Anthozoa: More on Morphology
Anthozoans completely lack a medusa stage; they are polyps throughout their life cycles. An anthozoan individual has a saclike body divided by radial partitions known as septa; these septa can easily be seen in corals, and their arrangement is an important character for classification.
Above is a diagram of the septal arrangement in two types of corals. On the left is a scleractinian (extant stony coral) showing hexagonal symmetry. The extinct rugose corals of the Paleozoic seas had a very different arrangement, diagrammed on the right; they were once called tetracorals (four-sided corals), but the arrangement is actually bilateral. Compare the above diagram with the corals shown below, Tubastraea, on the left, and Lithostrotionella, on the right.
Can you identify which is a scleractinian and which is a rugosan?