Term Paper on Higher Animals: Top 2 Papers | Biology

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Term Paper # 1. Phylum Arthropoda:

The arthropods, “joint-footed” animals, include such creatures as the crustaceans (crabs and lobsters), arachnids (spiders, ticks, mites, and scorpions), and insects; the latter constitute by far the largest group. Despite the huge size and diversity of the arthropod phylum, there are a number of features shared by all the members of this group.

Characteristics of the Arthropods:

The Exoskeleton:

First, all arthropods have an articulated (jointed) exoskeleton. This exoskeleton, or cuticle, is secreted by the underlying epidermis and is attached to it; it is made up of an outer waxy layer, composed of lipoprotein, and a middle horny layer and an inner flexible one, both composed principally of chitin. The exoskeleton not only covers the surface of the animal but also extends inward at both ends of the digestive tract and, in insects, lines the tracheae (breathing tubes) as well.

Muscles are attached to the various segments of the exoskeleton, just as they are attached to the various bones of the endoskeleton in vertebrates. When the muscles contract, the exoskeleton moves at its joints, rather than restricting the animal to moving inside of it, as with the shelled mollusks.

The cuticle may form a veritable coat of armor, as it does in beetles and in some of the crustaceans (in which it is often infiltrated with calcium salts), but at the joints it is flexible and thin, permitting free movement. It serves as protection against predators, and it is waterproof, keeping exterior water out and interior water in. It is used for food grinders in the foregut, for wings, and for tactile hairs. Cuticle even forms the lens of the arthropod eye.

The exoskeleton has certain disadvantages. It does not grow (as the bony vertebrate endoskeleton does) and so it must be discarded and re-formed many times as the animal grows and develops. Molting is dangerous; the newly molted animal is soft and hence particularly vulnerable to predators and, in the case of terrestrial forms, subject to water loss.

Many arthropods go into hiding until their new cuticle has hardened. Molting is also costly in terms of metabolic expenditures (although a number of insects and some freshwater crustaceans limit their losses by thriftily eating the old exoskeleton).

The fact that the exoskeleton is waterproof made possible the evolution of terrestrial forms among the arthropods. The arthropods are the only major invertebrate phylum with many species adapted to life on land in non-moist habitats.

Other Arthropod Characteristics:

All arthropods are segmented, a characteristic that suggests a common ancestry with the annelids. In, some of the arthropods the segments have become fused, forming a head, a thorax (sometimes fused with the head to form a cephalothorax), and an abdomen.

But the basic segmented pattern is often still clearly evident in the immature stages (witness the caterpillar) and can be discerned in the adult by examination of the appendages, the musculature, and the nervous system. Arthropods have a large number of jointed appendages. Especially among the insects and crustaceans, these appendages constitute a kit of highly specialised tools, including jaws, gills, poison fangs, tongs, egg depositors, sucking tubes, claws, antennae, paddles, and pincers.

The insects and some other terrestrial forms have unusual means of respiration, consisting of a system of cuticle-lined air ducts (tracheae) that pipe air directly into various parts of the body. Air flow is regulated by the opening and closing of special pores (spiracles) on the exoskeleton.

Terrestrial arthropods, such as spiders, that do not have tracheae have book lungs, structures that are also unique to this phylum. Excretion in terrestrial forms is by means of tubes (called Malpighian tubules) attached to and emptying into the midgut or hindgut. They absorb wastes from the body cavities. Respiration by tracheae, book lungs, or book gills and excretion by Malpighian tubules are found only in arthropods (although not all arthropods have these features).

Arthropods also share a number of characteristics, besides segmentation, with the annelids. Like them, they are bilaterally symmetrical, three-layered, and have a tubular mouth-to-anus gut. They have a coelom, as do the annelids, but the arthropod coelom is markedly reduced, consisting only of the cavities of the gonads and the nephridia. (the coelom serves as a hydrostatic skeleton in annelids, but the arthropods with their exoskeletons require less internal stiffening.)

The arthropods, like the bivalve mollusks, have an open circulatory system in which blood flows through free spaces among the tissues-the hemocoel-as well as through vessels. Blood returns from the hemocoel to the tubular heart through special valved openings.

An open blood system creates little in the way of turgor pressure, which accounts for the general squashiness of the internal organs of insects compared with the firmness, for example, of the vertebrate kidney.

The premise of an evolutionary relationship between the annelids and the arthropods is strengthened by the existence of a group of caterpillar-like worms called Onychophora. These worms have nephridia and reproductive tracts that resemble those of annelids but a heart and hemocoel and jaw like appendages resembling those of arthropods.

