No one knows how many different kinds of animals there are living today. When Aristotle (384-322 B.C.) wrote the first History of Animals he succeeded in rounding up only about 500 species in spite of the fact that Alexander the Great, his famous pupil, gave special instructions to his conquering armies to aid in collecting from the ends of the earth information about foreign animals which his old master so eagerly desired.
Since Aristotle's day explorers have stretched the horizon that then shut in the Mediterranean world, until now even Darkest Africa has been entirely criss-crossed, both poles have been trampled upon, and no considerable corner of the globe anywhere, on land or sea, is left from which authentic tales of animal life have not been brought back.
According to recent estimates upwards of 1,000,000 species of living animals are known to science. Of these probably 65,000 are chordates. In addition there are fossil remains of many more extinct animals that have no living representatives. As long ago as 1890, according to Ward, the manuscript catalogue of known plants at the Kew Gardens weighed over a ton. The inquirer who would be informed about the different kinds of living things might well be appalled at the prospect of passing in review within a single lifetime of study even a tithe of this wealth of animal and plant life.
John Malpet, who in 1567 wrote one of the first "natural histories" in the English language, started his treatise with the hopeful sentence, "Let us begin alphabetically with the adder." There is an easier way out of the situation, however, than by John Malpet’s alphabetical method. Even Aristotle recognized in the make-up of animals a unity of plan by which they could be placed in natural groups so that acquaintance with a single representative of a group would give a considerable working knowledge of all other kinds within that particular group. Familiarity with the mechanism and behavior of a house cat, for example, gives one a good idea of all other kinds of cats, such as lions, tigers, lynxes, leopards, ocelots, jaguars, wildcats, pumas, cheetahs, and panthers. In fact much of the fascination that goes with the study of biology lies in recognizing resemblances and differences between various sorts of plants and animals.
Although the number of kinds of plants and animals is very great, the different general types or plans of structure are relatively few, so that the student, by using the type-study method of sampling, may set out confidently and with a brave heart upon the ambitious quest of intellectually conquering all creation.
Limiting the survey solely to animal life, a list of the chief types of animals comprises: Protozoa, Coelenterata, Platyhelminthes, Nemathelminthes, Annelida, Arthropoda, Mollusca, Echinodermata, and Chordata.
Of all animal types the chordate type is of most immediate interest, since it includes man. Many of the riddles connected with that much studied animal find their solution in the lower forms.
For instance, the parietal body, a conical projection about the size of a cherry stone, is buried between the lobes of the human brain. Its origin and use baffled anatomists for centuries until Baldwin Spencer in 1886 dissected a New Zealand lizard, Sphenodon (Fig. 1), by some called a “living fossil.” He discovered that in the chordate type, the “parietal body,” or a part arising from it, is simply a degenerate median eye since in this curious primitive reptile it reaches, with retina and nerve complete, all the way to a transparent window in the roof of the skull and, in early life at least, may function as a third eye.
It is entirely true that often more may be learned of human development and structure by the intelligent examination of a dogfish, or some other lowly vertebrate, than by the direct study of the human body itself. This is due not so much to the greater availability of lower animals for dissection and experimentation, as to the fact that they furnish sidelight stages through which the human body has passed in arriving at its present degree of complexity, and thus give a clue for interpreting the why and wherefore of the “fearfully and wonderfully made” human mechanism. Herein lies the value of the study of comparative biology.
The indirect path has thus been often the shortest cut to unexpected attainment in the history of science. Inquisitive Ben Franklin, out in the thunder-storm with his key and kite, took the first step towards harnessing electricity; the Frenchman Daguerre, trying to discover some way to clean tarnished silver, blazed a path which has bccome a broad highway in photography and the colossal motion-picture industry; Alexander Graham Bell, attempting to aid the deaf to hear, led to the invention of the telephone ; Joseph Cushman, with insatiable curiosity about the variety of forms of microscopic shelled protozoans, hit upon a way to tell those who bore for oil when they were on the right track; while Pasteur, a thinking chemist interested primarily in the apparently remote subject of the shape of crystals, laid firm foundations for the far-reaching developments of bacteriology and modern medicine.
