Echinodermata - Regeneration in Invertebrates
Asteroids (starfishes), ophiuroids (brittle stars) and crinoids (sea lilies) can reproduce their lost arms and although parts of their disc. The arms of starfishes show autotomy. When disturbed or caught by a predator they cast off one or more arms near the base and regenerate them again. In Linckia species an arm completely devoid of any pan of disc regenerates the complete body. Such types of regenerating sea stars are called comets. Brittle stars and sea lilies as well practice autotomy in distress conditions and can regenerate their arms from the remaining part of the arm. Autotomy is specifically striking in sea cucumbers (Holothuroidea) which, when startled or disturbed, can eject by their anus all their internal organs, specifically the respiratory tree and alimentary canal. This spontaneous 'self-evisceration' is followed by complete regeneration of the viscera. If transversely bisected each half of the holothurians, regenerates the missing half to whole the body.