Animals with backbones, called vertebrates, form only a tiny fraction of all know creatures. In many environments, however, they exert an influence out of all proportion to their numbers, because they are in general larger than invertebrates and in many instances more mobile. Both size and mobility are made possible by a strong internal bony scaffolding, jointed to give flexibility and surrounded by muscles that hold it together and make it move. Structure and Development
As vertebrates ourselves, we tend to think that the possession of bones is the great divide between animals. This is not really so and vertebrates are in fact only part of a larger group, the Chordates (Phylum Chordata). Chordata animals have a stiff jelly-like rod, called a notochord, running longitudinally through the dorsal part of the body, and acting as an internal support to a series of muscles or muscle segments (myotomes). Above the notochord is a hollow nerve tube, which is usually folded at the anterior end to form the brain. Below the notochord lies the digestive tract. At some time in their lives all chordates have paired gill slits and a tail.
In true vertebrates the notochord is replaced by cartilage or bone. The latter is a strong, hard substance, formed mainly of calcium phosphate plus collagen and other protein fibers. Bone has great strength for its weight and is well suited to act as internal support for the body.
There are 5 classes of vertebrates: fishes, amphibians, reptiles, birds and mammals. The structure of fishes restricts them to water. During the late Devonian period (about 400 million years ago), however, some fishes, stranded in drought conditions, struggled towards new pools using large, strong stiltlike fins.
Since that time, vertebrate animals have become less and less dependent upon a water environment, though all still need water to live (even they only drink it). First among these terrestrial creatures are amphibians, which must return to the ancient habitat to lay their eggs. Their young generally look like tiny fishes but the adults are free to colonize the land.
Next in evolutionary sequence came the reptiles. These developed shelled eggs, enclosing their embryos in their own private pool (the watery white of the egg) during the larval ("tadpole") stage. The egg also supplies the embryo's nutrition, in the form of the yolk, which is the perfect complete food needed for growth to hatching point. Hatched reptiles show no trace of gills and their skin is hard and waterproof. Their metabolic pattern requires external warmth, so they are restricted to climatically favorable parts of the world.
In the Triassic Period, about 200 million years ago, mammals evolved from reptile species that are now extinct. The young of most mammal species are born at a relatively advanced stage of development, because a special maternal organ, the placenta, provides nourishment for the fetus while it grows within its mother's body. The placenta also removes excretory products. After birth, young mammals are fed on maternal milk which gives them all the nourishment they need at this stage. In many species, important maternal and social ties are formed and cemented during this period of suckling. Most mammals have relatively large and complex brains which enable the young to learn behavior, particularly from their parents.
During the Jurassic Period, about 160 million years ago, birds, the last of the vertebrate classes evolved from reptile stock far removed from the ancestors of mammals. Birds show many modifications of the general vertebrate pattern, the most obvious being the transformation of the forelimbs into wings. The body is maintained at a high temperature and is insulated by feathers. Reproduction still depends on reptile-like eggs which are laid individually and at intervals, because of the need to keep weight to a minimum for flight. Many birds care for their young for a prolonged period, which establishes social bonds comparable to those seen in mammals.
- Mike Janson and Joyce Pope (consultant editors). The Animal World
- Public Health Image Library (PHIL)
- U.S. Fish and Wildlife Service Multimedia database