The body is covered with a thin cuticle of chitin; however, there are no joints in the cuticle, which is soft enough so that it bends as the animal moves.

Subdivisions of the Phylum:

The arthropods are often classified in two subphyla, the Mandibulata and the Chelicerata, which can be clearly distinguished by even an inexperienced eye. There are conspicuous differences in the appendages. The most anterior appendages in the mandibulate (insects, crustaceans) are antennae, one or two pairs, and the next are mandibles (jaws).

The cheliceratas, which includes horseshoe crabs, sea spiders, and arachnids (spiders, mites, scorpions, and their relatives), have no antennae and no true mandibles. Their first pair of appendages consist of chelicerae (singular, chelicera), which usually take the form of pincers or fangs. In spiders, ducts from a pair of poison glands lead through the chelicerae, which are sharp and pointed and are used for biting and paralyzing prey.

Cheliceratas may have book lungs or book gills; these structures, which are not present in mandibulates, derive their name from their resemblance to the leaves of a partially opened book.

Each of these subphyla is in turn, divided into several classes. The three largest classes are the crustaceans, the insects (both mandibulates), and the arachnids (cheliceratas). Smaller classes include the horseshoe crab and sea spiders (xiphosurans), which are chelicerates, and the centipedes and millipedes, which are mandibulates.

Class Crustacea:

The crustaceans include crabs, crayfish, lobsters, barnacles, shrimp, prawns, Daphnia (water fleas), and a number of smaller forms found largely in freshwater and marine plankton, as well as some terrestrial forms such as the familiar pill-bugs or sow-bugs. Crustaceans are mandibulates. They differ from the insects, which are also mandibulates, in that they have legs or leg like appendages on the abdomen as well as the thorax, and have two pairs of antennae as compared to the insects’ one.

Class Arachnida:

The arachnids, which include spiders, ticks, mites, scorpions, and daddy longlegs, are cheliceratas. In this class, the first pair of appendages, the chelicerae, which are usually sharp and fanglike, are used for biting and paralyzing prey by the injection of a poison.

The second pair are the pedipalps, which, in scorpions, for example, are used for handling and tearing food. Male spiders also use pedipalps to transfer semen to the female. Arachnids have four pairs of walking legs.

Class Insecta:

The insects constitute the largest class, by far, of the arthropods. In fact, there are more species of insects than of all other animals combined. More than 700,000 are known.

Insects are the only invertebrates capable of flight. When insects began to fly-more than 240 million years ago-they were able to move into and exploit a life zone almost totally unoccupied by any other form of animal life. In terms both of numbers of species and of individuals, they are the dominant organisms of this planet.

Insect Characteristics:

Figure 12.6 shows a grasshopper. Here you can see many of the characteristic features of insects- three main divisions—the head, the thorax, and the abdomen; three pairs of legs; one pair of antennae; and a set of mouthparts similar to those of the lobster.

In the more primitive insects, such as the grasshopper, the mouth- parts are used for handling and masticating food, but in the more highly specialised groups, the mouthparts are often modified into sucking, piercing, slicing, or sponging organs. Some are exquisitely adapted to draw in nectar from the deep, tubular nectaries of specialised flowers.

Most adult insects have two pairs of wings made up of light, strong sheets of chitin; the veins in the wings are chitinous tubules that serve primarily for strengthening. In some entire orders, such as fleas and lice, wings are totally lost, returning the insect to the condition of its wingless ancestors. Other species may have short, nonfunctional wings in one or both sexes.

There are more than 20 orders of insects, of which we shall briefly describe four examples- Diptera, Lepidoptera, Hymenoptera, and Coleoptera. The Diptera (“two-winged”) include the familiar flies, gnats, and mosquitoes. The Lepidoptera (“scale wings”) are the moths and the butterflies.

Hymenoptera (“membrane-winged”) include ants, wasps, and bees, many species of which are social. The Coleoptera (“shield-winged”) are the beetles, most of which have a pair of hard protective forewings, which pivot forward out of the way during flight, and a pair of membranous hind-wings used for flying. Of the more than 700,000 classified insects, at least 275,000 species are beetles.

Digestive and Respiratory Systems:

The foregut and hindgut of the insect digestive tract are lined with chitin. Salivary gland fluids are carried with food into the crop, where digestion begins. The stomach, or mid-gut, which lies mainly in the abdomen, is the chief organ of absorption.

Insects have digestive enzymes as specialised as their mouthparts; the structure of the enzymes depends on whether the insect dines on blood, seeds, other insects, eggs, flour, cereal, glue, wood, paper, or your woolen clothes. Excretion is carried out through Malpighian tubules.

