animals have a very high surface area to volume ratio. Cnidarians prove themselves to be characterized by the highest surface area to volume ratios of any animal yet examined. While there are few fossils of cnidarians from the Burgess Shale or Chengjiang deposits, those that we have (such as the numerous examples of pennatulaceans, or Sea Pens) demonstrate the high surface to volume ratio of this phylum.
Sponges are another animal found in abundance in the early Cambrian deposits. Like the cnidarians, sponges show no respiratory structures, nor would we expect any. With a body plan built around a series of sacs (like the cnidarians but with even less organization: there are no true tissues in a sponge), all sponges show a very high surface area to volume ratio. In fact, sponges are like agglomerations of numerous, single-celled organisms, with each cell essentially in contact with seawater. But even with this advantage, sponges show an even more efficient way of gaining oxygen. Their main feeding cell, called a choanocyte, causes large volumes of water to pass through the sponge. Some sponge specialists have suggested that a sponge passes as much as 10,000 times its volume in seawater through its body each day. This puts the sponge in the category of having functional “pump gills,” since the feeding cell forces water past the cells at a phenomenal rate. Consequently, sponges are capable of living in extremely low-oxygen conditions.
So if most soft-part-covered animals, except for the very large ones, do not seem to require specialized respiratory structures, what of those with hard parts? As noted earlier in this chapter, the major groups of animals with hard parts in the Cambrian were the huge tribe of arthropods, followed in numerical importance (in most Cambrian marine strata) by brachiopods and then by a smaller number yet of mollusks and echinoderms. We have already discussed the respiratory system of arthropods, particularly trilobites, and will discuss these less numerous groups next.
First, Cambrian echinoderms make up a weird assemblage of small boxlike animals. All echinoderms have an outer layer of living flesh. The oxygen needs of this layer are taken care of by direct diffusion of oxygen from seawater and by the layer’s formation with gill-like elements emanating from the flesh in some modern forms, such as sea urchins, starfish, and sea cucumbers. All of these echinoderms also have