Evolution Of Invertebrate Nervous Systems
To survive, animals have to respond to changes in their internal and external environment. General responses are found in animals that have a simple nervous system and can only process information in a limited way. An example of this type of nervous system is found in the common freshwater Hydra, a cylinder-shaped inverebrate. It has a nerve net of neurons between the outer and inner layers of a sac-like body. The nerve net transmits impulses in all directions with no means of processing the information to make a specific response. In flatworms, such as planaria, there is a simple centralized nervous system. Here, neurons are organized into structures called ganglia which act to receive stimuli from the sensory structures and transmit them by way of a ladder-like arrangement of nerves to muscle cells. In this way, flatworms can make specific responses to stimuli, such as turning away from light, or curling up when touched. Higher invertebrates, such as annelids, arthropods, and mollusks, have a more complex nervous system with more highly developed sensory structures that allow the animals to receive, process, and respond to stimuli in a greater variety of ways. An example of this is the compound eyes of insects which send sensory information through nerve fibers to the ganglia in the head that serve as the brain. The information is then relayed to the other parts of the body through a nerve cord found on the ventral (lower) surface of the animal. The effectiveness of this arrangement is demonstrated by the rapid escape response of flies when attempts are made to kill them. In the octopus, a mollusk with well developed eyes and a central concentration of nerve cells, responses are highly specific, and the ability to learn how to perform complex tasks is evident.
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