In the grasshopper and many other insects, the nitrogenous and other wastes are eliminated in the form of nearly dry crystals of uric acid, an adaptation that promotes water conservation.

The respiratory system consists of a network of chitin-lined tubules through which air circulates to the various tissues of the body, supplying each cell directly. Muscular movements of the animal’s body improve the internal circulation of air. The amount of incoming air and also the degree of water loss is regulated by the opening and closing of the spiracles.

Metamorphosis:

Growing insects change not only in size but often in form, a phenomenon known as metamorphosis. The extent of change varies. In some species, the young, although sexually immature, looks like a small adult; it grows larger by a series of molts until it reaches full size.

In others, like the grasshopper, the newly hatched young are wingless but may gain wing-pads in later immature stages; otherwise they are similar to the adult. These immature, non-reproductive forms are known as nymphs. Almost 90 percent of the insects, however, undergo a complete metamorphosis, in which adults are drastically different from their immature forms.

The immature feeding forms are all correctly referred to as larvae, although they are also commonly known as caterpillars, grubs, or maggots, depending on the different species. Following the larval period, the insect undergoing complete metamorphosis enters an immobile pupal stage within which extensive remodeling of the organism occurs.

The adult (sexually mature) insect emerges from the pupa. Both eggs and pupae (which are non-feeding) can endure lengthy cold or dry seasons.

The insect that undergoes complete metamorphosis exists in four different forms in the course of its life history. The first form is the egg and the embryo. The second form is the larva, the animal that hatches from the egg; larvae eat and grow.

In many larvae, such as those of flies, growth takes place not by an increase in the number of cells, as in most animals, but by an increase in the size of the cells, in somewhat the same way that growth takes place in certain plant tissues. During the course of its growth, the larva molts a characteristic number of times-twice in the fruit fly, for example.

The stages between molts are known as instars. Then, when the larva is full-grown, it molts to form the pupa. During this outwardly lifeless pupal stage, many of the larval cells break down, and entirely new groups of cells, set aside in the embryo, begin to proliferate, using the degenerating larval tissue as a culture medium. These groups of cells are known as imaginal discs since they form the imago, the adult insect, which, according to Aristotle, is the perfect form or ideal image that the immature form is “seeking to express.” These imaginal discs develop into the complicated structures of the adult.

Molting and metamorphosis are under hormonal control. Like many hormone-controlled processes, they are the end result of interplay of several hormones- brain hormone, molting hormone (ecdysone), and juvenile hormone.

At intervals during larval growth, brain hormone, produced by neuro-secretory cells in the brain, is released into the blood. It stimulates the release, in turn, of molting hormone from a gland in the thorax.

The molting hormone stimulates not only molting but also the formation of a pupa and the development of adult structures. The latter are held in check, however, by a third hormone, the juvenile hormone. Only when the production of juvenile hormone declines, in later larval life, can metamorphosis to the adult form take place.

Reasons for Arthropod Success:

Among all the invertebrates, why are the arthropods, in general, and the insects, in particular, so spectacularly successful? One important reason is undoubtedly the nature of the exoskeleton, which waterproofs, provides protection, and makes possible the evolution of the many finely articulated appendages characteristic of this phylum.

A second reason, which applies especially to insects, is the high specificity of diet and other requirements of each species. As a consequence, many different species can live in a single small environment-in a few cubic centimeters of soil, on a small plant, or on or within the egg or body of a single animal—without competing with one another.

The varied and highly specialised mouthparts are a reflection of this specificity of diet. Among the insects with complete metamorphosis, even the larvae of a species do not compete with the adults for food and for the same territory.

A third reason for the success of the insects, is their capacity for flight, which gives them an extraordinary mobility, in three dimensions.

A fourth reason for success is undoubtedly the arthropod nervous system, with its fine control over the various appendages and the many extraordinarily sensitive sensory organs found in great diversity throughout the phylum.

The rest of this discussion deals with sensory perception among arthropods, especially insects, and some examples of arthropod behaviour.

Arthropod Senses and Behaviour:

Vision: The Compound Eye:

The most conspicuous sensory organ of the arthropods is the compound eye, which is an evolutionary development characteristic of this one phylum. The basic structural unit of this eye is the ommatidium. A dragonfly has some 30,000 ommatidia. Each ommatidium is covered by a cornea, usually with a round or hexagonal surface; these are visible under low-power magnification as individual facets of the eye.

Underlying the cornea is a group of eight retinular cells surrounded by pigment cells. The light-sensitive portion of the ommatidium is the rhabdom, which is the central core of the ommatidium.

