Elementary Zoology Part 7

The tentacles which surround the mouth-opening and serve to grasp food and carry it into the mouth, and the stinging or la.s.so threads with which these tentacles are provided are special organs possessed by most of these animals.

=Skeleton.=--Like the sponges, some of the Clenterata 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 as.e.xually and s.e.xually. The as.e.xual 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 medusae or jellyfish, which separate from the fixed polyp and swim off through the water.

These medusae 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 medusae 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 Clenterata 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.

=Cla.s.sification.=--The branch Clenterata is divided into four cla.s.ses: (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.

[Ill.u.s.tration: FIG. 13.--The Portuguese Man-of-War (_Physalia_ sp.).

(From specimen from Atlantic Coast.)]

=The polyps, colonial jellyfishes, etc. (Hydrozoa).=--To the cla.s.s Hydrozoa belongs the _Hydra_ already studied. There are a few other fresh-water polyps and they all belong to this cla.s.s. The most interesting members of the cla.s.s are the "colonial jellyfishes,"

const.i.tuting 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 cl.u.s.ters 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.

[Ill.u.s.tration: FIG. 14.--A colonial jellyfish (Siphonophora). (After Haeckel.)]

Usually the central zooid in a Siphonoph.o.r.e 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 Siphonoph.o.r.e 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 medusae. 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 pa.s.sive 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 ma.s.s penetrated by a food-carrying ca.n.a.l. 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 Siphonoph.o.r.e 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 cla.s.s. 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.

[Ill.u.s.tration: FIG. 15.--A jellyfish or medusa, _Gonionema vertens_, eating two small fishes. (From specimen from Atlantic Coast.)]

=The large jellyfishes, etc. (Scyphozoa).=--To the cla.s.s Scyphozoa belong most of the common large jellyfishes. When one walks along the sea-beach soon after a storm one may find many shapeless ma.s.ses 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 ma.s.s soon dries or evaporates away to a small shrivelled ma.s.s. 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 phosph.o.r.escent, and when irritated some emit a very strong light.

=The sea-anemones and corals (Actinozoa).=--Almost everywhere along the seash.o.r.e 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 sh.e.l.l 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 cla.s.s Actinozoa.

[Ill.u.s.tration: FIG. 16.--Sea anemones, _Bunodes californica_, open and closed individuals. The closed individuals in upper right-hand corner show the external covering of small bits of rock and sh.e.l.l, characteristic of most individuals of this species. (From living specimens in a tide-pool on the Bay of Monterey, California.)]

But in other oceans, along the coasts of other lands, especially those of the tropics and sub-tropics, there are some other members of the cla.s.s which are of unusual interest. They are the corals, or coral polyps. We know these animals chiefly by their skeletons (fig. 17).

The specimens of corals which one sees in collections, or made into ornaments, are the calcareous skeletons of various kinds of the coral polyps. Some of the corals live together in enormous numbers, forming branching colonies fixed as closely together as possible, and secrete while living a stony skeleton of carbonate of lime. These skeletons persist after the death of the animals, and because of their abundance and close ma.s.sing form great reefs or banks and islands. These coral reefs and islands occur only in the warmer oceans. In the Atlantic they are found along the coasts of Southern Florida, Brazil and the West Indies; in the Pacific and Indian Oceans there are great coral reefs on the coast of Australia, Madagascar and elsewhere, and certain large groups of inhabited islands like the Fiji, Society, and Friendly Islands are exclusively of coral formation. Coral islands have a great variety of form, although the elongated, circular, ring-shaped and crescent forms predominate. How such islands are first formed is described as follows by a well-known student of corals:

[Ill.u.s.tration: FIG. 17.--Skeleton of a branching coral, _Madrepora cervicornis_. (From specimen.)]

"A growing coral plantation, with its mult.i.tudinous life, oftentimes arises from great depths of the ocean, and the sea-bed upon which it rests is probably a submarine bank or mountain, upon which have lodged and slowly aggregated the hard skeletons of pelagic forms of life.

When, through various sources of increase, this submarine bank approaches the depth of from one hundred to one hundred and fifty feet from the surface of the water, there begins on its top a most wonderful vital activity. It is then within the bathymetric zone of the reef-building corals. Of the many groups of marine life which then take possession of the bank, corals are not the only animals, but they are the most important, as far as its subsequent history goes. As the bank slowly rises by their growth, it at last approaches the surface of the water, and at low tide the tips of the growing branches of coral are exposed to the air. This, however, only takes place in sheltered localities, for long before it has reached this elevation it has begun to be more or less changed and broken by the force of the waves. As the submarine bank approaches the tide level, the delicate branching forms have to meet a terrific wave-action. Fragments of the branching corals are broken off from the bank by force of the waves, and falling down into the midst of the growing coral below fill up the interstices, and thus render the whole ma.s.s more compact. At the same time larger fragments are broken and rolled about by the waves and are eventually washed up into banks upon the coral plantation, so that the island now appears slightly elevated above the tides. This may be called a first stage in the development of a coral island. It is, however, little more than a low ridge of worn fragments of coral washed by the high tides and swept by the larger waves--a low, narrow island resting on a large submarine bank."

