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ammocoete

British  
/ ˈæməˌsiːt /

noun

  1. the larva of primitive jawless vertebrates, such as the lamprey, that lives buried in mud and feeds on microorganisms

"Collins English Dictionary — Complete & Unabridged" 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Etymology

Origin of ammocoete

C19: from New Latin ammocoeteēs, literally: that lie in sand, from Greek ammos sand + koitē bed, from keisthai to lie