Nerve fibers carry the stimulus from each ommatidium to the brain. The pigment cells prevent light from traveling from one ommatidium to another. An ommatidium is much larger than a vertebrate photoreceptor, and so there are far fewer in an equivalent space. Hence, the image has less resolution, like a newspaper picture under high magnification.

Although the compound eye is deficient in acuity, offering less detail than the vertebrate eye, it is better for detecting motion because each ommatidium is stimulated separately and so has a separate visual field. Also, each ommatidium responds to stimuli more rapidly than does a vertebrate photoreceptor. Ability to detect motion can be measured accurately in the laboratory by testing a phenomenon known as flicker fusion.

In this test, a light is flicked on and off with increasing rapidity until the observer sees the flicker as a continuous beam. The beam is perceived as continuous because stimulation of any retinal cell persists for a brief period even after the stimulus disappears.

So, in effect, the flicker-fusion test is a measurement of how quickly the photoreceptor cell recovers from one stimulus and becomes sensitive to another. It is possible to test flicker-fusion rates in animals by training experiments in which the animal learns to associate a flickering light with a reward (usually food) and a steady beam with no reward, or vice versa. Such tests have proved that the compound eye greatly exceeds the camera eye of vertebrates in this respect.

A bee would see in clear outline a moving figure that we would see as blurred, and if the bee went to the movies, the film seen by us as a continuous picture would jerk along from frame to frame for the bee.

The ability to perceive motion is extremely important for an insect since it must be able to make out objects when it is flying at high speed (which, as far as the visual apparatus is concerned, presents the same problems as following a moving object).

In addition to, or instead of, compound eyes, many of the arthropods possess simple eyes, or ocelli, which seem generally to serve only for light detection. Most insects have two or three ocelli, and spiders, which do not have compound eyes, may have as many as eight ocelli, depending on species.

Touch Receptors:

The body surfaces of terrestrial arthropods are often covered with sensory-receptor units known as sensilla (singular, sensillum), or “little sense organs.” Most of the sensilla take the form of fine spines, or setae, composed of hollow shafts of chitin. At the bases of these shafts are sensory cells.

In their simplest form, the sensilla are touch receptors. In these, when the hair is touched or bent, the sensory cell responds and initiates nerve impulses. Such receptors are found, in particular, on the antennae and the legs. In addition to being stimulated by direct contact, they can also be stimulated by vibrations and air currents.

A spider monitors what is going on in its web by sensing vibrations transmitted through the threads when the web is touched. Soldier termites of certain species strike the ground or the walls of their nest with their heads when threatened or disturbed; the vibrations they produce warn their colony mates.

A fly perceives the air currents from the movement of a hand or fly swatter and so escapes; a fly in a glass jar is much less likely to be disturbed by such movements.

Proprioceptors:

Proprioceptors are sensory receptors that provide information about the position of various parts of the body and the stresses and strains on them. A type common in the arthropods is the campaniform sensillum. Campaniform sensilla are located in thin, stretchable areas of the cuticle. When the cells are twisted or stretched, a nerve fiber signals the central nervous system.

Touch receptors can also serve as proprioceptors. The praying mantis, for example, is capable of making a lightning-swift strike at a moving object. When it sights a potential victim, the insect moves its entire head to bring it into binocular range, since the eyes themselves do not move.

Movement of the head results in the stimulation of proprioceptive hairs on the head and thorax of the insect. On the basis of the impulses received from these hairs, the position of the prey and the movement of its own legs are automatically coordinated by the mantis. If these hairs are removed, the mantis can strike a moving object only if the object is directly in front of it.

Sound Receptors:

Arthropods have a variety of sound receptors. The simplest is a sensillum with a tactile hair that vibrates as a result of being “touched” by sound waves. The antennae of the male mosquito contain thousands of such hairs, which are responsive to the vibrations made by the wings of the female mosquito in flight and so serve to bring the sexes together.

When the male mosquito first emerges from its pupal shell, it is sexually immature and also deaf, with its antennal hairs lying flat along the shafts. When the male matures sexually, some 24 hours later, the hairs almost simultaneously become erect and are now free to vibrate when a female approaches.

Other insects have developed special groups of cells for hearing; these are known as tympanic organs. In these organs, a fine membrane, the tympanum (or eardrum) is stretched across an air-filled cavity. The tympanic membrane vibrates in response to sounds of certain frequencies, and this vibration is transmitted to underlying receptor cells.

Communication:

Communication by Sound:

Arthropods, particularly insects, have developed complex forms of sensory communication. A number of species, such as the locusts, grasshoppers, and crickets, call to one another by sounds made by rubbing their legs or wings together or against their bodies.

