One of the great delights of visiting the Galapagos is the opportunity to walk along beautiful shorelines such as those of Santiago Island, which we visit on Friday afternoons. We have ample opportunities to explore tidal pools if the tide is low, as well as observing the interesting objects the sea washes up for us. Among my favorites are the fascinating sea urchin tests, which display the most amazing pentameral (five-part) bilateral symmetry. This fascination of mine for these most perfect of natural objects undoubtedly comes from a collection my mother had as I was growing up, which I used to study for hours.

Sea stars (yes, please get out of the habit of calling them "star fish," as they are not related to fish at all!), sea urchins, sand dollars and sea cucumbers are all curiously beautiful representatives of the phylum Echinodermata, literally translated as the prickly-skinned organisms. These are a strange group, sharply distinguished from all other members of the animal kingdom. It is a very ancient group extending back to the Cambrian period (some 600 million years ago). They are, surprisingly, more closely related to vertebrates than any other major group of invertebrates!

One amazing feature is that they have successfully exploited radial symmetry, which allows them to engage the environment from any direction (even though the larval stages show that they are clearly descended from bilateral ancestors). Imagine that they are totally lacking a brain or anything resembling a head! They are built around a basic five-part plan arranged around a central oral-aboral axis and have a water-vascular system that uses hydraulic power to operate a multitude of tiny tube feet used in food-gathering and locomotion.

Well, they do say that variety is the spice of life! The two tests that we found on our beach walk this afternoon are of the slate-pencil sea urchin (Eucidaris thouarsii) and the little green sea urchin (Lytechinus semituberculatus), both common inhabitants of the Galapagos intertidal and subtidal areas.