n :  congenital partial or total absence of the skull .
n pl, in some classifications  :  a division of the phylum or subkingdom Chordata comprising the hemichordates, tunicates, and lancelets and including all the chordates without true heads and anterior brains - compare VERTEBRATA  .
 
 
 
Similar sounding terms:  Ac·a·ri·na  ac·a·rine 
 
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