Elementary Zoology Part 4

Observe the method of reproduction in _Hydra_. Commonly the parent produces small buds, which at first are only ev.a.g.i.n.ations of the body-wall, but which later develop tentacles and a mouth of their own.

Subsequently the bud becomes constricted at the base, separates from the parent, and the young _Hydra_ begins a distinct existence.

Another mode of reproduction takes place which, in distinction from the as.e.xual method just mentioned, is called s.e.xual reproduction. This last is the method common to most of the higher organisms. You may note that in some _Hydrae_ there is a swelling or bulging of the ectoderm of the body-wall in the region just below the tentacles. These are the _sperm-glands_. Within these are produced sperm-cells which break away in great cl.u.s.ters to fertilize the ova, or eggs. Note a larger bulging of the body-wall nearer the lower end of the body which, under high power, has a granular appearance. This is the _egg-gland_, in which develops a single _ovum_ or _egg_. The ovum breaks from its covering and is fertilized by sperm-cells from another individual. In forms like _Hydra_, where both s.e.xes are represented in a single individual, the organism is termed _moncious_ or _hermaphroditic_. In connection with reproduction Chapter XIII should be studied.

An instructive experiment can be performed by cutting a _Hydra_ into two or more parts, when (usually) each of the various parts will develop into a complete _Hydrae_. This process may be called reproduction by fission, but it rarely occurs naturally.

CHAPTER XI

THE SIMPLEST MANY-CELLED ANIMALS

=Cell differentiation and body organization in Hydra.=--From the examination of _Hydra_ we have learned that there are true many-celled animals which are much less complex in structure than the toad and crayfish. The body of _Hydra_, like the body of the toad, is composed of many cells, but these cells are of only a few different kinds; that is, show but little differentiation. There is relatively little division of the body into distinct organs. Still, certain parts of the body devote themselves princ.i.p.ally to certain particular functions. Thus all the food is taken in through the single "mouth-opening" at the apical free end of the cylindrical body, and there are certain organs, the tentacles, whose special business or function it is to find and seize food and to convey it to the mouth. After the food is taken into the cylindrical body-cavity it is digested by special cells which line the cavity. Some of these cells are unusually large, and each contains one or more contractile vacuoles. From the free ends of these cells, the ends which are next to the body-cavity, project pseudopods or flagella.

These protoplasmic processes are constantly changing their form and number. In addition to these large sub-amboid cells there are, in this inner layer of cells lining the body-cavity, and especially abundant near the base or bottom of the cavity, many long, narrow, granular cells. These are gland-cells which secrete a digestive fluid. The food captured by the tentacles and taken in through the mouth-opening disintegrates in the body-cavity, or digestive cavity as it may be called. The digestive fluid secreted by the gland-cells acts upon it so that it becomes broken into small parts. These particles are seized by the projecting pseudopods of the sub-amboid cells and taken into the body-protoplasm of these cells. The cells of the outer layer of the body do not take food directly, but receive nourishment only by means of and through the cells of the inner layer. The body-cavity of _Hydra_ is a very simple special organ of digestion.

In the outer layer of cells there are some specially large cells whose inner ends are extended as narrow pointed prolongations directed at right angles with the rest of the cell. These processes are very contractile and are called muscle-processes. Each one is simply a specially contractile continuation of the protoplasm of the cell-body.

There are also in this layer some small cells very irregular in shape and provided with unusually large nuclei. These cells are more irritable or sensitive than the others and are called nerve-cells. We have thus in _Hydra_ the beginnings of muscular organs and of nerve-organs. But how simple and unformed compared with the muscular and nervous systems of the toad and crayfish! There is no circulatory system, nor are there any special organs of respiration.

But _Hydra_ is far in advance of _Amba_ or _Paramcium_. Its body is composed of thousands of distinct cells. Some of these cells devote themselves especially to food-taking, some especially to the digestion of food; some are specially contractile, and on them the movements of the body depend, while others are specially irritable or sensitive, and on them the body depends for knowledge of the contact of prey or enemies. In the cnidoblast cells, those with the stinging threads, there is a very wide departure from the simple primitive type of cells. There is in _Hydra_ a manifest differentiation of the cells into various kinds of cells. The beginnings of distinct tissues and organs are indicated.

