Echinoderms have 2 major structural features:
- Calcified internal plates covered by muscle and thin skin.
- Water vascular system involved in gas exchange, locomotion and feeding. Seawater enters through a perforaed madreporite, then flows in a calcified canal leading to another canal ringing the esophagus. Other canals radiate from the ring canal, extending through the arms and connecting to tube feet.
Modern echinoderms oftentimes have bilaterally symmetrical ciliated larvae. These feed planktonically before settling and transforming into adults with pentaradial symmetry (symmetry in multiples of five).
There are 2 subphylum of modern echinoderms. These differ in the form of their water vascular systems. The 2 modern echinoderm subphylum are:
- Pelmatozoa (only class Crinodea, which includes sea lilies and feather stars, survives).
- Eleutherozoa (two lineages: urchins, sand dollars, sea cucumbers; sea stars & brittle stars).