The test (skeleton) of an ophiuroid is composed of calcite
and other organic minerals. It is organised into porous plates by
combing many 3-D crystals, forming a stereom lattice. The plates
interlock together forming the hard skeleton. In starfish these plates
are loosely linked but in brittle stars they are tightly bound together
making a very rigid frame (see picture).
In ophiuroids the test makes up the structure of the central disc, all
the internal organs are housed within this disc. The only major opening
in this disc is on the aboral surface (on the bottom) of the animal (see picture). The entrance of the mouth is on this surface, as is the madreporite (for the intake of water).
Dorsal view of the central disc
Ventral view of the central disc
The water
vascular system is a complex series of tubes and bladders, used like a
hydraulic pump. Water is pumped up through the madreporite (a small hole
on the ventral surface) and pumps water into the central disc and into the arms.
The water vascular system extends to
the tube feet which use this hydraulic system to suction things for
movement along surfaces and for feeding.
The internal organs of the brittle star are enclosed in the central disc.
These include the large sac-like stomach, the jaws, the genitals and
the connective muscle material. The mouth is rimmed by five jaws.
The arms of brittle stars are long and slender, they
are sharply separated form the central disc. The arms are constructed
of vertebral ossicles (similar to our own vertebrae) which work
together as ball and socket joints. Between these ossicles is the
connective tissue called mutable collagenous tissue (MCT), it is unique
to the echinoderms. MCT is stimulated by the nervous system into
contracting and releasing. This allows the movement in the arms. In
basket stars the joints are mobile enough to allow the curling and
branching of arms. It is also this process that allows the lashing or
rowing action that is seen when brittle stars are moving through the
water. As they have no defined head they can move in any direction.
Brittle stars drop their arms as a defence
when being preyed upon or in times of stress. This is done by the
nerves at the disc to arm interface stimulating the MCT to disintegrate
and sever the arm. Brittle stars can then regenerate the lost arm
within a few months.
The arms are used to capture food and pass it along to the mouth as well as in respiration and locomotion.