=Degrees in cell differentiation and body organization.=--In the study of the cellular const.i.tution of the tissues and organs of the toad, we found to what a high degree the differentiation of the cells may attain, and in the study of the anatomy of the toad we found how thoroughly these differentiated cells may be combined and organized into body-parts or organs. The body of the toad is made up of distinct organs, each composed of highly differentiated or specialized cells.

The body of _Hydra_ is composed of cells for the most part only slightly differentiated and hardly recognizably grouped or combined into organs. These two conditions are the extremes in the body-structure of the many-celled animals. Between them is a host of intermediate conditions of cell differentiation and body organization.

When we come to the study of other members of the great branch of simple many-celled animals to which _Hydra_ belongs (see Chapter XVII), it will be found that some of them show a slight advance in complexity beyond _Hydra_. Higher in the scale of animal life the forms will be found still more and more complex, with ever-increasing differentiation of the cells, with the combination of the differentiated cells into distinct organs, and the co-ordination of organs into systems of organs up to the extreme shown by the birds and mammals. And hand in hand with this increasing complexity of structure goes ever-increasing complexity or specialization of function.

Breathing is a simple function or process with _Hydra_, where each body-cell takes up oxygen for itself, but it is a complex business with the toad, or with a bird or mammal, where certain complex structures, the lungs and accessory parts, and the heart, blood-vessels and blood all work together to distribute oxygen to all parts of the body.

CHAPTER XII

DEVELOPMENT OF THE TOAD

FIELD AND LABORATORY EXERCISE

TECHNICAL NOTE.--As the work of this chapter, or some similar work in getting acquainted with the postembryonic development of a many-celled animal, should be done early in the course, and as most schools open in the fall, it will perhaps be impossible to make this first study of development from live specimens in the field. In such case the examination of a series of prepared specimens, previously obtained by the teacher, must be resorted to. In the spring the development of several kinds of animals, including the toad, can be studied from live specimens in the field or in breeding-cages and aquaria in the laboratory. The eggs of the toad may be found in April and May (the toads are heard trilling at egg-laying time) in ponds. The eggs look like the heads of black pins, and are in single rows in long strings of transparent jelly, which are usually wound around sticks or plant-stems at the bottom of the pond near the sh.o.r.e. Bring some of these strings into the schoolroom and keep them in water in shallow dishes. Keep them in the light, but not in direct sunlight. In the dishes put some small stones and mud from the pond, arranging them in a slope, thus making different depths of water. Stones with green algae on should be selected, for algae are the food of the tadpoles. The eggs will hatch in two or three days, and if too many tadpoles are not kept in the dish, and the little aquarium be well cared for, the whole postembryonic development of the toad can be well observed. For the study of the development from prepared specimens the teacher should have a complete series of stages from egg to adult toad in alcohol. The specimens may be examined by the students in connection with a talk from the teacher on the life-history of the toad.

If the study is made from prepared specimens, make drawings of egg-strings, and of a single egg magnified and shaded to indicate its color. Draw each specimen of the series of tadpoles, noting in the youngest the presence of gills and tail and absence of legs and eyes; in the older the appearance of eyes, the shrivelling of the gills, shrinking of the tail and development of legs; in the still older the characteristic shape, in miniature, of the adult toad.

In observing the course of development of the living specimens there should be made, in addition to the drawings, notes showing the duration of the egg stage, and the time elapsing between all important changes (as seen externally) in the body of the young. Observations and notes on the general behavior of tadpoles should also be made; note the swimming, the feeding, the gradual leaving of the water, etc.

In addition to the easily seen external changes in the body, very important ones in the internal organs take place during development.

Perhaps the most important of these concerns the lungs. The young gilled toad breathes as a fish does, but gradually its gills are lost, while at the same time lungs develop and the tadpole comes to the surface to breathe air like any lunged aquatic animal. The toad on leaving the water changes its diet from vegetable to animal food; a tadpole feeds on aquatic algae; a toad preys on insects. Correlated with the change in habit, the intestine during development undergoes some marked changes, becoming relatively diminished in length.

For an account of the development of the toad see Gage's "Life-history of a Toad" or Hodge's "The Common Toad."

