by Isaac Asimov
Around 1800, the French naturalist Georges Leopold Cuvier went beyond classes and added a more general category called the phylum (from a Greek word for “tribe”). A phylum includes all animals with the same general body plane (a concept that was emphasized and made clear by none other than the great German poet Johann Wolfgang von Goethe). For instance, the mammals, birds, reptiles, amphibia, and fishes are placed in one phylum because all have backbones, a maximum of four limbs, and red blood containing hemoglobin. Insects, spiders, lobsters, and centipedes are placed in another phylum; clams, oysters, and mussels in still another; and so on. In the 1820s, the Swiss botanist Augustin Pyrarnus de Candolle similarly improved Linnaeus’s classification of plants. Instead of grouping species together according to external appearance, he laid more weight on internal structure and functioning.
The tree of life now is arranged as I shall describe in the following paragraphs, going from the most general divisions to the more specific.
We start with the kingdoms, which for a long time were assumed to be two in number: animals and plants. (The assumption is still made in the popular game of “Twenty Questions,” in which everything is classified as “animal, vegetable or mineral.”) However, the growing knowledge concerning the microorganisms complicated matters, and the American biologist Robert Harding Whittaker suggested that living organisms be divided into no fewer than five kingdoms.
By Whittaker’s system, the plant kingdom and the animal kingdom are confined to multicellular organisms. The plants are characterized by the possession of chlorophyll (so that they are the so-called green plants) and the use of photosynthesis. The animals ingest other organisms as food and have digestive systems.
A third kingdom, the fungi, are multicellular and resemble plants in some ways but lack chlorophyll. They live on other organisms though they do not ingest them as animals do, but excrete digestive enzymes, digest their food outside the body, then absorb it.
The remaining two kingdoms contain one-celled organisms. Protista, a word coined in 1866 by the German biologist Ernst Heinrich Haeckel, includes the eukaryotes: both those that are made of cells resembling those that constitute animals (protozoa, such as the amoeba and the paramecium); and those that are cells resembling those that constitute plants (algae).
Finally, a kingdom known as moneta contain the one-celled organisms that are prokaryotes—the bacteria and the blue-green algae. Left out of this scheme are the viruses and viroids which are subcellular and might well form a sixth kingdom.
The plant kingdom, according to one system of classification, is divided into two main phyla—the Bryophyta (the various mosses) and the Tracheophyta (plants with systems of tubes for the circulation of sap), which includes all the species that we ordinarily think of as plants.
This last great phylum is made up of three main classes: the Filicineae, the Cymnospermae, and the Angiospermae. In the first class are the ferns, which reproduce by means of spores. The gymnosperms, forming seeds on the surface of the seed-bearing organs, include the various evergreen cone-bearing trees. The angiosperms, with the seeds enclosed in ovules, make up the vast majority of the familiar plants.
As for the animal kingdom, I shall list only the more important phyla.
The Porifera are animals consisting of colonies of cells within a pore-bearing skeleton; these are the sponges. The individual cells show signs of specialization but retain a certain independence, for when all are separated by straining through a silk cloth, they may aggregate to form a new sponge.
(In general, as the animal phyla grow more specialized, individual cells and tissues grow less “independent.” Simple creatures can regrow to entire organisms even though badly mutilated, a process called regeneration. More complex ones can regrow limbs. By the time we reach humans, however, the capacity for regeneration has sunk quite low. We can regrow a lost fingernail but not a lost finger.)
The first phylum whose members can be considered truly multicellular animals is the Coelenterata (meaning “hollow gut”). These animals have the basic shape of a cup and consist of two layers of cells—the ectoderm (“outer skin”) and the endoderm (“inner skin”). The most common examples of this phylum are the jellyfish and the sea anemones.
All the rest of the animal phyla have a third layer of cells—the mesoderm (“middle skin”). From these three layers, first recognized in 1845 by the German physiologists Johannes Peter Muller and Robert Remak, are formed the many organs of even the most complex animals, including man.
The mesoderm arises during the development of the embryo, and the manner in which it arises divides the animals involved into two superphyla. Those in which the mesoderm forms at the junction of the ectoderm and the endoderm make up the Annelid superphylum; those in which the mesoderm arises in the endoderm alone are the Echinoderm superphylum.
Let us consider the Annelid superphylum first. Its simplest phylum is Platyhelminthes (Greek for “flat worms”). This includes not only the parasitic tapeworm but also free-living forms. The flatworms have contractile fibers that can be considered primitive muscles, and they also possess a head, a tail, special reproductive organs, and the beginnings of excretory organs. In addition, the flatworms display bilateral symmetry: that is, they have left and right sides that are mirror images of each other. They move headfirst, and their sense organs and rudimentary nerves are concentrated in the head area, so that the flatworm can be said to possess the first step toward a brain.
Next comes the phylum Nematoda (Greek for “thread worm”), whose most familiar member is the hookworm. These creatures possess a primitive bloodstream—a fluid within the mesoderm that bathes all the cells and conveys food and oxygen to them. This allows the nematodes, in contrast to animals such as the flat tapeworm, to have bulk, for the fluid can bring nourishment to interior cells. The nematodes also possess a gut with two openings, one for the entry of food, the other (the anus) for ejection of wastes.