When the coral island rises thus a little above the surface of the water, the waves break up some of the coral into fine sand, which fills in the interstices, and offers a sort of soil in which may germinate seeds brought in the dried mud on the feet of ocean birds or carried by the ocean currents. With the beginning of vegetable growth the soil is more firmly held, is fertilized and ready for the seeds of plants which need a better soil than lime sand. Flying insects find their way to the island, especially if it be near the mainland, birds begin to nest on it, and soon it may be the seat of a luxuriant plant and animal life.

For an account of coral islands see Darwin's "The Structure and Distribution of Coral Reefs."

There are over 2000 kinds of coral polyp known, and their skeletons vary much in appearance. Because of the appearance of the skeleton certain corals have received common names, as the organ-pipe coral, brain coral, etc. The red coral, of which jewelry is made, grows chiefly in the Mediterranean. It is gathered especially on the western coast of Italy, and on the coasts of Sicily and Sardinia. Most of this coral is sent to Naples, where it is cut into ornaments.

There are other interesting members of the cla.s.s Actinozoa like the beautiful sea-pens, sea-feathers and sea-fans, delicate, branching, tree-like forms found all over the world.

=Ctenophora.=--The members of this cla.s.s are mostly small, peculiar jellyfishes which do not form colonies, and are extremely delicate, being usually perfectly transparent. They swim by means of cilia. They never appear in a polyp condition, but are always medusoid in shape.

CHAPTER XVIII

BRANCH ECHINODERMATA: STARFISHES, SEA-URCHINS, SEA-CUc.u.mBERS

STARFISH (_Asterias_ sp.)

TECHNICAL NOTE.--The species of _Asterias_ are widely distributed on both coasts of the United States and may be procured on almost any rocky sh.o.r.e at low tide. Teachers in inland schools can obtain preserved material from the dealers mentioned on p. 453. Most of the specimens should be placed in alcohol or 4% formalin. If fresh material can be had it is well to place at least one specimen for each student in a 20% solution of nitric acid in water for two or three hours, when all of the calcareous parts will have been dissolved, and after a thorough washing the specimen will be ready for use.

=External structure= (figs. 18 and 19.)--In a fresh specimen or one which has been preserved in alcohol or formalin note the raying out of parts of the body from a common centre. This is characteristic of the body organization of all Echinoderms, and is known as _radial symmetry_.

The lower surface of the body is called the _oral_ (because the mouth is on this surface), while the upper is called the _aboral_ surface. The central part of the body is called the _disk_. Note on the aboral surface of the disk a small striated calcareous plate, the _madreporite_ or _madreporic plate_. In the middle (or very nearly in the middle) of this surface of the disk there is a small pore, the _a.n.a.l opening_. The entire aboral surface as well as a greater part of the oral side is thickly studded with the calcareous _ossicles_ of the body-wall. These ossicles support numerous short stout _spines_ arranged in irregular rows. Note that some of the ossicles support certain very small pincer-like processes, the _pedicellariae_. In the inters.p.a.ces between the calcareous plates are soft fringe-like projections of the inner body-lining, the _respiratory caeca_. Note at the tip of each arm or ray a cl.u.s.ter of small calcareous ossicles and within each cl.u.s.ter a small speck of red pigment, the eye-spot or _ocellus_.

[Ill.u.s.tration: FIG. 18.--Dissection of a starfish (_Asterias_ sp.).]

Make a drawing of the aboral surface showing all these parts.

On the oral surface note the centrally-located _mouth_, the _ambulacral grooves_, one running longitudinally along each ray, and in each groove two double rows of soft tubular bodies with sucker-like tips. These are called the _tube-feet_ and are organs of locomotion.

Make a drawing of the oral surface.

=Internal structure= (figs. 18 and 19).--TECHNICAL NOTE.--Take a specimen which has been immersed for some time in the nitric acid solution, and with a strong pair of scissors, or better, bone-cutters, cut away all the aboral wall of the disk except that immediately around the madreporite and the a.n.u.s. Now begin at the tip of each ray and cut away the aboral wall of each, leaving, however, a single arm intact. When the roof of each arm has been carefully dissected away the specimen should appear as in fig. 18.

