Source : Webster's Revised Unabridged Dictionary (1913)

Acrania \A*cra"ni*a\, n. [NL., from Gr. 'a priv. + ? skull.]
   1. (Physiol.) Partial or total absence of the skull.

   2. pl. (Zo["o]l.) The lowest group of Vertebrata, including
      the amphioxus, in which no skull exists.