The next two phyla in this superphylum have hard external skeletons—that is, shells (which are found in some of the simpler phyla, too). These two groups are the Brachiopoda, which have calcium carbonate shells on top and bottom and are popularly called lampshells, and the Mollusca (Latin for “soft”), whose soft bodies are enclosed in shells originating from the right and left sides instead of the top and bottom. The most familiar molluscs are the clams, oysters, and snails.
A particularly important phylum in the Annelid superphylum is Annelida. These are worms, but with a difference: they are composed of segments, each of which can be looked upon as a kind of organism in itself. Each segment has its own nerves branching off the main nerve stem, its own blood vessels, its own tubules for carrying off wastes, its own muscles, and so on. In the most familiar annelid, the earthworm, the segments are marked off by little constrictions of flesh which look like little rings around the animal; in fact, Annelida is from a Latin word meaning “little ring.”
Segmentation apparently endows an animal with superior efficiency, for all the most successful species of the animal kingdom, including the human, are segmented. (Of the nonsegmented animals, the most complex and successful is the squid.) If you wonder how the human body is segmented, think of the vertebrae and the ribs; each vertebra of the backbone and each rib represents a separate segment of the body, with its own nerves, muscles, and blood vessels.
The annelids, lacking a skeleton, are soft and relatively defenseless. The phylum Arthropoda (“jointed feet”), however, combines segmentation with a skeleton, the skeleton being as segmented as the rest of the body. The skeleton is not only more maneuverable for being jointed; it is also light and tough, being made of a polysaccharide called chitin rather than of heavy, inflexible limestone or calcium carbonate. On the whole, the Arthropoda—which includes lobsters, spiders, centipedes, and insects—is the most successful phylum in existence. At least it contains more species than all the other phyla put together.
This accounts for the main phyla in the Annelid superphylum. The oth
er superphylum, the Echinoderm, contains only two important phyla. One is Echinodermata (“spiny skin”), which includes such creatures as the starfish and the sea urchin. The echinoderms differ from other mesoderm-containing phyla in possessing radial symmetry and having no clearly defined head and tail (though, in early life, echinoderms do show bilateral symmetry, which they lose as they mature).
The second important phylum of the Echinoderm superphylum is important indeed, for it is the one to which human beings themselves belong.
THE VERTEBRATES
The general characteristic that distinguishes the members of this phylum (which embraces the human being, ostrich, snake, frog, mackerel, and a varied host of other animals) is an internal skeleton (figure 16.1). No animal outside this phylum possesses one. The particular mark of such a skeleton is the backbone. In fact, the backbone is so important a feature that, in Common parlance, all animals are loosely divided into vertebrates and invertebrates.
Figure 16.1. A philogenetic tree, showing evolutionary lines of the vertebrates.
Actually, there is an in-between group which has a rod of cartilage called a notochord (“backcord”) in the place of the backbone (figure 16.2). The notochord, first discovered by Von Baer, who had also discovered the mammalian ovum, seems to represent a rudimentary backbone; in fact, it makes its appearance even in mammals during the development of the embryo. So the animals with notochords (various wormlike, sluglike, and mollusclike creatures) are classed with the vertebrates. The whole phylum was named Chordata in 1880, by the English zoologist Francis Maitland Balfour; it is divided into four subphyla, three of which have only a notochord. The fourth, with a true backbone and general internal skeleton, is Vertebrata.
Figure 16.2. Amphioxus, a primitive, fishlike chordate with a notochord.
The vertebrates in existence today form two superclasses: the Pisces (“fishes”) and the Tetrapoda (“four-footed” animals).
The Pisces group is made up of three classes: (1) the Agnatha (“jawless”) fishes, which have true skeletons but no limbs or jaws—the best-known representative, the lamprey, possessing a rasping set of files in a round suckerlike mouth; (2) the Chondrichthyes (cartilage fish), with a skeleton of cartilage instead of bone, sharks being the most familiar example; and (3) the Osteichthyes, or bony fishes.
The tetrapods, or four-footed animals, all of which breathe by means of lungs, make up four classes. The simplest are the Amphibia (“double life”)—for example, the frogs and toads. The double life means that in their immature youth (for example, as tadpoles), they have no limbs and breathe by means of gills; then as adults they develop four feet and lungs. The amphibians, like fishes, lay their eggs in the water.
The second class are the Reptilia (from a Latin word meaning “creeping”). They include snakes, lizards, alligators, and turtles. They breathe with lungs from birth, and hatch their eggs (enclosed in a hard shell) on land. The most advanced reptiles have essentially four-chambered hearts, whereas the amphibian’s heart has three chambers, and the fish’s heart only two.
