Any animal with a backbone, or
spinal column. made of interlocking units called
 vertebrae is known as a vertebrate. This strong but flexible structure supports the body and anchors the limbs, and it also protects the nerves of the spinal cord.
Vertebrates make up only about 2 percent of the animal species, and they
belong to just 1 or more than 30 phyla, or overall groups, in the animal
kingdom.
Any animal lacking a backbone
is known as an invertebrate. Invertebrates are by far
 the most numerous
animals on earth. Nearly 2 million species have been identified to date. These
2 million species make up about 98 percent of all the animals identified in
the entire animal kingdom. Some scientists believe that the true number of
invertebrate species may be as high as 100 million and that the work of
identifying and classifying invertebrate life has only just begun.
The temperature of cold
blooded animals, such as insects, reptiles, frogs and fish, varies with the
temperature of their surroundings. The temperature of cold blooded anim als is
always slightly below the outside temperature to prevent the loss of body
moisture through evaporation. Because their rate of metabolism declines with
a drop in the outside temperature, cold blooded animals becomes torpid in cold
weather. To avoid excessive body temperatures, they tend to favor cool, dark
places during the day.
The cells of the body in warm
blooded animals operate most efficiently within a narrow range of temperature.
In humans, body temperature is 37° C (98.6° F), although 36.4° and 37.2° C
(97.5° and 99° F) are within normal limits. If body temperature is too high,
the functions of the cells may become impaired or the cells themselves
damaged; if too low, the rate at which foodstuffs are metabolized decreases.
The body temperature is regulated by the rate at which heat is radiated from
the skin and by the evaporation of water vapor. Perspiring (evaporation
through pores in the skin) and panting (evaporation through pores in the
mouth) are common temperature regulators in warm-blooded animals. These
processes are controlled involuntarily by the brain.
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