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17
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Across Clues:

1.	This is what its called when a brittle star loses an arm on purpose (8)
6.	Skeletal plates whose closeness or looseness determine if a sea star feels stiff or soft (7)
9.	What do sea cucumbers breath with (4)
11.	Class-The animals belonging to this class have radial symmetry and have very skinny arms (11)
12.	It connects the water coming into the madreporite to each tube foot (10)
15.	microscopic pinchers that protect the skin and gills of echinoderms (12)
16.	Class-sea urcins and sand dollars (10)
17.	A system of canals that lead to the tube feet of a sea star and help it move (21)
18.	Class-These echinoderms are the only ones whose mouths face up (9)
19.	(10)
20.	Furrows that are bordered by tube feet on sea urchin tests (9)