THE WORLD OF LIFE with Douglas Drenkow

Various vertebrates live in marine, freshwater, or terrestrial environments. In addition, some vertebrates are aerial (Birds and bats can truly fly; some small lizards and mammals, with wing-like flaps of skin, can glide); and lampreys and some other primitive fish live as parasites on the bodies of other vertebrates.

DESCRIPTION

The body of a vertebrate contains "cartilaginous" (rubbery) or bony "vertebrae", segments forming a supportive "skeletal" structure -- the backbone -- that in the adult encases or, usually, replaces the "notochord" (a stiff, flexible "spine" present during the development of a vertebrate; present in other chordates; and presumably present in their evolutionary ancestors). Although a "coelom" (a membrane-lined body cavity outside of the gut) is present in vertebrates and although the body of aquatic species is supported in part by the buoyancy of the surrounding water, the body of a vertebrate is primarily shaped and supported by a cartilaginous or bony "endoskeleton", extending from the backbone. The skeleton can, in part, also act like an encasing "exoskeleton" (as in arthropods): The skull encases the brain; the vertebrae, hollow, encase the spinal cord (of nerves); and the rib-cage encases many of the vital internal organs.

See the individual classes of vertebrates (mentioned above) for descriptions of the diversity of vertebrates.

FEEDING HABITS

Various vertebrates are scavengers, filter-feeders (ex. whalebone whales), herbivores, carnivores, omnivores (eating almost anything), or parasites (ex. lampreys).

MOTION

The well-developed, "metameric" (segmented) muscles of vertebrates typically pull on the jointed bones of the skeleton in coordinated pairs (one muscle typically attached to the outside of the bone; the other, to the inside).

The backbone is an especially strong attachment for the muscles, maximizing their leverage.

Although legs have evolved in higher, terrestrial vertebrates (tetrapods), legs have been lost in the subsequent evolution of some of these higher vertebrates (such as snakes); and legs have been modified -- as into flippers or wings -- in the evolution of still other vertebrates. Once again, see the descriptions of the individual classes of vertebrates to appreciate our impressive "adaptive radiation".

DIGESTION

The gut of vertebrates is "complete", with both a mouth and an anus.

Typically, there is a mouth (with teeth and jaws), an esophagus, a stomach, an intestine, and accompanying digestive glands (such as the liver, gall bladder, and pancreas) aiding in the chemical breakdown and reworking of food molecules.

RESPIRATION

In modern fishes, gases are exchanged through internal, blood-filled gills. In amphibians, gases are exchanged through internal or external gills on the larvae and through the pulsing mouth, moist skin, and internal lungs of the adults. In higher vertebrates, gases are typically inhaled and exhaled through "nostrils" (at least partially bypassing the often full or closed mouth) and are exchanged across the typically large surface area of the infoldings of the lungs.

CIRCULATION

The vertebrate circulatory system is "closed". Blood pumped-out from the heart through the "artery" vessels filters through tiny "capillaries" (embedded within the tissues) and returns through likewise membrane-bound "veins" to the heart. The blood carries dissolved gases and -- often aided by a "lymphatic" system of ducts -- nutrients and other materials.

Lower vertebrates are "ectothermic" (so-called "cold-blooded") -- their body temperature controlled by the environment: These species (fishes, amphibians, and modern-day reptiles) often hibernate overwinter (or "aestivate" oversummer). However, birds and mammals are (and their prehistoric reptilian ancestors were) "endothermic" (so-called "warm-blooded") -- biochemical reactions release heat, carried by the blood throughout the tissues, producing a relatively high and constant temperature within the body (insulated from heat-loss by feathers, hair, or blubber): Warm-bloodedness is important for maintaining the relatively active lifestyles of us higher vertebrates.

Within the bloodstream, highly specialized "white blood cells", "antibody" proteins, and "complementary" proteins mount coordinated biochemical attacks on viruses and other micro-organisms invading the body as well as on "mutant" cells (which are genetically unusual) and "cancer" cells (which divide and grow in an uncontrolled fashion). Please note that bodily tissues are also protected from infection by the skin, cilia and mucous in the respiratory tract, enzymes, stomach acids, and intestinal "flora" (especially certain harmless bacteria, which out-compete most invaders).

EXCRETION

In modern fishes, many "nitrogen-rich" wastes (as in our urine) are secreted from the gills. In all vertebrates, dissolved wastes are cleansed from the blood by the many tubules within a pair of "kidneys"; and in most vertebrates, these wastes are excreted (through a "urinary bladder" in reptiles and most higher vertebrates) into a "cloaca" chamber. The cloaca also receives wastes from the digestive system and, thus, opens as the anus in most vertebrates; but in most mammals, the cloaca is divided during development into the "urethra" (the exit passage of the "urinary" system) and the "rectum" (the exit passage of the digestive system).

Terrestrial vertebrates typically excrete less water from their kidneys than do aquatic species.

COORDINATION

The growth, development, and activities of vertebrates are under genetic and hormonal control, influenced by the environment.

The "dorsal tubular nerve cord" (found within the upperside, or back, of the typical chordate body) develops into a brain and a "spinal cord" in vertebrates, which is an advantage for these typically active creatures. The typically well-developed brain processes the nervous inputs from elaborate organs of such senses as sight, sound (and balance), smell, taste, and touch (and vibration) and transmits the appropriate responses throughout the body; in particular, the nerve cells in the spinal cord (running through the backbone) act as the main conductors of this "electro-chemical" communication. Additionally, higher thought processes -- such as "consciousness" (awareness of self and environment) and "reasoning" (the mental ability to consider and solve problems) -- evolved in higher vertebrates, as the "cerebral cortex" of the brain enlarged and became more complexly folded (the increased surface area allowing for more of the electro-chemical processes of thought).

The populations of certain vertebrates are also under social control, as exhibited in schools of fish and flocks of birds to the typically caring family groups of mammals and even the national and international organizations of human beings. Typically, societies -- vertebrate or otherwise (See social insects) -- have as their ultimate goal the proper care of the young and, thus, the propagation of the species.

REPRODUCTION

Vertebrates reproduce sexually, with the sexes usually "separate" (each individual is either male or female). The reproductive systems are well-developed, with the sperms (male gametes) typically released from the testes (male gonads) through the end portions of the urinary system and the eggs (female gametes) released from the ovaries (female gonads) through "oviducts", which also typically open into the "cloaca" (the chamber receiving the outputs of both the urinary and digestive tracts in most vertebrates, except mammals -- as described above). Although fishes and amphibians typically release swimming sperm directly into environmental water, truly terrestrial vertebrates "copulate" (mate with physical contact).

The fertilized eggs of fishes and amphibians are thin-skinned and typically laid in watery environments, in which the larvae develop. The eggs of reptiles and birds are typically hard-shelled and laid in terrestrial environments -- the developing "embryo" within the egg is bathed and cushioned by an "amniotic sac", fed from a "yolk sac", and "oxygenated" by an "allantois" membrane. The fertilized eggs of birds are typically sat upon during their "incubation" (the period before hatching). The fertilized eggs of the few egg-laying mammals are typically laid in aquatic environments and also sat-upon during incubation (again an advantage, particularly for warm-blooded species). But the fertilized eggs of higher mammals develop within the "uterus" of the female.

Once again, please explore the individual classes of vertebrates (listed above) to discover our wonderfully varied -- and highly successful -- strategies of life.

Representatives

Chordates (Chordata)

Doug@DouglasDrenkow.com

Photo of Cells: H.D.A. Lindquist, US EPA

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