The final two groups of tetrapods are the Aves (birds) and the Mammalia (mammals). All are warm-blooded: that is, their bodies possess devices that maintain an even internal temperature regardless of the temperature outside (within reasonable limits). Since the internal temperature is usually higher than the external, these animals require insulation. As aids to this end, the birds are equipped with feathers and the mammals with hair, both serving to trap a layer of insulating air next to the skin. The birds lay eggs like those of reptiles. The mammals, of course, bring forth their young already “hatched” and supply them with milk produced by mammary glands (mammae in Latin)
In the nineteenth century, zoologists heard reports of a great curiosity so amazing that they refused to believe it. The Australians had found a creature that had hair and produced milk (through mammary glands that lacked nipples), yet laid eggs! Even when the zoologists were shown specimens of the animal (not alive, unfortunately, because it is not easy to keep it alive away from its natural habitat), they were inclined to brand it a clumsy fraud. The beast was a land-and-water animal that looked a good deal like a duck: it had a bill and webbed feet. Eventually the duckbilled platypus had to be recognized as a genuine phenomenon and a new kind of mammal. Another egglaying mammal, the echidua has since been found in Australia and New Guinea. Nor is it only in the laying of eggs that these mammals show themselves to be still close to the reptile. They are only imperfectly warm-blooded; on cold days their internal temperature may drop as much as 10 degrees centigrade.
The mammals are now divided into three subclasses. The egg-laving mammals form the first class, Prototheria (Greek for “first beasts”). The embryo in the egg is actually well developed by the time the egg is laid, and it hatches out not long afterward. The second subclass of mammals, Metatheria (“midbeasts”), includes the opossums and kangaroos. Their young, though born alive, are in a very undeveloped form and will die in short order unless they manage to reach the mother’s protective pouch and stay at the mammary nipples until they are strong enough to move about. These animals are called marsupials (from marsupium, Latin for “pouch”).
Finally, at the top of the mammalian hierarchy, we come to the subclass Eutheria (“true beasts”). Their distinguishing feature is the placenta, a blood-suffused tissue that enables the mother to supply the embryo with food and oxygen and carry off its wastes, so that she can develop the offspring for a long period inside her body (nine months in the case of the human being, two years in the case of elephants and whales). The eutherians are usually referred to as placental mammals.
The placental mammals are divided into well over a dozen orders, of which the following are examples:
Insectivora (“insect-eating”)—shrews, moles, and others.
Chiroptera (“hand-wings”)—the bats.
Carnivora (“meat-eating”)—the cat family, the dog family, weasels, bears, seals, and so on, but not including human beings.
Rodentia (“gnawing”)—mice, rats, rabbits, squirrels, guinea pigs, beavers, porcupines, and so on.
Edentata (“toothless”)—the sloths and armadillos, which have teeth, and anteaters, which do not.
Artiodactyla (“even toes”)—hoofed animals with an even number of toes on each foot, such as cattle, sheep, goats, swine, deer, antelopes, camels, giraffes, and so on.
Perissodactyla (“odd toes”)—horses, donkeys, zebras, rhinoceroses, and tapirs.
Proboscidea (“long nose”)—the elephants, of course.
Odontoceti (“toothed whales”)—the sperm whale and others with teeth.
Mysticeti (“mustached whales”)—the right whale, the blue whale, and others that filter their small sea food through fringes of whalebone that look like a colossal mustache inside the mouth.
Primates (“first”)—humans, apes, monkeys, and some other creatures with which we may be surprised to find ourselves associated.
The primates are characterized by hands and sometimes feet that are equipped for grasping, with opposable thumbs and big toes. The digits are topped with flattened nails rather than with sharp claws or enclosing hoofs. The brain is enlarged, and the sense of vision is more important than the sense of smell. There are many other, less obvious, anatomical criteria.
The primates are divided into nine families. Some have so few primate characteristics that it is hard to think of them as primates, but so they must be classed, One is the family Tupaiidae, which includes the insect-eating tree-shrews! Then there are the lemurs—nocturnal, tree-living creatures with foxlike muzzles and a rather squirrelly appearance, found particularly in Madagascar.
The families closest to humans are, of course, the monkeys and apes. There are three families of monkeys (a word possibly derived from the Latin homunculus, meaning “little man”).
The two monkey families in the Americas, known as the New World monkeys, are the Cebidae (for example, the organ-grinder’s monkey) and the Callithricidae (for
example, the marmoset). The third, the Old World family, are the Cercopithecidae; they include the various baboons.
The apes all belong to one family, called Pongidae. They are native to the Eastern Hemisphere. Their most noticeable outward differences from the monkeys are, of course, their larger size and their lack of tails. The apes fall into four types: the gibbon, smallest, hairiest, longest-armed, and most primitive of the family; the orangutan, larger, but also a tree-dweller like the gibbon; the gorilla, rather larger than a man, mainly ground-dwelling, and a native of Africa; and the chimpanzee, also a dweller in Africa, rather smaller than a man and the most intelligent primate next to humans themselves.
As for our own family, Hominidae, it consists today of only one genus and, as a matter of fact, only one species. Linnaeus named the species Homo sapiens (“man the wise”), and no one has dared change the name, despite provocation.
Evolution
It is almost impossible to run down the roster of living things, as I have just done, without ending with a strong impression that there has been a slow development of life from the very simple to the complex. The phyla can be arranged so that each seems to add something to the one before. Within each phylum, the various classes can be arranged likewise; and within each class, the orders.
