The phylum Cnidaria is made up of two sister clades, Anthozoa and Medusozoa. The two groups have distinctive differences in life history, anatomy, mitochondrial genome structure, and presently available DNA sequences. Cnidaria has more than 11,000 valid species, with well over half representatives of Anthozoa. In terms of higher taxa, Anthozoa contains a single class, whereas Medusozoa is comprised of the classes Cubozoa, Hydrozoa, Scyphozoa, and Staurozoa. The distinguishing characteristic of Cnidaria, and the feature that gives the group its name, is the cnida (the primary type is known as a nematocyst). Cnidae are capsular organelles with eversible tubules, and they are present in all cnidarians. When nematocysts explode, they release venom for prey capture and predator avoidance as the barb-like tubule everts in one of the fastest motions known in nature.
While many cnidarians exhibit radial symmetry, there is quite a bit of variation in this character. For instance, many cnidarians are biradial, some are bilateral, and some are even directionally asymmetric. Another feature often thought to be diagnostic for Cnidaria is the planula, the stage between embryo and sessile juvenile polyp in most cnidarian life cycles. While the planula is usually ciliated, sausage-shaped, and non-feeding, significant departures from this basic (probably ancestral) pattern exist in different cnidarian groups. The polyp stages of cnidarians also exhibit great variability. Cnidarian polyps are usually sessile, but they vary greatly in structure (with or without various types of musculature, mineralized skeletons, or even tentacles) from group to group. Polyps are often solitary, but many are colonial, and within colonial forms polyps can be either monomorphic or polymorphic.