Starfish are the best-known echinoderms, but the group also features feather and brittle stars, as well as sea urchins and sea cucumbers. While they vary in shape, all these creatures have an internal body structure formed from calcium and tube feet that enable them to move over the seabed.
There are few more diverse invertebrate groups than the mollusks. Marine gastropod mollusks possess a protective shell and a “foot” for locomotion, just like the land snails.
Defining a fish is harder than it seems. To most people, it is an animal that lives and breathes in water. Some fish, however, emerge onto land, breathe air, and use their fins like legs. And many other animals, including amphibians, mammals, and invertebrates, live in water. To add further confusion, some creatures called fish—starfish, jellyfish, and cuttlefish—are not really fish at all.
Most people regard catfishes as purely freshwater fishes but there are marine and brackish water species as well. Imported under the names Arms seemani and Ariusjordani, they are collectively known in the trade as the shark catfishes, and have been a regular, if somewhat seasonal, import over the last twelve years or so. Their common name probably derives from their vague resemblance to sharks when they are swimming.