that could be called a “body plan.” For example, all vertebrates have a backbone with a nerve cord, so this characteristic becomes the body plan of all vertebrates. All sponges show only two cell layers with a similar kind of cell allowing them to pump water through the body. This is another kind of body plan. Ultimately, the naturalists could find only 32 distinct kinds of body plans among animals and these became formalized later as the 37 animal phyla. Other examples of phyla are arthropods, all with a body plan with a jointed exoskeleton; Cnidaria, all with two cell layers and tentacles with stinging cells called nematocysts; and mollusks, with a soft body but a hard calcium carbonate shell at some time in their life.
The concept of the phylum has changed little since then. For example, one of the best modern descriptions of body plans comes from James Valentine in On the Origin of Phyla:
The body plans of phyla have been much admired as representing exquisite products of evolution in which form and function are combined into architectures of great aesthetic appeal. While the phyla seem relatively simple in their basic designs, most contain branches that form important variations on their structural themes and some body types display remarkable embellishments in their morphological details. Presumably these variants reflect something of the ranges of ecological roles and environmental conditions in which the various phyla have evolved and functioned.
Why do the various phyla of life on Earth have the body plans they do? In other words, why did animals evolve the respective body plans seen today? Surely the answers lie in adaptations to conditions on Earth at the time of the evolution of the various animal body plans, no earlier than about 600 million years ago. It was a world different from today, with more oceans and less land surface, higher temperatures, more ultraviolet radiation, more atmospheric carbon dioxide, and less oxygen. There were no large predators or herbivores, and hence predation and competition to the emerging animals were only from their own kind. So what drove the body plans that emerged? My contention is that respiration was perhaps the most important driver of animal body plans. And yet in Rudy Raff’s The Shape of Life and James Valentine’s On the Origin of Phyla, two recent excellent treatments of this topic, respiration gets only a brief mention as a driving factor in the first and is not even listed in the index of the second. The emphasis