Sea stars belong to the Phylum Echinoderms. That means they are related to sand dollars. Sea urchins, and sea cucumbers. All echinoderms have five-point radial symmetry, which means that their body plan has five sections arranged around a central disk.

 Although sea stars live underwater and are commonly called “Starfish“, they are not fish. They do not have gills, scales, or fins like fish do and they move quite differently from fish. While fish propel themselves with their tails, sea stars have tiny tube feet to help them move along

 While the five-armed varieties of sea star are the most well known, not all sea stars have 5 arms. Some have many more. Take the sun star for instance, which has up to 40 arms!

 ring canal  Circular canal in which filtered water enters through the madreporite and branches out into the radiated canals.  rectal cecum  Lateral duct of the terminal part of the digestive tract, where waste is stored before being expelled through the anus.  anus  Terminal orifice of the digestive tract allowing waste to be ejected; most of the undigested food is regurgitated rather than expelled through the anus.  stomach  Dilated section of the digestive tract preceding the intestine; it receives food to be digested.  gonad  Each of the two glands located in each arm, producing gametes (spermatozoids or ovules) depending on the sex of the starfish.  pyloric cecum  Radiated duct of the digestive tract producing digestive enzymes and also allowing digested food to be stored.  gonopore  Dorsal opening through which gametes (spermatozoids and ovules) are expelled into the water to be fertilized.  intestine  Section of the digestive tract between the stomach and the anus where absorption of nutrients is carried out and waste is transformed into fecal matter.  radial canal  Canal running the length of the arm; it receives water from the annular canal, which then passed into the tube feet.  ampulla  Bulb that contracts to let water enter the tube foot, allowing it to extend; when it dilates, the foot retracts.  esophagus  Muscular membranous channel of the anterior section of the digestive tract; it allows food to reach the stomach.  mouth  Anterior cavity of the digestive tract located on the ventral surface that allows food to be ingested.

 Internal anatomy of a starfish : marine animal with radiating symmetry. They are equipped with numerous tube feet. Sieve plate : entrance to the water-vascular system of a starfish. Anus : end of the digestive tract of a starfish. Rectal cecum : cul-de-sac related to the rectum. Ray : limb of a starfish. Ring canal : tube that forms a ring of a starfish. Tube feet : organ of locomotion of a starfish. Digestive gland : organ of a starfish that secrets digestive enzymes. Cardiac portion of the stomach : part of the stomach of a starfish used for pumping blood. Pyloric portion of the stomach : part of the stomach of a starfish connected to the duodenum.

 Starfish vary in size. They usually have five to ten inches in length and weigh up to 11 pounds.  Common feature for all starfish is that their body is radially symmetrical. That means that their body can be divided in five equal pieces (even if they have more than five arms).  Another impressive characteristic of the starfish is their ability to regenerate different (missing) parts of their body. If predator eats part of the starfish, remaining part (one arm for example) will develop missing arms and bring back previous look after some time. This process is not always fast and it may last up to one year.  Starfish is a carnivore who likes to eat clams, shells and mussels. It has two stomachs, one of which can be pushed outside the body during eating. This tactic allows them to eat large prey (that cannot be swallowed with their small mouth).  Starfish mouth is located on the underside of its body. Anus is located on the upper side of the body.

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