Note the large _alimentary ca.n.a.l_, which is divided into several regions. Note the short _sophagus_ leading from the _mouth_ on the oral surface directly into a large membranous pouch, the _cardiac_ portion of the _stomach_. By a short constriction the cardiac portion is separated from the part which lies just above, i.e., the _pyloric_ portion of the stomach. From the pyloric portion large, pointed, paired glandular appendages extend into each ray. These are the _pyloric caeca_. Their function is digestive, and oftentimes they are spoken of as the _digestive glands_ or "livers." The pyloric caeca, as well as the cardiac portion of the stomach, are held in place by paired muscles which extend into each arm. Note two sets of these muscles, one set for thrusting the cardiac portion of the stomach out through the mouth and another for pulling it back, the _protractor muscles_ and _retractor muscles_, respectively. The starfish obtains its food by enclosing it in its everted stomach and then withdrawing stomach and food into the body. Note that the pyloric portion of the stomach opens above into a short _intestine_ terminating in the _a.n.u.s_, and observe that there is attached to the intestine a convoluted many-branched tube, the _intestinal caec.u.m_.

Carefully remove a pair of pyloric caeca from one of the rays and note the short duct which connects them with the pyloric chamber of the stomach. Note in the angle of each two adjoining rays paired glandular ma.s.ses which empty by a common duct on the aboral surface. These glands are the _reproductive organs_. Note the small bulb-like bladders extending in two double rows on the floor of each ray. These are the water-sacs or _ampullae_, and each one is connected directly with one of the locomotor organs, the tube-feet.

Make a drawing of the organs in the dissection which have so far been studied.

TECHNICAL NOTE.--For a careful study of the locomotor organs a fresh starfish should be injected. This can usually be accomplished by cutting one ray off squarely, and inserting the needle of a hypodermic syringe (which has been previously filled with a watery solution of carmine or Berlin blue), into the end of the radial water-tube which runs along the floor of the ray. By injecting here, the whole system of vessels, tube-feet, and ampullae are filled.

Note a ring-shaped ca.n.a.l which pa.s.ses around the alimentary ca.n.a.l near the mouth from which radial vessels run out beneath the floor of each ray and from which a hard tube extends to the madreporite. This hard tube is the _stone ca.n.a.l_, so called because its walls contain a series of calcareous rings, while the circular tube is the _ring ca.n.a.l_ or _circ.u.m-oral water-ring_ from which radiate the _radial ca.n.a.ls_. In some species of starfish there are bladder-like reservoirs, _Polian vesicles_, which extend interradially from the ring ca.n.a.l.

Note that the ampullae and tube-feet are all connected with the radial ca.n.a.ls. By a contraction of the delicate muscles in the walls of the ampullae the fluid in the cavity is compressed, thereby forcing the tube-feet out. By the contraction of muscles in the tube-feet they are again shortened while the small disk-like terminal sucker clings to some firm object. In this way the animal pulls itself along by successive "steps." This entire system, called the _water-vascular system_, is characteristic of the branch Echinodermata. In addition to the fluid in the water-vascular system there is yet another body-fluid, the _perivisceral fluid_, which bathes all of the tissues and fills the body-cavity.

[Ill.u.s.tration: FIG. 19.--Semi-diagrammatic figure of cross-section of the ray of a starfish, _Asterias_ sp.]

TECHNICAL NOTE.--Take a drop of the perivisceral fluid from a living starfish and examine under high power of microscope, noting the amboid cells it contains.

The perivisceral fluid is aerated through outpocketings of the thin body-wall which extend outward between the calcareous plates of the body. These outpocketings have already been mentioned as the respiratory caeca (see p. 109). Surrounding the stone ca.n.a.l is a thin membranous tube, and within it and by the side of the stone ca.n.a.l is a soft tubular sac. The function of these organs is not certainly known.

Work out the _nervous system_; note, as its princ.i.p.al parts, a nerve-ring about the mouth, and nerves running from this ring beneath the radial ca.n.a.ls along each arm.

=Life-history and habits.=--The starfishes are all marine forms. They hatch from eggs, and in their early stages are very different in appearance from the adults. At first they are bilaterally symmetrical, their radial symmetry being acquired later. Thousands of eggs and sperm-cells are extruded into the sea-water, where fertilization and development take place. The young swim freely in the open sea, feeding on microscopic organisms, and then undergo very radical changes in the course of their development. The adults are for the most part carnivorous, feeding on crabs, snails, and the like. The live prey is surrounded by the extruded stomach which secretes fluids that kill it, after which the soft parts are digested. (See general account of the life-history of Echinoderms on p. 119.)