海星

Listen to a talk by a marine biologist in an aquarium. Welcome to our aquarium. As we begin our tour, the first animal we will see today is the starfish. You probably have seen pictures of the starfish, but in a few minutes you will see some live ones and learn a little about their structure and life cycle. First of all, starfish are not really fish, they belong to the family of echinoderms which are spiny skinned sea animal that is, their skin is covered with thorny bumps. Most starfish have five arm-like extensions on their bodies. And so they look like a five-pointed star. But some other kinds have as many as 40 or more arms. Starfish, like other members of the echinoderm family, have what’s called radial symmetry. All that means is that the body parts of these animals are arranged around the center, kind of like spokes of a wheel around a hub. One of the special features of the starfish is that it can drop off arms as a defensive reaction, to get away from an attacker, for example. They can then grow new arms to replace the old ones. Starfish reproduce by releasing eggs into the sea. These eggs develop into larvae, and can swim freely. There early forms which are what larvae are, differ from adult starfish, because the larvae have bilateral symmetry. That means that the two halves of the larvae look exactly the same, which makes them look a lot different from the later form of the starfish. Eventually the larvae sink to the ocean bottom and change into the adult radial form. If you don’t have questions, we will go in now and see some of the creatures in person.