Five distinct types of calls are known:

(1) Calling by males and

(2) Calling by females, both of which are long-range sounds;

(3) Courtship sounds by males and

(4) Aggressive sounds by males, both of which are short-range; and

(5) Alarm sounds, which may be given either by males or by females.

Recognition of and response to the sound seem to be based on pattern and on rhythm because insects apparently are not able to distinguish frequencies, or the differences between high and low notes, and so are essentially “tone deaf.”

The effectiveness of calling songs is often increased by group singing, such as the famous chorus of male seventeen-year cicadas, which can attract females from distances far greater than an individual “voice” would reach. Insects produce songs and respond to appropriate songs without ever having heard a song before.

Communication by Pheromones:

The use of chemicals for communication is common among animals, and the substances employed range from the sex attractants of the little algal cell Chlamydomonas to Chanel No. 5. Many insects commu­nicate by chemicals; such chemicals are known as pheromones. Pherom­ones are chemical messengers.

They are usually produced in special glands and are discharged into the environment, where they act on other members of the same species. Among the best studied of the pheromones are the mating substances of moths.

One female gypsy moth, by the emission of minute amounts of a pheromone commonly known as gyplure, can attract male moths that are several kilometers downwind.

In fact, a single female contains enough gyplure, somewhere around a millionth of a gram, to attract more than a billion males, supposing that it were distributed with maximum efficiency. Since, the male can detect as little as a few hundred molecules per cubic centimeter of the attractant, gyplure is still potent even when it has become widely diffused.

The male moth characteristically flies upwind, and the pheromone, of course, disperses downwind. Therefore, when a male moth detects the odour of a female of the species, he will fly toward the source. If he loses the scent, he flies about at random until he either picks it up again or abandons the search.

It is not until he is quite close to the female that he can fly “up the gradient” and use the intensity of the odour as a locating device.

Insect Behaviour:

Complex patterns of unlearned genetically transmitted behavior, such as web-building, among spiders are another arthropod characteristic. This rigid programming of behaviour may be a necessary correlate to the shortness of the life span of the smaller arthropods.

It provides an interesting contrast to the more flexible behaviour patterns of the higher mammals, with their comparatively long life spans, long periods of learning, and much larger brains.

Term Paper # 2. Phylum Chordata:

The phylum Chordata comprises three subphyla- the Cephalochordata, or lancelets, which includes Branchiostoma (formerly called Amphioxus); the Tunicata, or tunicates, of which the most familiar are the sea squirts; and the Vertebrata, or vertebrates.

Branchiostoma is a small, blade-shaped, semitransparent animal found in shallow marine waters all over the warmer parts of the world. Although it can swim very efficiently, it spends most of its time buried in the sandy bottom, with only its mouth protruding above the surface.

This animal exemplifies all four of the salient features of the chordates. The first is the notochord, a rod that extends the length of the body and serves as a firm but flexible axis. The notochord is a structural support. Because of it, Branchiostoma can swim with strong undulatory motions that move it through the water with a speed unattainable by the flatworms or aquatic annelids.

The second chordate characteristic is the dorsal, hollow nerve cord, a tube that runs beneath the dorsal surface of the animal above the notochord.

The third characteristic is a pharynx with gill slits. The pharyngeal gill slits become highly developed in fishes, in which they serve a respiratory function, and traces of them remain even in the human embryo. In Branchistoma, they serve primarily for collecting food.

The cilia around the mouth and at the opening of the pharynx pull in a steady current of water, which passes through the pharyngeal slits into a chamber known as the atrium and then exits through the atrial pore. Food particles are collected in the sieve like pharynx, mixed with mucus, and channeled along ciliated grooves to the intestine.

The fourth characteristic is a tail, posterior to the anus, consisting of blocks of muscle around an axial skeleton. Most of the body tissue of Branchiostoma is made up of blocks of muscles, the myotomes.

Although Branchiostoma usefully exemplifies the chordates, many biologists believe it is more likely to be a degenerate form of primitive fish rather than a truly primitive member of the phylum. A more probable candidate for the ancestral form is the tunicate. Although the adult form does not have all of the typical chordate features, the larva, which resembles Branchiostoma, is clearly a chordate possessing the four chordate characteristics.

Subphylum Vertebrata:

The vertebrates are a large (about 43,000 species) and familiar subphylum of the chordates. All vertebrates have a backbone, or vertebral column, as their structural axis-a flexible bony support that develops around the notochord, supplanting it entirely in most species. Dorsal projections of the vertebrae encircle the nerve cord along the length of the spine.

