Lacking a spinal cord or bone marrow; relating to the absence of marrow tissue.
From Greek a- (without) + myelos (marrow) + -ous (adjective suffix meaning 'full of' or 'characterized by'). The term emphasizes the quality of being without marrow.
While most spinal cord defects let some marrow form, truly 'amyelous' organisms would be unable to function—which is why you basically never see this in living animals except in the most severe birth defects.
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