CLASSIFICATION OF ANIMALS—VERTEBRATES AND INVERTEBRATES—PROTOZOA—HYDROZOA—ACTINOZOA.

Zoology treats of life—of organized beings which are capable of voluntary motion. Plants exist, animals live and move. Both are organic beings, but the latter possess the faculty of will and spontaneous movement. The animal can leave a place and enjoy other surroundings, the plant cannot. We have already crossed the borderland which connects the plant and the animal. We have seen plants almost animals. We could commence this section with animals which are almost plants, so closely do the divisions approach each other. Zoology is the science of the knowledge of animals as Botany is of the knowledge of plants.

Where there is vegetation there are animals, not quadrupeds or bipeds necessarily, but numbers of small, it may be invisible, creatures which exist upon the vegetable kingdom—the algÆ and minute creations of globules and cells, the infusoria already mentioned, the corals, etc. And in the “protozoa,” or first specimens of animal life, we have a similarity to the vegetable kingdom; we then get by gradual steps to other more perfect beings, each after his kind, till we arrive at the most perfect animal—Man.

Animals are divided into two families, the Invertebrate and the Vertebrate. The former has no spine nor skeleton; the latter has both. These again are divided into sub-families, classes, and orders, as follows.

Man is an animal—but what is an animal? We can scarcely tell in a few words. LinnÆus defined the difference between the animal and the plant, for the former, said he, live, grow, and feel, while the latter live and grow. We have protozoa in the animal kingdom consisting of a single cell or blood corpuscle, some others without mouths or digestive organs, some have no head; some, as in the tape-worm, only a so-called head, with suckers or attachments, after which it develops joints, which are at first imperfect, but gradually mature as they are pushed farther away by new-issuing joints.

Animals, therefore, do not all possess organs, nor is there any common organ by which all animals can be classed. The indispensable in one is absent in another, and while our mouths and digestive apparatus are all important, in other animals suckers and no digestive apparatus at all is quite sufficient. Some have one mouth, some several; some have mouths and a proboscis to assist them, some only the trunk and no mouth—so called—at all, as in some insects.

The organisms which could not be distinguished from vegetables were termed zoophytes, or plant animals, and, were space available, a comparison might be instituted between the extremes of growth of the animals and plants, from the largest whales to the tiny microscopic protozoa, and from the mould upon jam to the gigantic trees of California, one leaf of which it is said will shelter twenty men from rain.

Cuvier spent many years in perfecting his systematic arrangements of animals, and this classification, though many rearrangements have been made as modern discovery progressed, may be regarded as the fundamental system of all. Professor Agassiz adopted it with modifications. Professor Nicholson has made a somewhat different arrangement, but essentially there will be found but slight difference between them. We append both these arrangements for comparison:—

In the vertebrated animals the blood is red in consequence of the minute cells (corpuscles) which contain the colouring matter. In invertebrate animals these red cells are absent, and so the animals are white-blooded. Some animals, again, are cold-blooded like the fish; birds and mammalia have warm blood. It is worthy of remark that the higher we advance in the scale the fewer the offspring of the animal. The animalcules multiply at the rate of many billions a day, and even one codfish is stated to contain more than nine millions of eggs. A mackerel will produce 500,000; and so on, as we rise, we find mammals with seldom more than ten young at a time, down to one single offspring.

We could fill pages with the account of the differences existing between animals created for such different purposes and fitted to inhabit different climates, their mode of feeding and catching prey. The manner of bringing forth and rearing the young, and the temperament and temper of the animal creation would fill a volume, but we cannot now stay to examine these various characteristics. The following is the arrangement now usually adopted:—

We will adopt the latter order as being the more modern, and endeavour to make the various classes of the invertebrates clear to the mind, if we cannot present them to the vision, of the reader.

In our sketch of Botany we remarked upon the similarity existing between the cells of plants and animals, and although there are, of course, differences, there are many points of resemblance in these cells.

Plants have their lowest representatives called Protophytes. Animals which correspond to this class are termed Protozoa, from the Greek, proton, first, and zoÖn, animal. The former are, as already mentioned, seen amongst the algÆ, consisting of simple cells, and protozoa cannot easily be distinguished from them except in the matter of nutriment, for some protozoa have no mouth except in the infusoria class. The cells are very much alike, and Dr. Carpenter sums them up briefly as follows:—

“The animal cell, in its most complete form, is comparable in most parts of its structure to that of the plant, but differs from it in the entire absence of the ‘cellulose wall’ or of anything that represents it, the cell-contents being enclosed in only a single limitary membrane, the chemical composition of which, being albuminous, indicates its correspondence with the primordial utricle. In its young state it seems always to contain a semi-fluid plasma, which is essentially the same as the protoplasm of the plant, save that it does not include chlorophyl granules, and this may either continue to occupy its cavity (which is the case in cells whose entire energy is directed to growth and multiplication), or may give place, either wholly or in part, to the special product which it may be the function of the cell to prepare. Like the vegetable cell, that of animals very commonly multiplies by duplicative subdivision, it also (especially among protozoa) may give origin to new cells by the breaking up of its contents into several particles.”

