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PRECAMBRIAN PERIOD

630-600 million years ago

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BILATERAL SYMMETRY

There are a few examples among modern cnidarians of directional movement and of a concentration of neural tissue at one end of the organism. More than 600 million years ago, some radially symmetrical organisms designated one end of their body as the "front" and propelled that part forward more often than not. These became the first bilateral animals, the first worms. In their descendants, neural tissue and sense organs would accumulate in this front end forming a brain and head.

Cnidarians (jellyfish, coral, Hydra ), only achieve the tissue level of organization and also have the primitive condition of radial symmetry. Rather than having right and left halves, there is more than one plane of symmetry around a central axis. In these radially symmetrical animals, the nervous system consists of a diffuse nerve net. Since sensory structures are simple and equally distributed on all sides of the body, there is no need for a concentration of nervous tissue (that is, a brain) in any one part of the body.

radially symmetric organism

A group of jellyfish called the ctenophores (comb jellies) are elongated and possess a gravity-detecting statocyst at one end of the body. There is some evidence that ctenophores are the sister group of bilaterans. There are a few examples of a concentration of neural tissue in cnidarians. Marginal ganglia of Scyphomedusae show a slight centralization and in ctenophores the ganglion is associated with a sense organ (aboral) at one pole of the body.(Beklemishev, vol. 2 p. 79-80). In cnidarian larvae, nervous tissue may concentrate in anterior end of the body (Hyman). Hydra and bilaterans both express homeogenes of the Prd group, nuclear orphan receptors COP-TF, msh (or the Antennapedia group) and thrombospondin in the developing head or neural tissue (Miljkovic-Licina, 2004).
All animals with organs have bilateral symmetry: the only way to divide them into two equal halves is to divide them into right and left halves. Sense organs tend to be located at one end, the end that is usually propelled forward. With sense organs concentrated at the front end of early bilaterans, nervous tissue began to accumulate there to process this sensory input, thus forming a brain.

worm with bilateral symmetry