The Collaborative International Dictionary of English v.0.48:

Acrania \A*cra"ni*a\, n. [NL., from Gr. 'a priv. + ? skull.]
   1. (Physiol.) Partial or total absence of the skull.
      [1913 Webster]

   2. pl. (Zool.) The lowest group of Vertebrata, including the
      amphioxus, in which no skull exists.
      [1913 Webster]