fig. 16). Touch the fringe of tentacles with your fingertip and feel how they cling to it and see how they close in so as to carry what they feel into the mouth-opening. A host of individuals there are, and scores of different kinds; some small, some large, some with the body covered outside with tiny bits of stone and shell so that they are hardly to be distinguished from the rock to which they cling; some of bright and showy colors. These are the most familiar members of the class Actinozoa.