The brain is similarly enclosed and protected by bony skull plates. Between the vertebrae are cartilaginous disks, which give the vertebral column its flexibility. One of the great advantages of this bony endoskeleton, as compared with the exoskeletons of the invertebrates, is that it is composed of living tissue that can grow with the animal.

In the developing embryo, the skeleton is largely cartilaginous, with bone gradually replacing cartilage in the course of maturation. In the vertebrates, the growing parts of the bones remain cartilaginous until the animal reaches its full adulthood.

There are seven living classes of vertebrates- the fish (comprising three classes), the amphibians, the reptiles, the birds, and the mammals.

Classes Agnatha, Chondrichthyes, and Osteichthyes: Fish:

The first fish were jawless and had a strong notochord running the length of their bodies. Today these jawless fish (class Agnatha), once a large and diverse group, are represented only by the hagfish and the lampreys.

They have a notochord throughout their lives, like Branchiostoma. Although their ancestors had bony skeletons, modern agnaths have a cartilaginous skeleton. Lacking true bones, they are very flexible; a hagfish can actually tie itself in a knot.

Many cyclostomes (“round mouths”), as they are called, are highly predatory, attaching to other fish by their sucker like mouths and rasping through the skin into the viscera of their hosts. The juvenile lamprey, which resembles Branchiostoma, however, feeds by sucking up mud containing microorganisms and organic debris—as, most probably, did the primitive Agnatha.

The sharks (including the dogfish) and skates, the Chondrichthyes, the second major class of fish, also have a completely cartilaginous skeleton. Like agnaths, their ancestors were also bony animals. Their skin is covered with small, pointed teeth (denticles), which resemble vertebrate teeth structurally and give the skin the texture and abrasive quality of coarse sandpaper.

The third major class of fish includes those with bony skeletons, the Osteichthyes. This group includes the trout, bass, salmon, perch, and many others most of the familiar freshwater and saltwater fish.

According to present evidence, fish evolved in fresh water. The chondrichthyans returned to the sea early in their evolution, while the bony fish went through most of their evolution in fresh water and spread to the seas at a much later period.

Some still recapitulate in each lifetime this difficult physiological transition. Salmon, for example, return to fresh water to spawn, while eels leave the fresh waters of Europe and North America to return to the Sargasso Sea at breeding time, from which distant point the young begin the long, difficult journey, often lasting many years, back to the rivers and lakes.

The Transition to Land:

Another characteristic of the bony fishes is that the early forms seem to have had lungs or lung like structures, as well as gills. These lungs, however, were not efficient enough to serve as more than access­ory structures to the gills.

They were a special adaptation to fresh water, which, unlike ocean water, may become stagnant (depleted of oxygen), because of decay of organic matter or of algal bloom. Lunged fish were the most common fish in the later Devonian seas, apparently evolving independently several times.

In most of them, the lung evolved into an air bladder, or swim bladder; many modern osteichthyans have gas-filled swim bladders that serve as flotation chambers or organs of sound production. A fish raises or lowers itself in the water by adding gases to or removing them from the air bladder via the bloodstream. Still other primitive fish evolved into the modern lungfish.

These fish can live in water that does not have sufficient oxygen to support other fish life. Lungfish surface and gulp air into their lungs in much the same way that certain aquatic but air-breathing snails bob to the surface to fill their mantle cavities.

In yet others, skeletal supports evolved that served to prop up the thorax of the fish. These fish could gulp air even when their bodies were not supported by water. These osteichthyans could waddle, dragging their bellies on the ground, along the muddy bottom of a drying stream bed to seek deeper water or perhaps even make their way from one water source to another one nearby. Thus the transition to land began as an attempt to remain in the water.

Class Amphibia:

Amphibians descended from air-breathing lunged fish. Modern amphibians include frogs and toads (which are tailless as adults) and salamanders (which have tails throughout their lives).

They can readily be distinguished from the reptiles by their thin, usually scale-less skins, which serve as breathing organs. (Frogs have lungs as adults. Some salamanders have lungs, but others breathe entirely through their skins and the mucous membranes of their throats.)

Because water evaporates rapidly through their skins, some amphibians can readily die of desiccation in a dry environment. Those found in deserts spend the drier times of the day far below the surface of the sand.

Most frogs in cold climates have two life stages (hence their name, from am phi and bias, meaning “two lives”). The eggs are laid in water and are fertilised externally. They hatch into gilled larvae (tadpoles). The tadpoles later develop into adults that lose their gills and develop lungs.

The adults may live out of the water, at least in/the summer. However, there are many variations on this theme. Some of the American salamanders fertilise their eggs on land; the males deposit sperm packets that are picked up by the females. Many amphibians are now known to skip the free-living larval stage.