CHAPTER XIII

MULTIPLICATION AND DEVELOPMENT.--MULTIPLICATION OF ONE-CELLED ANIMALS

=Multiplication.=--We know that any living animal has parents; that is, has been produced by other animals which may still be living or be now dead or, as with _Amba_, may have changed, by division, into new individuals. Individuals die, but before death, they produce other individuals like themselves. If they did not, their kind or species would die with them. This production of new animals constantly going on is called the reproduction or multiplication of animals. The process is well called multiplication, because each female animal normally produces more than one new individual. She may produce only one at a time, one a year, as many of the sea-birds do or as the elephant does, but she lives many years. Or she may produce hundreds, or thousands, or even millions of young in a very short time. A lobster lays 10,000 eggs at a time. Nearly nine millions of eggs have been taken from the body of a thirty-pound female codfish. As a matter of fact but very, very few of these eggs produce new animals which reach maturity. From the 10,000 eggs produced by the lobster each year an average of but two new mature lobsters is produced. There is always a struggle for food and for place going on among animals, for many more are produced than there are food and room for, and so of all the new or young animals which are born the great majority are killed before they reach maturity. In a later chapter more attention will be given to this great struggle for life.

In the preceding paragraph it has been stated that "we know that any living animal has parents; that is, has been produced by other animals which may still be living or be now dead." This is a statement, however, which has found complete acceptance only in modern times. It is a familiar fact that a new kitten comes into the world only through being born; that it is the offspring of parents of its kind. But we may not be personally familiar with the fact that a new starfish comes into the world only as the production of parent starfish, or that a new earthworm can be produced only by other earthworms. But naturalists have proved these statements. All life comes from life; all organisms are produced by other organisms. And new individuals are produced by other individuals of the same kind. That these statements are true all modern observations and investigations of the origin of new individuals prove. But in the days of the earlier naturalists the life of the microscopic organisms like _Amba_ and _Paramcium_, and even that of many of the larger but unfamiliar animals, was shrouded in mystery. And various and strange beliefs were held regarding the origin of new individuals.

=Spontaneous generation.=--The ancients believed that many animals were spontaneously generated. The early naturalists thought that flies arose by spontaneous generation from the decaying matter of dead animals. Frogs and many insects were thought to be generated spontaneously from mud, and horse-hairs in water were thought to change into water-snakes. But such beliefs were easily shown to be based on error, and have been long discarded by zoologists. But the belief that the microscopic organisms, such as bacteria and infusoria, were spontaneously generated in stagnant water or decaying organic liquids was held by some naturalists until very recent times. And it was not so easy to disprove the a.s.sertions of such believers. If some water in which there are apparently no living organisms, however minute, be allowed to stand for a few days, it will come to swarm with microscopic plants and animals. Any organic liquid, as a broth or a vegetable infusion, exposed to the air for a short time becomes foul through the presence of innumerable microscopic organisms. But it has been certainly proved that these organisms are not spontaneously produced in the water or organic fluid. A few of them enter the water from the air, in which there are always greater or less numbers of spores of microscopic organisms. These spores germinate quickly when they fall into water or some organic liquid, and the rapid succession of generations soon gives rise to the hosts of bacteria and one-celled animals which infest all standing water. If all the active organisms and inactive spores in a gla.s.s of water are killed by boiling the water, and this sterilized water be put into a sterilized gla.s.s, and this gla.s.s be so well closed that germs or spores cannot pa.s.s from the air without into the sterilized liquid, no living animals will ever appear in it. We know of no instance of the spontaneous generation of animals, and all the animals whose life-history we know are produced by other animals of the same kind.

=Simplest multiplication and development.=--The simplest method of multiplication and the simplest kind of development shown among animals are exhibited by such simple animals as _Amba_ and _Paramcium_. The production of new individuals is accomplished in _Amba_ by a simple division or fission of its body (a single cell) into two practically equivalent parts. An _Amba_ which has grown for some time contracts all of its finger-like processes, the pseudopodia, and its body becomes constricted. This constriction or fissure increases inwards so that the body is soon divided fairly in two. There are now two _Ambae_, each half the size of the original one; each, indeed, actually one-half of the original one. The original _Amba_ was the parent; the two halves of it are the young. Each of the young possesses all of the characteristics and powers of the parent; each can move, eat, feel, grow, and reproduce by fission. The only change necessary for the young or new _Amba_ to become like its parent, is that of simple growth to a size about twice its present size. The development here is reduced to a minimum. Just as the simplest animals perform the other life-processes, such as taking and digesting food, breathing and feeling, in an extremely primitive simple way, so do they perform the necessary life-process of reproduction or multiplication in the simplest way shown among animals.

