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Morphology continued...

Internal Morphology

Not all of the body cavity is occupied by internal organs, there is also empty space, the perivisceral coelom. The body cavity does contain the water vascular system that operates the tube feet, and the hemal system.

Circulation and respiration

Circulation occurs in three places on the sea star. These are the perivisceral coelom, the water vascular system, and the hemal system.

The water-vascular system, which is a complex internal apparatus of tubes and bladders containing fluid, has extensions emerging through the skeleton to the outside as the podia.

Water enters the madreporite due to an ionic imbalance and flows through the stone canal and then enters the circular ring canal. The water vascular system uses cilia and the constantly contracting ampullae to keep things moving.Water then is separated into five radial canals that branch into double rows of bulb like structures called ampullae, which are on each side of the ambulacral ridge. 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.The ampullea are connected to sucker-like tube feet. Contraction of the ampullae causes the tube feet to stretch as water is brought into them. This whole process allows for movement, and is quite powerful but extremely slow .

The figure to the left, after Clarkson 2001, is a cross section of an asteroid arm showing the plating structure and external position of the radial water vessel.








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 beats like a very inefficient heart to help transfer nutrients from the digestive tract. The gonads are situated inside the sea star underneath the hepatic caeca.


Dissection of Asterias rubens. 1 - Ambulacral ossicles and ampullae. 2 - Madreporite. 3 - Stone Canal. 4 - Pyloric caecae. 5 - Rectal glands. 6 - Gonads. - reproduced with permission from Wikipedia.

Sea stars have two stomachs, the cardiac stomach and the pyloric stomach, in which digestion takes place.
The cardiac stomach, which is a sack like stomach located at the centre 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 mollusc in place.
Due to this ability to digest food outside of its body, the sea star is able to hunt prey much larger than its mouth would otherwise allow including molluscs, arthropods, and even small fish.

The pyloric stomach, inside the sea star then accepts the partially digested food and continues the process. the sea star's arms are filled with digestive glands called pyloric caeca or hepatic caeca. Some echinoderms 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 a complex nervous system and although their sensory inputs are not well defined they are sensitive to touch, light, temperature, orientation, and surrounding water status. Specifically the tube feet, spines, and pedicellariae are sensitive to touch, and the eyespots on the ends of the rays are light sensitive. All echinoderms have a nerve plexus, which lies within and below the skin. The oesophagus is surrounded by a number of nerve rings, which send radial nerves that often parallel the branches of the water vascular system. The ring nerves and radial nerves coordinate the starfish's balance and directional systems.


Most starfish reproduce in a method similar to the sponge. They gather in a large group and release their gametes into the water, where hopefully, they connect with gametes from the opposite sex.

After fertilisation, there are a variety of ways that the eggs can proceed:

  1. Small eggs, without much yolk, grow into free-swimming larvae which feed on small organisms until they metamorphose into juvenile sea stars and can begin living on the ocean floor.
  2. Eggs with larger yolks can develop into a similar planktonic larvae, but feed on its yolk instead of other organisms.
  3. 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.

Sea stars are developmentally (embryologically) known as deuterostomes. Since echinoderms and chordates share this same embryological pattern, they are thought to be closely related. Nevertheless, as these creatures are invertebrates and not actually fish, most marine biologists are pushing to completely replace the term starfish with sea star.


Sea stars have a remarkable ability to regenerate. Some species can regenerate lost arms and regrow an entire new arm 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. One genus particularly noted for its regeneration ability is Linckia. These sea stars can cast off an arm that regrows into an entire organism, as a means of asexual reproduction.

go on to the next page to find out about specialised characteristics

or go back to external morphology

Author: Elizabeth Sweet
Last updated: 22nd November 2005
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