The Collaborative International Dictionary of English v.0.48:

Zygobranchia \Zyg`o*bran"chi*a\, n. pl. [NL., from Gr. zygo`n a
   yoke + ? a gill.] (Zool.)
   A division of marine gastropods in which the gills are
   developed on both sides of the body and the renal organs are
   also paired. The abalone (Haliotis) and the keyhole limpet
   (Fissurella) are examples.
   [1913 Webster]