The protozoa are microscopic creatures consisting of one or more cells, and are infinitely little, thousands existing in a drop of water. They have no distinction of sexes, and their generation takes place by subdivision or blending of cells. The infusoria are the highest of the protozoa, and were formerly included amongst the radiata. Their numbers are infinite, and in a drop of water (see fig. 828) some very interesting specimens will be found. These infusoria are merely sarcode, or a jelly-like substance, and some have cilia, or hairy appendages, with which they agitate the water and cause a kind of current which brings them food. It is this partaking of food which has served to divide the lowest animal from the lowest vegetable creations. There is no progressive increase of development from the lowest plant to the highest animal. The animal begins by himself, as it were, as the plant, and both grow up in different directions. The protozoa exist upon organic substances, while plants absorb inorganic substances and assimilate them.

The GregarinidÆ are very tiny cells, and though microscopically minute, they sometimes develop into worm-like or elongated oval bodies. They inhabit the intestines of crustacea, worms, and cockroaches, as well as of higher animals. They are capable of certain motion, but are not furnished even with the “false feet” (pseudo podia) of the rhizopoda, the next animal in these very low scales of creation.

Of the Rhizopoda the AmebÆ are very interesting, and we find them in our veins as well as in the stagnant green water of the pond. They are simply sarcode or jelly, and, as the name implies, the amebÆ can change their appearance (amoibos, changing). They possess a kind of crawling, progressive motion, and under the microscope will be perceived to develop a tiny bud, as it were, which is the “false foot” that assists its progress. These amebÆ are in our blood moving about, and are always altering their form, and when warm they move more quickly in the red blood corpuscles or cells, but excessive heat will kill them.

These curious creatures feed by the foot they protrude; and by drawing in the “process” as it is termed, they can collect within themselves the nourishment they require. Of course they have no mouth, and if we can conceive a creature of this kind which thrusts out from a jelly-bag a tiny lump, and pulls it in again at any time and place it likes, we have an idea of an ameba.

The pond ameba is somewhat different from the others, inasmuch as it possesses an outer and inner portion or layer which are different in density. There is what is termed a contractile vesicle which “beats” as a heart beats, but this is very primitive. There is really no structure whatever in these rhizopoda, and, as we have seen, their shape is always undergoing change. The outer and inner layers of the amebÆ are called “the ectosarc” and “endosarc” respectively; the latter contains the darker portion—the nucleus.

The Foraminifera have already been mentioned in the chapters on Geology. We find these minute creatures must have had a great deal to do with the building up of rocks, as they have the power to make tiny coverings for themselves, which have been built into rocks by the addition of sandy particles, and consolidated by pressure. Here we have a most wonderful instance of the tiniest creatures producing the greatest masses of the earth. The body is merely sarcode, the shell is carbonate of lime. The foraminifera produce false feet in abundance, which surround the cell like fine hairs or rays. They live in the sea, and when they die the shells descend upon the ocean floor, where they undergo many changes and become converted into rock. The ooze of the great oceans is composed of these shells, and is practically a chalky deposit; the shells are being built up as in former ages with the curious nummulites of the Eocene formation, which are amongst the most interesting of fossils.

Sponges. We must go on at once to the Sponges, which form such an interesting subject as they are so familiar to us all. Sponge is not often regarded by the public as an animal, and though perhaps authorities may not have yet concluded in what category they should be placed, we may consider them here according to the list.

We find the spongida both in fresh and salt water, and they have given rise to much discussion as to whether they should be classed as animals at all. But that question having been finally settled, we can proceed to examine a sponge in its native state, and we shall find both skeleton and “flesh.”

The skeleton is hard and composed of needles of “tiny” texture. The flesh is “sarcode,” and the animal possesses no mouth, but is full of holes (pores) and canals through which the water is continually distributed. The outer layer of the sponge is formed of ultimate components of the living substance of the sponge (like the amebÆ we have been considering). Each contains a nucleus, and when joined together form the outer layer of the body. Beneath is a wide cavity communicating with the exterior by means of minute holes, and filled with water. The cavity separates the superficial layer from the deeper substance, which is of the same character. In the water passages of the sponge are cilia which induce a cement, and the interior canals develop into chambers lined with sponge particles, and the water carries particles to the sponge, which represents a kind of sub-aqueous city, where the people are arranged about the streets and roads in such a manner that each can easily appropriate his food from the water as it passes along.38