In the case of _Paramcium_ the process of multiplication is slightly more complex than that of _Amba_ in the fact that sometimes before the simple fission of the body takes place the interesting phenomenon of conjugation occurs. _Paramcium_ may reproduce itself for many generations by simple fission, but a generation finally appears in which conjugation takes place. Two individuals come together and each exchanges with the other a part of its nucleus. Then the two individuals separate and each divides into two. The result of the conjugation, or the coming together, of two individuals with mutual interchange of nuclear substance is to give to the new _Paramcia_ produced by the conjugating individuals a body which contains part of the body-substance of two distinct individuals. If the two conjugating individuals differ at all--and they always do differ, because no two individual animals, although belonging to the same species, are exactly alike--the new individual, made up of parts of each of them, will differ slightly from both. Nature seems intent on making every new individual differ slightly from the individual which precedes it.

And the method of multiplication which Nature has adopted to produce the result is the method which we have seen exhibited in its simplest form in the case of _Paramcium_--the method of having two individuals take part in the production of a new one.

The development of the new _Paramcia_ is a little more complex than that of _Amba_. Not only must the new _Paramcium_ grow to the size of the original one, but it must develop those slight, but apparent, modifications of the parts of its body which we can recognize in the full-grown, fully developed _Paramcium_ individual. A new mouth-opening must develop on the new individual formed of the hinder half of the original _Paramcium_ and new cilia must be developed.

Thus there is a slight advance in complexity of development, just as there is in complexity of structure in _Paramcium_ as compared with _Amba_. In the many-celled animals this complexity of development is carried to an extreme.

=Birth and hatching.=--When a young animal is born alive, it usually resembles in appearance and structure the parent, although of course it is much smaller, and requires always a certain time to complete its development and become mature. A young kangaroo or opossum is carried for some time after its birth in an external pouch on the mother's body and is a very helpless animal. A young kitten is born with eyes not yet opened and must be fed by the mother for several weeks. On the other hand young Rocky Mountain sheep are able to run about swiftly within a few hours after birth.

Most animals appear first as eggs laid by the mother. This is true of the birds, the reptiles, the fishes, the insects, and most of the hosts of invertebrate animals. This egg may be cared for by the parent as with the birds, or simply deposited in a safe place as with most insects, or perhaps dropped without care into the water as with most marine invertebrates. The young animal which issues from the egg may at the time of its hatching resemble the parent in appearance and structural character (although always much smaller) as with the birds, some of the insects, and many of the other animals. Or it may issue in a so-called _larval_ condition, in which it resembles the parent but slightly or not at all, as is the case with the gill-bearing, legless, tailed tadpole of the frog or the crawling, wingless, wormlike caterpillar of the b.u.t.terfly, or the maggot of the house-fly.

=Life-history.=--Any animal which hatches from an egg has undergone a longer or shorter period of development within the egg-sh.e.l.l before hatching. The development of an animal from first germ-cell to the time it leaves the egg, for example, the development of the embryo chick from the first cell to time of hatching, is called its _embryonic_ development; and the development from then on, for example, that of the chick to adult hen or rooster, or that of tadpole to frog, is called the _post-embryonic_ development. Beginning students of animals cannot study the embryonic development (_embryology_) of animals readily, but they can in many cases easily follow the course of the post-embryonic development, and this study will always be interesting and valuable.

When the "life-history" of an animal is spoken of in this book, or other elementary text-book of zoology, it is the history of the life of the animal from the time of its birth or hatching to and through adult condition that is meant, not the complete life-history from beginning single egg-cell to the end. In all of the study of the different kinds of animals to which the rest of this book is devoted, attention will be paid to their life-history.

PART II

SYSTEMATIC ZOOLOGY

CHAPTER XIV

THE CLa.s.sIFICATION OF ANIMALS

=Basis and significance of cla.s.sification.=--It is the common knowledge of all of us that animals are cla.s.sified: that is, that the different kinds are arranged in the mind of the zoologist and in the books of natural history, in various groups, and that these various groups are of different rank or degree of comprehensiveness. A group of high rank or great comprehensiveness includes groups of lower rank, and each of these includes groups of still lower rank, and so on, for several degrees. For example, we have already learned that the toad belongs to the great group of back-boned animals, the Vertebrates, as the group is called. So do the fishes and the birds, the reptiles and the mammals or quadrupeds. But each of these const.i.tutes a lesser group, and each may in turn be subdivided into still lesser groups.

