Definitions for "Hydrozoa"
The Acalephæ; one of the classes of cœlenterates, including the Hydroidea, Discophora, and Siphonophora.
A class of phylum Cnidaria. The primary life stage is nonmotile and has a sac-like body composed of two layers of cells and a mouth that opens directly into the body cavity. A second life stage, the free-living medusa, often resembles the common jellyfish.
coelenterates typically having alternation of generations; hydroid phase is usually colonial giving rise to the medusoid phase by budding: hydras and jellyfishes