Sponge, then, is a mass of living organisms—tiny living creatures capable of feeding and of movement, resembling amebÆ or perhaps infusoria, with cilia, to enable them to obtain nourishment by a kind of inhalation or respiration. They are reproductive by sexual and a-sexual processes which produce spongellÆ. The living sponge is a beautifully coloured animal, and grows upon almost any solid foundation; and in the autumn the parent sponge displays a number of yellow dots or “gemmules,” which are the young. These are soon cast off and left to shift for themselves, and seek their fortunes, helpless as they appear, in the wide and stormy sea. At last they find a resting-place, and fix themselves for ever, growing up and reproducing their species until they are carried off to be sold and used in civilized countries for domestic purposes.

We must leave these curious animalculÆ and glance at the Infusoria, which constitute a higher branch, but microscopic and universal, and include those called Flagellate, Ciliate, and Tentaculate. The first have whip-like cilia, or feelers, or filaments, which are ever in motion to cause a movement of the water and carry food to the animal. You will find plenty of infusoria in any stagnant water, and when placed under the microscope a mouth may be perceived, but no stomach, nor any apparent receptacle for food, which appears to enter at once into the body substance. The other kinds capture their food by seizure by the tentacles, or by agitating the cilia, like the flagellata, and thus whipping the nourishment towards the mouth, as children will draw in a toy boat to land by agitating the water in the given direction. These cilia, or hairs, serve for organs of locomotion as well as of capture. These creatures are called Infusoria, because they exist in vegetable “infusions” exposed to the atmosphere.

Decaying vegetable or animal substances, such as the leaves of trees, grass, a piece of flesh, etc., affused with water and exposed to air and warmth, will speedily, upon microscopic examination, be found peopled with numbers of most active minute creatures of the most varied forms. These animalcules are found also in the stagnant pools around our cities, in the waters of rivers, harbours, and lakes, and even in the ocean.

In reference to the origin of these animalcules, the view was long entertained that they were generated spontaneously, that the decaying vegetable and animal substances were decomposed and resolved into these simple beings. More accurate experiments have shown, however, that the infusoria are produced from ova, or germs, which are probably carried about in the dried-up state, in the form of minute particles of dust,39 ready to develop themselves in any spot which may afford them the requisite moisture and nutriment. In this respect they resemble the microscopic fungi, whose germs are diffused in the same way. When once they have obtained the means of development, they multiply with incredible celerity. If the decaying vegetable or animal substances be carefully excluded from contact with the air, or if the air be heated before it is admitted to them, no infusoria will appear. They are rarely developed on mountains of a certain height, where the atmosphere is free from foreign bodies.

Though these animalcules are so exceedingly minute, yet the forms exhibited by them are extremely various, and some of them present also considerable variety in the forms assumed by the same individual under different circumstances. In many species the soft body is enclosed in a firm integument, strengthened by a deposit of siliceous matter; these envelopes, which are often preserved after the death of the animals, are termed the shields, and the animalcules encased in them are called loricated infusoria. The remarkable discovery has been made that large distinct beds of former formations are entirely made up of the accumulated remains of these animalcules.

We arrive at the Hydrozoa after leaving the Infusoria, and find ourselves in the sea, and far from land, where it will be difficult for us to ascertain the characteristics of these interesting animals. But fortunately we can obtain much nearer home, and occasionally in a private aquarium, a specimen of the hydrozoa which will serve our purpose, as it has served before to introduce readers to the study of these water-polypes, some of which are so like plants that they are frequently mistaken for them.

The hydrozoa present a “definite histological structure,” says Professor Huxley; “the body always exhibits a separation into at least two distinct layers of tissue, an outer and an inner.” The Hydras, or fresh-water polypes, which may be found in nearly every pond adhering to the duckweed, appears like tubes, and if touched will curl up into tiny knobs. But if let alone they will adhere to a glass by their single foot, or sucker, which can be moved at pleasure.

The foot, or sucker, is continued to a slender cylindrical stalk, from the end of which radiate a number of tentacles, or “feelers,” growing around the mouth, and serving to convey or attract food to the animal which is, so to speak, all stomach. There is no breathing apparatus, and what food it cannot digest is expelled from the mouth. The peculiarity which has given the hydra its name is, that no matter into how many pieces you cut this polype, the parts cut off will all develop into little polypes perfect as their parent.40 But germination is carried on naturally by buds thrown out, and cast (by “gemmation”), or by the ordinary sexual production of ova.

The outer and inner skins of the hydra are called the ectoderm and endoderm, and the animal is quite capable of locomotion, walking, or rather moving, backwards, by raising and planting its sucker or foot, and by swimming. The prey is captured by the tentacles and by the darting out of tiny spears from the cells or “thread cells” which contain them on the surface of the body. The well-known “Portuguese man-of-war,” an ocean polype, has these “harpoons” greatly developed, and can inflict serious pain as of many stinging nettles; the sensation is exceedingly painful, and lasts some time.