In the early days of the study of animals and plants their cla.s.sification or division into groups was based on the resemblances and the differences which the early naturalists found among the organisms they knew. At first all of the cla.s.sifying was done by paying attention to external resemblances and differences, but later when naturalists began to dissect animals and to get acquainted with the structure of the whole body, the differences and likenesses of inner parts, such as the skeleton and the organs of circulation and respiration, were taken into account. At the present time and ever since the theory of descent began to be accepted by naturalists (and there is practically no one who does not now accept it), the cla.s.sification of animals, while still largely based on resemblances and differences among them, tells more than the simple fact that animals of the same group resemble each other in certain structural characters. It means that the members of a group are related to each other by descent, that is, genealogically. They are all the descendants of a common ancestor; they are all sprung from a common stock. And this added meaning of cla.s.sification explains the older meaning; it explains why the animals are alike. The members of a group resemble each other in structure because they are actually blood relations. But as their common ancestor lived ages ago, we can learn the history of this descent, and find out these blood relationships among animals only by the study of forms existing now, or through the fragmentary remains of extinct animals preserved in the rocks as fossils. As a matter of fact we usually learn of the existence of this actual blood relationship, or the fact of common ancestry among animals, by studying their structure and finding out the resemblances and differences among them. If much alike we believe them closely related; if less alike we believe them less closely related, and so on. So after all, though the present-day cla.s.sification means something more, means a great deal more, in fact, than the cla.s.sification of the earlier naturalists means, it is largely based on and determined by resemblances and differences just as was the old cla.s.sification. Sometimes the fossil remains of ancient animals tell us much about the ancestry and descent of existing forms. For example, the present-day one-toed horse has been clearly shown by series of fossils to be descended from a small five-toed horse-like animal which lived in the Tertiary age.

=Importance of development in determining cla.s.sification.=--A very important means of determining the relationships among animals is by studying their development. If two kinds of animals undergo very similar development, that is, if in their development and growth from egg-cell to adult they pa.s.s through similar stages, they are nearly related. And by the correspondence or lack of correspondence, by the similarity or dissimilarity of the course of development of different animals much regarding their relationship to each other is revealed.

Sometimes two kinds of animals which are really nearly related come to differ very much in appearance in their fully developed adult condition because of the widely different life-habits the two may have. But if they are nearly related their developmental stages will be closely similar until the animals are almost fully developed. For example, certain animals belonging to the group which includes the crabs, lobsters, and crayfishes, have adopted a parasitic habit of life, and in their adult condition live attached to the bodies of certain kinds of true crabs. As these parasites have no need of moving about, being carried by their hosts, they have lost their legs by degeneration, and the body has come to be a mere sac-like pulsating ma.s.s, attached to the host by slender root-like processes, and not resembling at all the bodies of their relatives the crabs and crayfishes. If we had to trust, in making out our cla.s.sification, solely to structural resemblances and differences, we should never cla.s.sify the _Sacculina_ (the parasite) in the group Crustacea, which is the group including the crabs and lobsters and crayfishes. But the young _Sacculina_ is an active free-swimming creature resembling the young crabs and young shrimps. By a study of the development of _Sacculina_ we find that it is more closely related to the crabs and crayfishes and the other Crustaceans than to any other animals, although in adult condition it does not at all, at least in external appearance, resemble a crab or lobster.

=Scientific names.=--To cla.s.sify animals then, is to determine their true relationships and to express these relationships by a scheme of groups. To these groups proper names are given for convenience in referring to them. These proper names are all Latin or Greek, simply because these cla.s.sic languages are taught in the schools and colleges of almost all the countries in the world, and are thus intelligible to naturalists of all nationalities. In the older days, indeed, all the scientific books, the descriptions and accounts of animals and plants, were written in Latin, and now most of the technical words used in naming the parts of animals and plants are Latin. So that Latin may be called the language of science. For most of the groups of animals we have English names as well as Greek or Latin ones and when talking with an English-speaking person we can use these names. But when scientific men write of animals they use the names which have been agreed on by naturalists of all nationalities and which are understood by all of these naturalists. These Latin and Greek names of animals laughed at by non-scientific persons as "jaw-breakers," are really a great convenience, and save much circ.u.mlocution and misunderstanding.