Final answer:
The correct statement about phylum Cnidaria is that its members have bodies organized around a gastrovascular cavity, and they exhibit radial or biradial symmetry.
Step-by-step explanation:
Among the options listed about the phylum Cnidaria, the correct answer is E: The bodies of its members are organized around a gastrovascular cavity. Cnidarians are marine invertebrates characterized by radial or biradial symmetry and are diploblastic, developing from two embryonic layers. They possess specialized cells called cnidocytes which contain nematocysts used for capturing prey. It is incorrect to say that all cnidarians except corals are in the medusa stage, as some such as sea anemones and corals only exhibit the polyp stage. Additionally, the phylum does not have more species than any other phylum, nor do cnidarians exhibit bilateral symmetry or cephalization.
The phylum Cnidaria includes animals that exhibit radial or biradial symmetry and are diploblastic, meaning that they develop from two embryonic layers, ectoderm and endoderm. Nearly all (about 99 percent) cnidarians are marine species. These animals have specialized cells called cnidocytes, which contain nematocysts that can immobilize prey with toxins. Cnidarians have two distinct body plans: polyp and medusa, and the life cycle can alternate between these forms.