When such facts as these are recalled, nothing about the structure or activities of any animal, however familiar or strange, becomes insignificant or trivial to the seeker after truth concerning man.
Essential Features of Every Animal Type
Every form of life, whether plant or animal, must possess machinery of some sort for accomplishing two fundamental processes, namely, metabolism and reproduction.
Metabolism includes all activities that concern the upkeep of the individual, such as the intake of energy by way of food, its release in the form of action which constitutes “living,” and the disposal of waste products incident thereto.
Reproduction provides for the continuation of the species upon the earth, often at the cost of the individual life. The former function may be designated as selfish and egoistic, the latter as unselfish and altruistic.
A typical insect, for example, is made up of three easily distinguishable regions, in order of relative importance, the abdomen, thorax, and head (Fig. 2). In the large abdomen is lodged the principal machinery for metabolism and reproduction, that is, most of the digestive apparatus, the respiratory, excretory, and circulatory machinery, and the reproductive organs.
The thorax is devoted primarily to locomotion, furnished as it is with three pairs of legs and usually with two pairs of wings together with the muscles necessary to work them, and is thus enabled to transport the important abdomen to places where it can selfishly procure energy-producing food and unselfishly provide for the next generation. Finally, there is the head, with its battery of directive sense organs and a controlling brain, which tells the thorax and abdomen where to go and what to do upon arrival. Many animals get along comfortably without a head or locomotor devices, but none can dispense with the all-important abdomen or something corresponding to it. Even in man that crowning glory, the head, which he is quite apt to regard as important, as well as the locomotor legs, becomes quite subsidiary when the trunk, that corresponds to the insect’s abdomen, sends out the imperious call of hunger or of sex.
The function of metabolism is usually accomplished in a different way by animals than it is in plants, with the result that most plants remain stationary, while most animals move about. The reason for this difference lies in the fact that green plants possess the power, in the presence of sunlight, of building up their foods out of universally distributed materials, such as carbon dioxide in the air, water, and various inorganic compounds in the soil. No animal can do this, so it comes about that all animals must find their energy, either directly or indirectly, in the stored supply already captured by green plants from the sun. This is why most animals are forever fated, like the Wandering Jew, to be travelers, a condition which necessitates in animal types some adequate device for locomotion, and consequently an accompanying directive sensory equipment. The fact that certain animals like oysters and corals are sedentary is the exception to the rule. “Their strength is to sit still.” Even in the case of these animals the indirect dependence on green plants is quite as complete as among locomotor forms, since they feed upon microscopic green plants that form the floating meadows of the ocean, and in consequence have developed secondary devices for bringing this floating food to them. Most animals digest their food in an alimentary canal, which begins in a mouth near the anterior end of the body and terminates posteriorly in an anus.
The science of the form and shape of organisms is called Morphology, a term coined by the many-sided Goethe in 1817. It is closely related to the mathematical science of Solid Geometry, with the difference that the mathematician has little occasion to inquire why one figure is a cube and another a sphere except to determine the relation of the different dimensions to each other, while the biologist is constantly being challenged to explain why each organism is shaped as it is, in relation to the particular life that it leads. Moreover, the shapes and forms with which the geometrician deals are arbitrary creations of the human mind, not particularly related to the environment, having no modifying past and no forward look to a future in which modifications may take place. The forms of animals and plants which a biologist considers are the products of an actual historical sequence that has taken place, of ancestral shapes that have in succession all left their determining impress.
There are no animals with less than three dimensions, although some of the lower forms are so small and thin as to necessitate very delicate instruments to determine their length, breadth, and thickness.
Three fundamental shapes and forms are recognized and, as a result, three general types of symmetry, namely, spherical, radial, and bilateral. Each of these types may be camouflaged in various ways by secondary modifying qualifications.
