Question

Describe the range of variation in the nervous systems of invertebrates.

Answer 1

Invertebrate Nervous Systems

Although the invertebrate nervous system is usually much simpler than the nervous systems found in vertebrates, there is still a broad range in complexity depending on the type of invertebrate.

The simplest type of nervous system is found in hydras and jellyfish (cnidarians) and is referred to as a "nerve net." Nerve nets do not have distinct central or peripheral regions, and lack anything that resembles a brain. Instead, the scattered nerve cells form loose networks in each cell layer of the body wall. Some of these neurons carry information from sensory organs that detect touch, light, or other changes in the environment. These neurons in turn contact neurons that control movement of the organism, such as swimming.

Unlike the hydras and jellyfish, invertebrates such as sea stars (echinoderms) display some centralized organization of the nervous system. A ring of neurons is located in the center of the sea star, and simple bundles of neurons called radial nerves extend from the ring to the tip of each arm. In each arm, extensions of the radial nerves form nerve nets as in the jellyfish. This arrangement permits coordinated movement of each arm and the tube feet located on the surface of the arm.

A distinct separation of peripheral and central nervous systems is found in invertebrates such as worms, insects, and mollusks, like the squid. Neuron cell bodies are grouped into clusters called ganglia , which are usually located along the animal's midline. The peripheral component of the nervous system is formed by the extensions of the cells in these ganglia; some carry sensory information from the environment to the ganglia, while others carry signals from the ganglia to produce a response (such as movement). This type of organization permits segmentation, in which each ganglion responds to and controls an individual segment of the body. To coordinate the segments, these ganglia are connected to each other in a chainlike fashion by a nerve cord, which is a bundle of neurons that runs the length of

The nerve signal can travel in both directions in the hydra (a), a cnidarian. A planarian (b) has two nerve cords and two clusters of nerve cell bodies at its anterior end. In annelids, such as the earthworm (c), two cords are fused and run down the ventral surface of the body. Arthropods such as the crayfish (d) also have a double ventral nerve, in addition to clusters of nerve cells in the area of the head.
The nerve signal can travel in both directions in the hydra (a), a cnidarian. A planarian (b) has two nerve cords and two clusters of nerve cell bodies at its anterior end. In annelids, such as the earthworm (c), two cords are fused and run down the ventral surface of the body. Arthropods such as the crayfish (d) also have a double ventral nerve, in addition to clusters of nerve cells in the area of the head.
the animal. Some organisms have more than one nerve cord connected by transverse nerves, resembling a ladder.
In many invertebrates, the nerve cord is enlarged at the anterior (or head) end of the organism. This enlargement can be considered a primitive brain, and together with the nerve cord comprises the central nervous system. Without any type of brain, the coordination between different segments of the organism is limited at best, and the nervous system primarily produces simple reflexive movements. The presence of a brain allows the organism to receive a wide array of information from the environment, analyze it, and generate a coordinated and complex response. For example, the large brain of a squid enables it to process visual information and rapidly generate coordinated responses to capture prey. In fact, this invertebrate nervous system is so specialized, it closely resembles some vertebrate nervous systems.



My Source: http://www.biologyreference.com/Mo-Nu/Nervous-Systems.html

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