Evolution Sunday

The cnidarians (formerly called the coelenterates) are a successful animal phylum. They are almost exclusively marine, except for some aquatic representatives of the sub-pylum hydrozoa (hydras). Beachgoers are very familiar with the jellyfish. Corals and sea anenomes are also cnidarians, as are some less familiar creatures.

The cnidarians possess a nerve network, but no brain -- no central processing unit. Unlike the chordates, to which we belong, and the many animal phyla more closely related to us, they have only two embryonic tissue layers, as opposed to our three. They have an important asset belonging to no other phylum, called the nematocyst -- a mechanism in specialized cells that shoots out a stiff thread. In most cases, this thread is a poisonous dart. Nematocysts are typically found in the tentacles which surround cnidarians' mouths, and are used to kill prey which the tentacles then transport to the mouth. But nematocysts also function for adhesion, locomotion, and other purposes.

While we may pity the cnidarians for their lack of a brain, they are at an even greater disadvantage from lack of an anus. We three-layered creatures have a tube running the length of our body cavities. Food enters at one end, is systematically disassembled, and its useful contents assimilated. The dreck eventually exits at the other end. Cnidarians have a much less efficient system. They have only one opening to the digestive cavity. In order to ingest a meal, they must open the door, so that whatever contents are already present may escape, whether or not they are fully digested. When cnidarians swim, the cavity is inevitably distorted and its contents may be expelled. Even less happily, from our idiosyncratic point of view, cnidarian gametes -- sperm and ovae, essentially -- are discharged into the same cavity. To be exchanged with the opposite sex -- or a fellow hermaphrodite -- the gametes must be expelled into the oceanic environment along with lunch.

It's sad that we have so little appreciation for one of our most important assets. It seems an undeserved honor to label Pat Robertson, Jerry Falwell, James Dobson, and their friend in the White House by a name for this essential body part.