The eggs, which may be laid on land, in a hollow log or cupped leaf, or even carried, by the parent, hatch into miniature versions of the adult. Some salamanders, such as the mud puppy and the axolotl, never complete their metamorphosis, remaining essentially aquatic larval forms.

In some species, these larva like forms can be induced to metamor­phose into adult forms by administration of hormones, indicating that the genetic capacity for this later developmental stage has not been lost.

Class Reptilia:

The vascular plants freed themselves from the water by the development of the seed. Analogously, the vertebrates became truly terrestrial with the evolution in the reptiles of the amniotes egg, an egg that retains its own water supply and so can survive on land.

The reptilian egg, which is much like the familiar hens egg in basic design, contains a large yolk, the food supply for the developing embryo; abundant albumen; and a water supply. A membrane, the amnion, surrounds the developing embryo with a liquid-filled space that substitutes for the ancestral pond.

The gilled stage is passed in a shelled egg or in the maternal oviduct or uterus. In mammals also, although their eggs typically develop internally, the embryos are enclosed in water within an enveloping membrane, the amnion, and pass through a gilled stage (although the gills are never functional) before birth.

Reptiles are characteristically four-legged, although the legs are absent in most snakes and some lizards. In keeping with their terrestrial existence, reptiles have a dry skin, usually covered with protective scales. Modern reptiles, of which there are 5,400 species, include lizards, snakes, turtles, and crocodiles.

Evolution of the Reptiles:

By late in the Carboniferous period, the first reptiles had begun to evolve from closely similar amphibian ancestors. During the succeeding Permian period, there was an explosive increase in the number of reptilian species. (During this same period, conifers began to replace the ferns and other “amphibious” plants, suggesting that a drier climate may have been a primary selective force in both of these events.)

During the Permian and much of Triassic time, the dominant land vertebrates were mammal-like reptiles, an abundant and diverse group, members of which were later to give rise to the mammals. Around the beginning of the Triassic period, several of the more specialised reptile groups arose, which include, among the surviving forms, turtles and lizards, and also, the thecodonts, ancestors of the Archosauria, or ruling reptiles, perhaps the most spectacular of all the land’s inhabitants so far.

The origin and rise of the Archosauria was associated with improvements in reptilian locomotion. Vertebrates first came to the land with all four limbs sprawled far out to the side; turtles have retained this sprawling gait. Among the early Archosauria, there was a progressive tendency toward bipedalism, with a great increase in the strength and running capabilities of the hind legs, with the concomitant freeing of the front legs for other purposes—for example, flight.

There were three major groups of archosaurs- the pterosaurs or flying reptiles; the crocodilians, which reverting to four-legged posture became our modern crocodiles and alligators; and the dinosaurs, a varied and splendid group of reptiles.

The largest of the dinosaurs, Brachiosaurus, was 25 meters in length and weighed, it is estimated, 50 tons, far larger than any land animal that has succeeded it. Throughout the long Mesozoic era, the dinosaurs dominated the life of the land, rulers of the earth for 150 million years.

Then, they vanished; the reasons for their extinction are not known, but it did occur during a period of climatic change and may have been related to problems of temperature regulation or changes in the vegetation or the disappearance of swampy habitats. They left only a single line of descendants, the birds.

Class Aves: Birds:

Birds are essentially reptiles specialised for flight. Their bodies are lightened by air sacs and also by having hollow bones. The frigate bird, a large seagoing bird with a wingspread of more than 2 meters, has a skeleton that weighs only 110 grams (about 4 ounces).

The most massive bone in the bird skeleton is the keel, or breastbone, to which are attached the huge muscles that operate the wings. Flying birds have jettisoned all extra weight; the female’s reproductive system has been trimmed down to a single ovary, and even this becomes large enough to be functional only in the mating season.

Birds have feathers which are their outstanding, unique physical characteristic, and they maintain a high and constant body temperature, which distinguishes them from most of the modern reptiles (although a few, such as leatherback turtles, show some degree of endotherm).

In modern birds, feathers serve both to aid in flight and as insulation. (Only animals that are endothermic require insulation; animals that warm their bodies by exposure to the environment would find insulation a disadvantage.) Birds also have scales, a reminder of their reptilian ancestry. Many birds that are born at an immature stage require a long period of parental care.

Evolution of Right:

How did flight evolve? Biologists agree that evolution occurs by a series of small changes, each of which, to be conserved by natural selection, must be of survival value. Being able to fly not very well is a dubious advantage. Until recently, the most popular theory for the origin of flight has been that the ancestors of the birds were tree- dwelling reptiles and that flight evolved as a way to extend or break jumps from branch to branch.

