The Collaborative International Dictionary of English v.0.48:

Balanoid \Bal"a*noid\, a. [Gr. ba`lanos acorn + -oid.] (Zool.)
   Resembling an acorn; -- applied to a group of barnacles
   having shells shaped like acorns. See Acornshell, and
   Barnacle.
   [1913 Webster]