In simple animals like the jellyfish sensory neurons transmit their signals directly to motor neurons. But only a little higher on the evolutionary ladder-for example, in Ascaris, a parasitic roundworm-a third class of neurons intervenes between the sensory and the motor neurons. These intermediate neurons, called interneurons, process the signals sent to them by sensory neurons and by other interneurons. Then, on the basis of all the information they receive, they may or may not send a signal to the motor neurons for transmission to the muscles. Clearly, this three- stage system is capable of producing more complex forms of behavior than a two-stage system.
In Ascaris and other invertebrates, bunches of interneurons form clumps called ganglia. In general, both the proportion of neurons in the interneuron class and the total number of cells in the nervous system are greater in the more highly developed species.
By the time we get to the vertebrates (fish amphibians, reptiles birds and mammals), the ganglia have become a full-fledged brain. We like to believe that the brain has reached its highest state of development in the human species-and perhaps it has, judging by some of our achievements. But it is well to remember that humans have neither the largest brains in the animal kingdom (elephants and porpoises have larger brains) nor even the highest brain weight-to-total-weight ratio (it is about 1:50 in humans, but 1:20 in certain monkeys). There is no need, though, to feel overly humble. The human brain has somewhere between 10 and 10 neurons, or between 10 billion and 100 billion-as many as there are stars in our galaxy!
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