BRITISH CHALK ECHINODERMS
The Echinoderms are a major division of the animal kingdom, and a highly important component of the British Chalk. Echinoderms have several unifying characters;
- The skeleton is composed of many calcareous elements, each composed of the unique honeycomb microstructure known as stereom. Each element (plate, spine, ossicle, etc..) is a single crystal of calcite which is typically recrystallised in the chalk, with the loss of the stereom microstructure.
- Echinoderms possess a water-vascular system, extensions of which, the tube-feet, protrude through holes in the skeleton and aid in locomotion, feeding and respiration. Tube feet are organised into rows called ambulacra. Water enters the system via a sieve-like plate called the madreporite (absent in crinoids).
- Echinoderms also display penta-radial (five rayed) symmetry, most obvious as the five rows of ambulacra and the five arms of Asteroids (starfish) and Ophiuroids (brittle stars).