User:Tonei/Notepad/Sea star

Digestion and excretion
When feeding, sea stars are able to extrude their cardiac stomachs out if their mouth and onto their prey, allowing them to digest creatures far too large to fit inside of the starfish's body. The prey is partially digested outside of the body, after which the stomach is retracted and the digestion process is completed internally with the pyloric stomach. Sea stars often grasp on to a mussel shell and force it open with their tube feet, and then remove their stomachs from their bodies and insert them into the shell to kill and digest the mussel. Some echindoderms have been shown to live for several weeks without food under artificial conditions - it is believed that they may receive some nutrients from organic material dissolved in seawater.

Nervous System
Echinoderms have rather complex nervous systems. All echinoderms have a nerve plexus (a network of interlacing nerves) which lies within as well as below the skin. The esophagus is also surrounded by a number of nerve rings, which send radial nerves that are often parallel with the branches of the water vascular system. The ring nerves and radial nerves coordinate the starfish's balance and directional systems. Although the echinoderms do not have many well-defined sensory inputs, they are sensitive to touch, light, temperature, orientation, and the status of water around them. The tube feet, spines, and pedicellariae found on starfish are sensitive to touch, while eyespots on the ends of the rays are light-sensitive.

Reproduction
Most starfish reproduce in a method similar to the sponge. The starfish gather in a group (using environmental signals to coordinate the timing), and release their gametes into the water, where they will hopefully connect with gametes from the opposite sex.

After fertilization, there are a variety of ways that the eggs can proceed. Small eggs (those without much yolk) grow into free-swimming larvae which feed on small organisms until they metamorphose into juvenile starfish and can begin living on the ocean floor. Eggs with larger yolks can develop into a similar larvae which is planktonic, but feeds on its yolk instead of other organisms. Some eggs may go through direct development, where the yolk is abundant and the egg passes directly into a juvenile form, without a larval stage.

Circulation and respiration
There are three places on the sea star where circulation occurs. These are the perivisceral coelom (the space inside the body not occupied by the organs), the water vascular system, and the hemal system. Hemal channels form rings around the mouth (the oral hemal ring), closer to the top of the starfish (the aboral hemal ring), and around the digestive system (the gastric hemal ring). The axial sinus, a portion of the body cavity, connects the three rings. Each ray also has hemal channels running next to the gonads. There is a dorsal sac connected to the hemal system which pulsates like a very inefficient heart to help transfer nutrients from the digestive tract.

The water vascular system uses cilia and the constantly contracting ampullae to keep things moving. An ionic imbalance causes water to flow into the madreporite, entering the water vascular system. Some of this water is diverted into the periviscerial coelom (the large cavity in which major organs are suspended), where it is circulated by the beating of cilia. Most oxygen enters the starfish via diffusion into the tube feet (with the water vascular system), or the papulae (small sacs covering the upper body surface.

Locomotion
Sea stars move using a water vascular system. Water comes into the system via the madreporite. It is then circulated from the stone canal to the ring canal and into the radial canals. The radial canals carry water to the ampullae and provide suction to the tube feet. The tube feet latch on to surfaces and move in a wave, with one body section attaching to the surfaces as another releases.

Symmetry and special characteristics
Like all echinoderms, sea stars feature radial symmetry. In the sea star, this is shown in the five rays of the star. This is known as pentaradial symmetry. It is believed that sea stars (and other echinoderms) evolved from bilaterally symmetrical creatures.

Sea stars have a remarkable ability to regenerate. If a starfish loses a ray, it can grow an entire new one in time. Most species must have the central part of the body intact to be able to regenerate, but a few can grow an entire starfish from a single ray. These species will regenerate several starfish from a single one which is torn apart.

Resources
"starfish." Encyclopædia Britannica. 2005. Encyclopædia Britannica Premium Service 16 May 2005 .

"echinoderm." Encyclopædia Britannica. 2005. Encyclopædia Britannica Premium Service 16 May 2005 .

Dale, Jonathan. "Starfish Science." Madreporite Nexus. 10 May 2000. 17 May 2005 .

"Sea star." Wikipedia, the free encyclopedia. 7 May 2005, 02:31 UTC. 16 May 2005 .