However, John Ostrom of Yale, having studied the anatomy of the five known specimens of Archaeopteryx and many related forms as well, has come to support a second theory: that the protobirds were ground-dwelling reptiles. Archaeopteryx, according to the fossil evidence, is a close relative of small, bipedal, carnivorous dinosaurs known as theropods.

The only major distinctions are that Archaeopteryx has feathers and fused collar bones (“wish bone”), like modern birds. But why should feathers evolve in a ground-dwelling organism? According to Ostrom, feathers were originally an adaptation not for flight but for insulation. This raises an interesting question, because “cold-blooded” animals, such as reptiles, warm themselves from outside, and so, to these organisms, insulation is a disadvantage.

Only the “warm-blooded” organisms, such as birds and mammals, require insulation as a Help in conserving heat produced by a high metabolic rate. In other words, the theropods, according to this hypothesis, were warm-blooded.

Evidence that the original function of feathers was not flight is provided by anatomical studies showing that the wing feathers apparently were not attached to the bones of the “hand” in Archaeopteryx, as they are in modern birds, but were merely embedded in the skin. Also the breastbone and its keel are lacking, indicating that wing muscles were not well developed.

Archaeopteryx was clearly not a good flyer, if indeed it could fly at all. As Ostrom reconstructs it, the early stages in flight began with a feathered, warm-blooded, carnivorous dinosaur running after its prey, flapping its long feathered arms, and leaping.

The long “wing” feathers may have originated to serve as cage like traps natural nets-for capturing prey; some modern predatory birds use their wings in this way. Thus flight is seen as the culmination of a long, successful predatory leap.

Class Mammalia:

Mammals also descended from the reptiles. Characteristics distinguishing mammals from other vertebrates are that mammals- (1) have hair, (2) provide milk for their young from specialised glands (mammae), and (3) like birds, but unlike other vertebrates, maintain a high body temperature by generating metabolic heat.

In nearly all mammalian species, the young are born alive, as they are in some fish and reptiles, which retain the eggs in their bodies until they hatch. Some very primitive mammals, however, the monotremes, such as the duckbilled platypus, lay eggs with shells but nurse their young after hatching.

The marsupials, which include the opossum and the kangaroo, also bear their young alive, but they differ from the major group of mammals in that the infants are born at a tiny and immature stage and, in some species, are kept in a special protective pouch in which they suckle and continue their development.

Most of the familiar mammals are placentals, so called because they have a more efficient nutritive connection, the placenta, between the uterus and the embryo. As a result, the young develop to a much more advanced stage before birth. Thus the young are afforded protection during their most vulnerable period.

The earliest placentals were small, shy, and probably nocturnal, thus avoiding the carnivorous dinosaurs. They undoubtedly lived mostly on insects, grubs, worms, and eggs. Shrews, which are believed to closely resemble these primitive mammals, have retained their elusive habits.

Mammals have fewer, but larger, skull bones than the fish and reptiles, an example of the fact that “simpler” and “more primitive” may have quite opposite meanings. In the mammals, a bony platform or partition has developed that separates nasal and food passages far back in the throat, making it possible for the animal to breathe while eating.

The lower mammalian jaw consists of a single bone. Mammals, unlike snakes or lizards, cannot move the upper jaw in relation to the brain case. They have lost the capacity of many reptiles to unhinge their jaws-an ability that makes it possible for a large anaconda, for example, to swallow a pig whole.

The primates, the order to which we belong, are placental mammals characterised by three kinds of teeth (canines, incisors, and molars), opposable first digits (thumbs and usually toes), two pectoral mammae, expanded cerebral cortex, and a tendency toward single births. We are distinguished from the other primates by our upright posture, long legs and short arms, high forehead and small jaw, and lack of general body hair.

Among all the mammals, we are one of the least specialised. Humans are omnivores. Unlike the Carnivora, which are meat eaters, and the several orders of herbivores, we eat a wide variety of fruits, vegetables, and other animals.

Our hands closely resemble those of a primitive reptile, in contrast to the highly specialised forelimbs developed by, for example, whales, bats, and horses. We cannot see as well as an eagle. Our sense of smell is much less keen than a dog’s, and our sense of taste far less sensitive than a housefly’s.

Many animals can run faster, swim more powerfully, and climb trees with more agility (though few can do all three). Humans have, however, one area of extreme specialisation the brain. Because of our brain, we are unique among all the other animals in our capacity to reason, to speak, to plan, to learn, and so, to some extent, to control our own future and those of the other organisms with which we share this planet.