WE'VE NEVER BEEN so selfconscious about ourselves as we seem to be these days. The popular magazines are filled with advice on things to do with a self: how to find it, identify it, nurture it, protect it -- even, for special occasions, weekends, how to lose it transiently. There are instructive books, best sellers on self-realization, self-help, self-development. Groups of self-respecting people pay large fees for three-day sessions together, learning self-awareness. Self enlightenment can be taught in college electives.

You'd think, to read about it, that we'd only just now discovered selves. Having long suspected that there was something alive in there, running the place, separate from everything else, absolutely individual and independent, we've celebrated by giving it a real name. My self.

We tend to think of our selves as the only wholly unique creations in nature, but it is not so. Uniqueness is so commonplace a property of living things that there is really nothing at all unique about it. A phenomenon can't be unique and universal at the same time. Even individual, free-swimming bacteria can be viewed as unique entities, distinguishable from each other even when they are the progeny of a single clone.

Beans carry self-labels, and are marked by these as distinctly as a mouse by his special small. The labels are glycoproteins, the lectins, and may have something to do with negotiating the intimate and essential attachment between the bean and the nitrogen-fixing bacteria which live as part of the plant's flesh, embedded in root nodules. The lectin from one line of legume has a special affinity for the surfaces of the particular bacteria which colonize that line, but not for bacteria from other types of bean. The system seems designed for the maintenance of exclusive partnerships. Nature is pieced together by little snobberies like this.

Coral polyps are biologically self-conscious. If you place polyps of the same genetic line together, touching each other, they will fuse and become a single polyp, but if the lines are different, one will reject the other.

Fish can tell each other apart as individuals, by the smell of self; so can mice.

The only living units that seem to have no sense of privacy at all are the nucleated cells that have been detached from the parent organism and isolated in a laboratory dish. Given the opportunity, under the right conditions, two cells from wildly different sources, a yeast cell, say, and a chicken erythrocyte, will touch, fuse, and the two nuclei will then fuse as well, and the new hybrid cell will now divide into monstrous progeny. Naked cells, lacking self-respect, do not seem to have any sense of self.

The markers of self, and the sensing mechanisms responsible for detecting such markers, are conventionally regarded as mechanisms for maintaining individuality for its own sake, enabling one kind of creature to defend and protect itself against all the rest. Selfness, seen thus, is for self-preservation.

In real life, though, it doesn't seem to work this way. The self-marking of invertebrate animals in the sea, who must have perfected the business long before evolution got around to us, was set up in order to permit creatures of one kind to locate others, not for predation but to set up symbiotic households.

The anemones who live on the shells of crabs are precisely finicky; so are the crabs. Only a single species of anemone will find its way to only a single species of crab. They sense each other exquisitely, and live together as though made for each other.

Sometimes there is such a mix-up about selfness that two creatures, each attracted by the molecular comfiguration of the other, incorporate the two selves to make a single organism. The best story I've ever heard about this is the tale told of the nudibranch and medusa living in the Bay of Naples.

When first observed, the nudibranch, a common sea slug, was found to have a tiny vestigial parasite, in the form of a medusa, or jellyfish, permanently affixed to the ventral surface near the mouth. In curiosity to learn how the medusa got there, some marine biologists began searching the local waters for earlier developmental forms, and discovered something amazing.

The attached parasite, although apparently so specialized as to have given up living for itself, can still produce offspring, for they are found in abundance at certain seasons of the year. They drift through the upper waters, grow up nicely and astonishingly and finally become full-grown, handsome, normal jellyfish.

Meanwhile, the snail produces snail larvae, and these too begin to grow normally, but not for long. While still extremely small, they become entrapped in the tentacles of the medusa and then engulfed within the umbrella-shaped body. At first glance, you'd believe the medusae are now the predators, paying back for earlier humiliations, and the snails the prey. But no.

Soon the snails, undigested and insatiable, begin to eat, browsing away first at the radial canal, then the borders of the rim, finally the tentacles, until the jellyfish becomes reduced in substance by being eaten while the snail grows correspondingly in size. At the end, the arrangement is back to the first scene, with the full-grown nudibranch basking, and nothing left of the jellyfish except the round, successfully edited parasite, safely affixed to the skin near the mouth.

It is a confusing tale to sort out, and even more confusing to think about. Both creatures are designed for this encounter, marked as selves so that they can find each other in the waters of the Bay of Maples. The collaboration, if you want to call it that, is entirely specific; it is only this species of medusa and only this kind of nudibranch that can come together and live this way. And, more surprising, they cannot live in ant other way; they depend for their survival on each other. They are not really selves, they are specific others.

The thought of these creatures gives me an odd feeling. They do not remind me of anything, really. I've never heard of such a cycle before. They are bizarre, that's it, unique. And at the same time, like a vaguely remembered dream, they remind me of the whole earth at once. I cannot get my mind to stay still and think it through.