Spherical symmetry in organisms is rare. It is to be found only among microscopic animals, such as the Heliozoa, or “sun animalcules” of the protozoan type which float without contact with anything solid, surrounded by water on all sides. Many floating animals, on the other hand, become attached, during a part of their life at least, and lead a sedentary plantlike existence. Such anchored animals are usually headless, and frequently develop a crown of radiating arms or tentacles that enable them to reach out in every direction to explore as far as possible their immediate neighborhood. This headless plan is the radial type of symmetry, which in general is characteristic of trees and other stationary plants, as well as of attached or sessile animals, whose food is brought to them floating in water. In all of these organisms the body possesses polarity, being organized along a longitudinal axis with an attached end and a free end.
On land, where food does not float in a transporting medium, animals have to travel to obtain it when they are hungry. This has made necessary a directive head. Although a head end is characteristic of certain water animals such as fishes, it becomes an absolute necessity for locomotor land animals. Whenever an animal moves persistently in one direction with reference to its own body, in other words whenever a true head end is established, bilateral symmetry results, and a stagnant life of watchful waiting ceases. With the appearance of this type of symmetry animals usually develop the habit of keeping one particular side of the body either in contact with the substrate or facing downwards. This undersurface is the ventral side of the animal, while the upper surface is the dorsal side. The body presenting this type of symmetry may be divided into halves by means of three planes which can be arranged with reference to length, breadth, and thickness.
In radial symmetry, on the other hand, the number of planes dividing the animal into similar halves is practically infinite, like the number of ways in which a cylinder may be split lengthwise into two equal parts.
The three planes (Fig. 3) bisecting length, breadth, and thickness divide any bilaterally symmetrical animal into definite regions, very useful as landmarks in description, as follows:
Sagittal plane dividing the body into right and left halves, mirror images of one another;
Transverse plane dividing the body into anterior and posterior halves;
Frontal plane dividing the body into dorsal and ventral halves.
The sagittal and frontal planes are so named because of certain sutures in the human sltull with which they coincide. It is obvious that upright man moves forward with the ventral body-half in front, instead of the anterior body-half, because he is a bilaterally symmetrical animal tipped up on end.
Although the comparative anatomist uses the above terms, the student of human anatomy sometimes uses a different set. The head-end may be known as the superior portion in man, while the lower part of the body is the inferior portion. The terms anterior and posterior are then used as the synonyms of “ventral” and “dorsal,” respectively, of comparative anatomy.
The body of an annelid worm, an arthropod, or an embryonic chordate consists, in basic plan, of a series of similar, repeated divisions (metameres or segments) arranged one behind the other. An adult annelid (e.g., Nereis) very closely approximates this basic plan, with the the metameres clearly marked off externally. Each metamere, except the first and last, possesses a pair of appendages. Internally, at the level of each external constriction, there is found a cross-partition (septum). Other internal structures, such as nephridia, tend to be repeated in each metamere. Chordates, however, never have external constrictions, although most of them exhibit internal segmentation of a number of embryonic organs, for example, the skeletal muscles. The axial skeleton and the nervous system show a modified metameric organization.
The chordates, in common with annelids, echinoderms, and some other invertebrate phyla, possess a coelom (the so-called true body cavity), lined with mesodermal tissue and lying between the digestive tract and the body wall. Thus great freedom is permitted for the development and activity of embryonic organs as well as for the movement of such adult parts as the heart, stomach and intestine.
The backboned animals (Vertebrata), together with a few closely related animals which do not possess a backbone, are ordinarily included in the Phylum Chordata. In the preceding sections certain chordate features, also commonly possessed by members of other animal phyla, have been considered. These primitive characters of chordates include: (1) reproductive glands; (2) alimentary canal; (3) polarity; (4) bilateral symmetry; (5) metamerism; and (6) coelom.
Chordates also possess certain other features, which distinguish them from all other animals. The chief diagnostic characters of chordates are the following:
During the embryonic development of every chordate there appears a supporting rod, the notochord, which lies immediately dorsal to the digestive tract. In some chordates this structure persists throughout life; in others it is partially or completely replaced by a skull and a “backbone” made up of separate bony elements, or vertebrae, as the name “vertebrate” indicates. Essentially the notochord consists of a tough connective tissue sheath in which soft cells are packed so tightly that the whole structure possesses a certain turgor, somewhat like that when sausage meat is crowded into a casing.
