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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 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.
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.
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:
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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Websites produced by students on the MSc Palaeobiology programme in the Department of Earth Sciences at the University of Bristol for academic year 2005-6