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Sea cucumbers are football shaped creatures that lay on their side at the bottom of the ocean. They have five rows of tube feet running lengthwise. Its mouth is surrounded by tentacles that are really tubed feet. Unlike sea stars, the vascular system is not filled with sea water. Instead, sea cucumbers use a special body fluid.

from Brain Pickings

Mark Laita’s Breathtaking Photos of Sea Creatures

Sea stars (group name Stelleroidea) are sometimes called starfish, though they are not real fish (they lack both vertebrae and fins). There are two sub-types of sea stars: Asteroideas are the true sea stars and sun stars. Ophiuroideas are brittle stars and basket stars.