The Collaborative International Dictionary of English v.0.48:

Hydrobranchiata \Hy`dro*bran`chi*a"ta\, n. pl. [NL., fr. Gr.
   "y`dwr water + ? gills.] (Zool.)
   An extensive artificial division of gastropod mollusks,
   including those that breathe by gills, as contrasted with the
   Pulmonifera. -- Hy`dro*bran"chi*ate, a.
   [1913 Webster]