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Characters and Morphology

Starfish sea stars and brittle stars, as they are popularly called, are members of the Stelleroidea or Asterozoa which are characterised by a flexible star shaped body which has a central disc with five or more radiating arms. Each arm contains a radial branch of the water vascular system, which supports numerous paired tube feet, or podia, which extrude through the underside of each arm, alongside the ambulacral groove, the feeding groove. The mouth is located on the underside of the disk, in the centre where the radiating ambulacra intersect.

External Morphology

 Recent specimen from Glamorganshire.

This specimen is 4cm across

Click on the pictures to see a large version.

Photos by the author.

 Ventral, or adapical, view of Pilaster sp.    Dorsal, or aboral, view of Pilaster sp.

All echinoderms have internal mesodermal skeletons of porous calcite plates, which are normally spiny and covered both in and outside by a thin protoplasmic skin.
Typically the skeletons have a
pentameral symmetry.
tube feet have a variety of functions, notably locomotion, respiration and feeding.
The tube feet are arranged in grooves along the arms. They operate by hydraulic pressure. They pass food to the ventral mouth at the centre, and can attach to surfaces. A starfish that is overturned simply turns one arm over and attaches it to a solid surface, and levers itself the right way up.
All asterozoans have a central disc that extends laterally into five, or more arms. The mouth is situated on the
adapical surface. Not all species of asterozoa have fully digitated arms some retain a basic star shape, like Metopaster parkinsoni.

On the upper surface there are a series of plates the top down are the carinals, dorso-laterals and marginals. These may interlock, making the test rigid, but they are frequently flexible, which enables the arms to curl, a useful adaptation for climbing.
The plates are larger on the lower surface and organised into a double column of ambulacral plates. Outside these are the ambulacral plates which adjoin the marginals. All the adoral plates unite around the mouth in a rigid peristomal ring.
The respiratory papulae, extensions of the coelom project through the
ossicles on the upper surface.


on to internal morphology

go back to subclasses in subphylum Asterozoa

Author: Elizabeth Sweet
Last updated: 22nd November 2005
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