Dorsal, Hollow, Central Nervous System
The chordate nervous system develops on the dorsal side of the body by a process known as invagination (Fig. 4). In this structure, even in adult animals, there is a cavity which is continuous from near the anterior end of the brain to the posterior end of the nerve cord. The central nervous system of non-chordates, on the other hand, is formed on the ventral side of the body and is solid. In the vertebrate members of the chordate phylum, the anterior end of the central nervous system is much enlarged into a brain with which there are associated three pairs of major sense organs: olfactory (nose); optic (eyes) ; and otic (ears).
Pharyngeal Breathing Device
Fishes have several porthole-like passage-ways, or gill slits, penetrating through the lateral walls of the food tube on either side of its anterior end. Within these gill slits in water-dwelling chordates hang feathery tufts of capillaries, or gills, which rob the circulating water of some of its dissolved air, thus accomplishing the function of breathing.
Gill slits, or traces of them, are present, at least in embryonic life, in all chordates, whether dwelling in water or out of it, and even in reptiles, birds, or mammals which never breathe by means of gills. Whenever breathing is accomplished by lungs, such organs develop as side alleys from this same anterior pharyngeal region of the food tube where the gills originate. No non-chordate breathes in this way, although many kinds of animals employ “gills” of various sorts. Pharyngeal gills and gill slits, or traces of them, are peculiar to chordates.
These first three chordate characteristics are present at some time during the life of every individual of the chordate phylum.
The heart, which is the headquarters of the circulatory system, is ventrally located in chordates. In other animals, when a heart is present, it is on the dorsal side of the body.
Closed Blood System
In chordates the blood courses through a continuous system of tubes from heart, to arteries, to capillaries in the various tissues, to veins, and back to the heart again. Most non-chordates, on the other hand, have an open blood system, that is, one in which the blood may pass freely back and forth between the blood vessels and surrounding spaces or sinuses. The contrast is remotely like that between the waterworks of a modern city with water and sewage confined to pipes and mains, and the open ponds and streams of the countryside.
Hepatic Portal System
Although venous systems, beginning in capillaries in the tissues of the body, ordinarily terminate at the heart, there are places where veins not only begin but also end in capillaries. Such a group of veins is known as a portal system. In most chordates, the food-laden blood from the digestive tract passes through a strainer-like capillary network, the liver, before it arrives at the heart to be sent over the hungry body. The group of veins beginning in capillaries in the digestive tract and ending in hepatic (hepat-, liver) capillaries is known as the hepatic portal system.
Although other animals have organs that are called “livers” by courtesy, only chordates have a true liver, or clearing house, where the strained blood is reorganized by addition and subtraction of various substances before being distributed to different parts of the body.
A Post-Anal Tail
A true tail may be defined as a continuation of the body axis posterior to the anal exit of the food tube. That part of a lobster, for example, which is sometimes erroneously called the “tail,” is not a true tail at all, but the abdomen, since the anus opens at the end of it. Each vertebrate has a true tail, either throughout life or embryonically and ancestrally. Even tailless man has in his early fetal stages an unmistakable tail (Fig. 5), and there are numerous well-authenticated cases reported in medical literature of human tails that persist beyond embryonic life.
Red Blood Cells
The red respiratory pigment of the blood may be dissolved in the liquid part of the blood or may be confined in red blood cells. In the chordates the pigment is always in cells. In non-chordates it is usually in the plasma, but in a number of species, scattered among various invertebrate phyla, the red pigment is in cells. Among the non-chordates possessing red blood cells are: Area, a bivalve mollusc; Glycera, a polychaete annelid; and Thyone, a sea cucumber of the echinoderm phylum. These particular examples are chosen because they are relatively common animals representative of three different phyla. No evolutionary significance is to be attached to the occurrence of this chordate feature in widely scattered species of invertebrates.
Comparative Diagrams of Chordates and Non-Chordates
A visualized diagrammatic summary of some of the outstanding points of contrast between a generalized chordate and a corresponding non-chordate is presented in Fig. 6.