Sea star: Encyclopedia II - Sea star - Internal Anatomy

Sea star - Internal Anatomy

Inside the sea star underneath the hepatic caeca are the gonads which are involved in reproduction. The space inside the body not occupied by the internal organs is known as the perivisceral coelom. The body cavity also contains the water vascular system that operates the tube feet, 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.

Sea star - Digestion and excretion

Sea star digestion is carried out in two separate stomachs, the cardiac stomach and the pyloric stomach. The cardiac stomach, which is a sack like stomach located at the center of the body may be everted - pushed out of the organism's body and used to engulf and digest food. Some species take advantage of the great endurance of their water vascular systems to force open the shells of bivalve molluscs such as clams and mussels, and inject their stomachs into the shells. Once the stomach is inserted inside the shell it digests the mollusk in place. The sea star's anus is located at the center top of the animal.

Because of this ability to digest food outside of its body, the sea star is able to hunt prey that are much larger than its mouth would otherwise allow including arthropods, and even small fish in addition to molluscs.

Partially-digested food is passed to the inside of the sea star where digestion continues in the pyloric stomach. Due to all of this digestive demand, the sea star's arms are filled with digestive glands called pyloric caeca or hepatic caeca. 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.

Sea star - 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.

Sea star - 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.

Adapted from the Wikipedia article "Internal Anatomy", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki