SIMPLE NERVE NETS IN CNIDARIANS AND ECHINODERMS

Simple Nerve Nets in Cnidarians and Echinoderms

Cnidarians (jellyfish and anemones, also sometimes called coelenterates) have neurons but no centralized brain. They still manage to organize fairly complex behaviors. In cnidarians a net of axons lies between the inner and outer body walls. Sensory nerves project to the surfaces of the animal, and nerves also attach to contractile cells, which function similarly to muscles in other animals. This nerve net allows transmission of information from one part of the animal to another, and coordination of simple movements.

The stinging organs of cnidarians, nematocysts, discharge without nervous control. Well-fed cnidarians are less, likely, though, to discharge their nematocysts. Nervous connections to the nematocysts may affect the likelihood of discharge, depending on the need for food.

The green anemones on the left are underwater, with their tentacles and mouth exposed. On the right, the mass of anemones sits above the tideline, exposed to the sun, air, and possibly predators. They have retracted their tentacles into their their bodies so that the leathery outer covering protects them from dessication and other threats.

Seastars, echniderms, also have minimal nervous systems. Yet, they can organize complex behavior, moving through reefs and tidepools in search of prey.