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Please begin with an informative title:

The nervous system of animals (one of the characteristics that distinguishes between plant and animal) is a marvelous evolutionary development.

While we, like bilateral animals in general, have a central nerve cord, cephalization (having a head), and ganglia (swellings containing concentrations of neurons - including the brain) that serve as control centers, such a system is not the only way to organize a nervous system.  In radially organized animals, which lack a head, such directional structure is unnecessary. Coral animals, sea anemones, hydroids, jellyfish and related forms (Cnidaria - formerly Coelenterata), plus echinoderms like starfish and sanddollars, all radially organized, have a nerve net instead of a centralized nervous system. Cnidarians contract and expand utilizing single nerve cells that connect with others in a network that is truly beautiful to see. The net is especially evident on the mesenteries (muscle tissue) and can be made visible by the use of certain staining dyes, such as methylene blue.

For one summer I served as a undergraduate assistant to Professor Peter Pickens at the University of Arizona, taking care of the livestock for his invertebrate neurophysiology laboratory and also trying to accurately reproduce a staining technique on the nerve nets of sea anemones.  It was frustrating work as the staining quite often did not occur for reasons that we were unable to explain.  The reduced solution of methylene blue had to be made by mixing a byproduct of Walsh coal mines, Rongalite ( Na+HOCH2SO2,) with water, and then buffered to pH 7.  For quite some time I was unsuccessful until I finally determined that we needed to use a more purified form of the chemical, known as sodium formaldehyde sufoxylate.

After the preparation of the staining solution,I dissected the mesenteries from anesthetized sea anemones and placed them in petri dishes containing the stain.  After a short time, the preparation was examined under a dissecting microscope. We used several genera of anemones, all from the Gulf of California.  Most were impossible to work with as they produced huge amounts of mucus, which kept the solution from touching the nerve fibers, but one species of burrowing anemone in the genus Callimactus occasionally produced beautiful results, with the single nerve axons forming a spiderweb across the tissue. I was irritated, however, when I showed a particularly beautiful preparation to my supervisor and he didn't get around to photographing the result.  We never did understand why the vital staining could not be easily reproduced, even in Callimactus.

The nerve net is what transmits nervous impulses that makes anemones contract and allows jellyfish to swim in their characteristic pulsating manner.  We had better get used to some of these creatures as according to some research jellyfish may take over the oceans and replace many fish in our diet as global warming procedes. For primitive creatures they seem to have survived almost unchanged from at least the Cambrian, and probably much before that time. On the other hand, coral animals, which are also cnidarians, depend on calcium carbonate to build their colonies and so will probably decline as seas become more acid.

The study of such simple nervous systems helps us to understand more complex ones.  The nerve net in Cnidaria is generally one neuron wide so each nerve cell can be examined in detail more easily than systems involving thousands or millions of nerve cells.  Because of their easy culture the freshwater polyps in the genus Hydra and its relatives have served as important laboratory animals for many years.  These simple polyps were first studied closely by the Swiss scientist Abraham Trembley, and his observations were published in his 1744 work "Mémoires pour servir à l'histoire d'un genre de polypes d'eau douce."  Trembly could not see the nerve net, but did record the behavior of Hydra and performed the first experiments on this simple, but fascinating, animal.  The nerve net was first described in detail by Hadzi in 1909, and many publications have appeared since (See: Koizumi 2002 for a review).

The first wild Hydra I ever encountered was on a piece of floating wood in Rose Canyon Lake in the Santa Catalina Mountains above Tucson, Arizona. Much later when we set up an aquatic insect aquarium at the university there was suddenly a huge number of these tiny polyps all over the glass sides.  I have never seen such a large number before or since!

We have yet much to learn from even the simplest of organisms. The best science produces more questions than answers, but it does produce some answers as we slowly understand a tiny bit more about the universe. Nerve nets are one small area of research that produce a few of both these answers and questions.

Reference:

Koizumi, Osamu. 2002. Developmental neurobiology of hydra, a model
animal of cnidarians. Canadian Journal of Zoology, 80: 1678–1689.  http://www.biochem.uci.edu/...    

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Originally posted to Desert Scientist on Sat Jun 15, 2013 at 08:54 AM PDT.

Also republished by Backyard Science, SciTech, and Community Spotlight.

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