2. Starfish is the most representative class of echinoderms.

3, the body is flat, mostly five-radiation symmetry, and the boundary between the body plate and the wrist is not obvious. Live with your mouth down and your mouth up. There is a stepped groove on the ventral side of the wrist, and the tube foot extends out of the groove. The bone plate of endoskeleton is connected by connective tissue and flexible and elastic. There are spines and forked spines on the body surface, which are protrusions of bones. Membranous vesicles protruding from between bone plates are covered with epithelium and lined with body cavity epithelium, and their inner cavities are connected with secondary body cavities, which are called dermgills, and have the functions of breathing and diffusing metabolites to the outside world. The water pipe system is developed. During ontogeny, there are larvae with feathers and short carpels.

4. Radial diameter 1-65cm, mostly 20-30cm. The wrist is hollow and covered with short thorns and forked thorns. There are rows of pipe legs (some with suction cups at the ends) in the ditch below, so that starfish can crawl in any direction and even climb up the steep surface. Lower starfish feed on food particles along the entrance of wrist groove. Higher kinds of stomachs can be turned upside down as bait for in vitro digestion or swallowed whole. The endoskeleton consists of calcareous bone plates. Breathe through the skin. There is a bright spot on the wrist. Most of them are hermaphroditic, and a few are hermaphroditic; Some can reproduce asexually.

5. Existing species 1600, 300 fossil species, widely distributed in sandy seabed, soft mud seabed, coral reefs and oceans of various depths.