Nematodes (roundworms) have a simple brain and nervous system consisting of approximately 300 nerve cells, or neurons. Sensory neurons located in the head end of the animal detect stimuli from the environment and pass messages to the brain. The brain then sends out impulses through a ventral nerve cord to muscles which respond to the stimulus. The way that the interneurons of the brain process the data determines the response.
The earthworm and other annelids, as well as insects and other arthropods, have more complex nervous systems. In these animals, there are paired ventral nerve cords that run from head to tail on the animal's underside. Cell bodies of neurons in the cords form pairs of ganglia in each body segment. Four of the most anterior ganglia fuse in the head to form a brain. As a result, the brain ganglia are larger than the segmental ganglia, and also contain a larger proportion of sensory motor neurons. The brain ganglia have some dominance over the segment ganglia. The ventral nerve cords, brain, and segmental ganglia comprise the central nervous system. Neuronal fibers in the cords, bundled into nerves that carry communications between ganglia, make up the peripheral nervous system.
The earthworm's brain consists of paired ganglia in the head end. An impulse, such as touch, light, or moisture, is detected by receptor cells in the skin. A pair of nerves in each of the earthworm's segments carries the signal to the brain and smaller ganglia in each segment, where the signals are analyzed. The central nervous system then transmits impulses on nerves that coordinate muscle action, causing the earthworm to move.
In insects, specialized sense organs detect information from the environment and transmit it to the central nervous system. Such sense organs include simple and compound eyes, sound receptors on the thorax or in the legs, and taste receptors. The brain of an insect consists of a ganglion in the head. Some of the segmental ganglia are fused, allowing better communication between the segments. The information that insects use for behaviors such as walking, flying, mating, and stinging is stored in the segmental ganglia. In experiments in which heads are cut off of cockroaches and flies, these insects continue to learn.
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