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Sessile or sedentary marine animals with radial symmetry as adults


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Echinoderms
Biology AP

2003-4
Phylum Echinodermata

-Sessile or sedentary marine animals with radial symmetry as adults

-Internal and external parts radiate from the center, often as 5 spokes

-Thin skin covers a hard platelike exoskeleton

-Most have bumps or spines which serve various functions


Water vascular system

-Unique to echinoderms

-Network of hydraulic canals which branch into tube feet; water is taken in through a central disk and moves through radial canals to the tube feet

-Function in locomotion, feeding and gas exchange

-A foot is extended when fluid is forced into it by the contraction of the ampulla

-Suction cups at the end of the foot attach to a substrate and the muscles contract to shorten the foot


Class Asteroidea

-Sea stars which have 5 or more arms extending from the central disc

-Have a strong ability to regenerate

-Tube feet are used to grasp the shells of bivalves; as the shell is pulled open, the sea star everts its stomach into the shell and secretes digestive enzymes; it then absorbs the digested particles


Class Ophiuroidea

-Brittle stars

-Have smaller central discs and longer, more flexible arms than sea stars

-No suckers on their tube feet

-Move by thrashing their arms
Class Echinoida

-Sea urchins and sand dollars

-Don’t have arms but their bodies are divided into 5 sections

-Have 5 rows of tube feet for movement

-Spines have muscles that allow them to pivot and aid locomotion

-Have a 5 part jaw-like structure (Aristotle’s lantern) used for feeding


Class Holothuroidea

-Sea cucumbers

-Have no spines and a reduced endoskeleton

-Have an elongated body



-Have 5 rows of tube feet some of which have developed into feeding tentacles

-Can expel its internal organs in response to predators


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