Characteristics

Free living exclusively marine forms.

Adults are radially symmetrical while larvae are bilaterally symmetrical. Body is represented by a central disc covered by ossicles with spines called pedicellaria.

Disc may bear extensions called arms.

Digestive system is complete.

A unique ambulacral or water vascular system is present.

Tube feet are present for locomotion and respiration. Tube feet are extended and retracted by variation in hydraulic pressure of the fluid in them and contraction of their muscles.

Nervous system has a central nerve ring with five radiating nerves.

Reproduction is sexual. Sexes are separate. Development is indirect. Show very high power of regeneration.

Tube feet are for feeding as well.