thin-walled tube feet, used for locomotion but also serving for respiration, as oxygen can diffuse across the tube feet and then pass inward through small holes in the skeleton (the ambulacra) to the interior of the box. There were many curious forms related to the more familiar kinds in our world. All of these echinoderms surely were covered with a thin epidermis as well, so that only their interiors needed enhanced respiratory techniques. All had hard skeletons of outer plates that surely inhibited oxygen uptake, thus requiring some form of adaptation for respiration for the interior. This need for interior respiration is somewhat alleviated by the fact that most echinoderms have very little flesh in their boxes. A sea urchin, for instance, while having a voluminous space in its spherical skeleton, actually has little flesh within; our measurements of Puget Sound urchins show that only about 20 percent of the volume within their box is composed of living protoplasm. The rest is seawater or the unique water vascular system that enables the tube feet.
More common than echinoderms were mollusks. Most during the Cambrian were small in size. Each of the major classes of mollusks (gastropods, bivalves, and cephalopods) is found in Cambrian strata. The most common mollusks were monoplacophorans, a minor class today but common in the Cambrian. They had a limpet-like shell and a snail-like body with a broad, creeping foot. Most interestingly, alone among mollusks of the time, they showed a body organization that suggests segmentation. From looking at muscle scars on the fossil shells and from comparing anatomy of the still living forms, it looks as if the Cambrian monoplacophorans had multiple gills. Modern-day gastropods have a single pair of gills or sometimes a single gill. But the Cambrian monoplacophorans, which lived a very snail-like existence in all likelihood, found it necessary to have multiple gills. The respiratory adaptations of the mollusks are distinctive enough to warrant additional discussion, below.
Second in abundance to the arthropods were brachiopods, a phylum related to bryozoans that are routinely mistaken for bivalved mollusks. Yet while the shells of bivalves and brachiopods show a superficial similarity, the internal anatomies of the two groups are radically different. The major feature of a brachiopod is a feeding organ known as a