University of Bristol logo and link Fossil Groups banner and link

Ophiuroidea Anatomy

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
Dorsal view of the central disc   Ventral view of the central disc

Line diagram of the internal water vascular system
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.
Picture of tube on the leg of an echinoid

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.
Drawing of the internal organs of a brittle star

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.
cross section of the brittle star arm

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.

Author: Lorna O'Brien
Last updated: 19/11/2006
Return to Fossil groups home page

All images on this page are copyright free.

Websites produced by students on the MSc Palaeobiology programme in the Department of Earth Sciences at the University of Bristol for academic year 2006-7