This image shows
a partially dissected view of the aboral (upper) surface of a
plastic model of a starfish. The image contains the principal
internal organs and some elements of the water-vascular system.
Asteroids normally have five rays, or arms, (1) arranged around
a central disc (2).
have a complete digestive system with a mouth (found on the oral
surface of the starfish), a cardiac stomach (3), a pyloric
stomach (4), pyloric ceca (5) that take up most of the space in
the arms and an anus (6). Although radially symmetrical as
adults, echinoderms possess a well developed coelom (7) lined
with cilia that keep water moving through the organism.
Structures of the
water vascular system that can be seen include the entrance to
the system called a madreporite (8) that leads to a vertical
stone canal (9). From there water enters a ring canal that
surround the mouth, then into radial canals in each arm that
deliver water via lateral canals to the bulb-like structures
called ampullae (10). It is the contractions of these ampullae
that deliver fluid under pressure to extend the tube feet so
that they can make contact with the substrate or a food item.
The lateral canals are contained within bony elements called
ambulacral ossicles (11) that form an ambulacral ridge in each
arm (seen as ambulacral grooves on the oral surface of each
arm). Starfish also have a well developed internal skeleton made
up of bony elements called dermal ossicles (12). Also seen on
the model are several of the gonads (13), which are occupy a
proximal position in each arm.