Evolution Of The Nervous System Through Nine Animal Phylum
Evolution Of The Nervous System Through Nine Animal Phylum
The only multi-cellular animals without a nervous system are sponges. They do not have any nerve cells or sensory cells. Despite this, touch or pressure to the outside of a sponge will cause a local contraction of its body.
Cnidarians
The jellyfish and most all other forms of cnidarians, such as the hydra and sea anemone, are characterized by a nerve net. A nerve net is a series of interconnected nerve cells that conduct impulses around the jellyfish’s entire body. The strength of the jellyfish’s response is proportional to the strength by which the jellyfish is being stimulated. So, in other words, the stronger the stimulus is the stronger the reaction will be.
Platyhelminthes
The nervous system of the planaria, similar to that of many platyhelminthes, is only a step above the simple nerve net of the Cnidarians. In addition to the nerve net, the platyhelminthes have long nerve cords that connect the different sections of nerve nets. All of the nerve cords come together at a location near the head called the cerebral ganglion. The central nervous system has been described as ladder-like because of the nerves connecting the cords.
Nematoda
The nematoda have a simple nervous system. It consists of a ring of nervous tissue around the pharynx that gives rise to dorsal and ventral nerve cords running the length of the body. The dorsal nerve cord runs along the top of the intestine and the lateral nerve cord runs to the left of the intestine. When the nerve cords are stimulated they cause the longitudinal muscle, which runs all along the outside of the body, to contract.
Annelida
The brain of most annelids is relatively simple in structure. In some, the brain is divided into a forebrain, midbrain, and hindbrain. Sensory nerves leave the brain and run forward into the prostomium and first segment. A single pair of circumesophageal connectives leaves the brain, surround the anterior gut, and connect with the ventral nerve cord.
The most primitive annelids have a pair of ventral nerve cords joined by transverse connectives; the most advanced forms have the cords fused to form a single cord. A ganglionic swelling of the cord is found in each body segment. Two to five pairs of lateral nerves leave each ganglion to innervate the body wall of that segment....