The MedusidÆ are known to the seaside visitor as the jelly-fish, and the other AcalephÆ, the “hidden-eyed” medusÆ, include the Portuguese man-of-war mentioned above, and many other umbrella-like animals. They have received the name of medusÆ from Medusa, whose long, snaky locks the tentacles of the animals are supposed to be like. Some of these “floating umbrellas” are very dangerous, and will inflict severe stings upon any one in their vicinity. The tentacles or filaments extend for a long distance, and bathers should be cautious. We have often watched them, and they are beautiful to contemplate particularly at night, and in Kingstown Harbour, near Dublin, many exceedingly fine specimens have been obtained. The pulsation of the “umbrella” or bell, enables the animal to swim, and the even undulations of this beautiful covering are apparently caused by nervous contractions.

The jelly-fish have no resemblance to “fish,” and scarcely appear to exist; they are of no use to man, and when removed from the water dwindle by little and little to a tiny film and nothing more—they dissolve into air and water. Cases have been known and tales told of how farmers collected hundreds of these jelly-fish for manure, and when the cart reached the field, to the man’s astonishment, nothing was left but what appeared cobweb in the place of the load of fish.

The Cyclippe is a very common specimen, and moves by means of its cilia; Cestum Veneris—the zone or girdle of Venus—is another curious example. It appears like a glass ribbon about five inches wide and perhaps four or five feet long. The cilia when in motion are very brilliant in colouring, and the creature undulates through the water in a remarkable manner.

The luminosity of the medusÆ is clearly perceived, the so-called phosphorescence being due chiefly to the minute jelly-fish which abound near the surface of the sea. It appears impossible, for most, at any rate, if not all, these medusÆ to sink beneath the surface, for they can be found in hundreds cast ashore, melting away into film. We might imagine that they would be provided with some means of sinking themselves, but being apparently only air and water, it is necessary for them to remain upon the surface to exist at all.

The term AcalephÆ, by which they are known, means “stinging” fish or sea-nettles, the Greek word meaning nettle.

The Actinozoa comprise corals and the popular sea anemones (actinidÆ). They resemble the hydrozoa in possessing tentacles, and also the two inner and outer tissues of the body. But they differ from the hydrozoa in their interior arrangement in the possession of a kind of stomach between the “body cavity” and the mouth which the hydrozoa do not possess. The appearance of the sea anemone is well known. It fixes itself by the flat base and hangs out its tentacles to obtain food. When we touch an anemone with a stick we perceive how it contracts itself, but there is no nervous system nor any respiration. The reproduction of its species is carried on within, not as in other animals, like the hydra, by exterior budding.

The corals belong to the same class as the sea anemones, and are called zoanthidÆ. We have already in previous portions of this volume mentioned the “coral” building polypes, but we may again describe them here. We have the black coral or antipathidÆ, which live in masses and are united by a stem. They grow upon this fleshy trunk and cover it in time “just as a trunk of a tree is covered by the bark.” This stem is called a cÆnosarc, which secretes the coral, or skeleton. The madrepores are the greatest producers of the coral of commerce.

“If we examine a simple coral of this group,” says Professor Nicholson, “we find that we have to deal with an animal in all important respects identical with an ordinary sea anemone, but having a more or less complicated skeleton developed in its interior.” This skeleton is the corallum, and it is composed, as most people are aware, of calcareous matter deposited within the polype itself; in the former case the development or formation is exterior to the polype. A single polype will thus secrete a deposit, and a colony of them produce a compound skeleton, and as they throw out buds or young polypes, the manufacture of skeletons goes on by secretion.

The Tubipores are like pipes, and the coral has been termed the “organ-pipe.” It is formed cylindrically and joined externally. As under Geology we have examined the question of coral reefs, we need not here recapitulate the descriptions given in that section.

Doctor Bariel writes of these animals as follows:—“By far the greater part of the Zoanthoid polypes, as they grow, deposit in the cellular substance of the flesh of their back an immense quantity of calcareous matter which enlarges as the animal increases in size, and, in fact, fills up those portions of the substance of the animal, which by the growth of new parts are no longer wanted for its nourishment, and in this manner they form a hard and strong case, amongst the folds of which they contract themselves so as to be protected from external injury, and by the same means they form for themselves a permanent attachment which prevents their being tossed about by every wave of the element in which they live. The stony substances so formed are called corals, and their mode of formation causes them exactly to represent the animal which secretes them. The upper surface is always furnished with radiating plates, the remains of the calcareous particles which are deposited in the longitudinal folds of the stomach.”