What is "zygobranchia"

Zygobranchia \Zyg`o*bran"chi*a\, n. pl. [NL., from Gr. zygo`n a yoke + ? a gill.] (Zo["o]l.) 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.