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Recent specimen from Glamorganshire. This specimen is 4cm across Click on the pictures to see a large version. Photos by the author. |
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| 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.
The 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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