n.coelenterates typically having alternation of generations; hydroid phase is usually colonial giving rise to the medusoid phase by budding: hydras and jellyfishes
· 水螅纲 （ Class Hydrozoa ） 是本门中最原始的一纲，只有二胚层，生殖细胞在外胚层形成，可行有性、无性的繁殖，有世代交替的现象，如僧帽水母、西班牙..
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class Hydrozoa
n.coelenterates typically having alternation of generations; hydroid phase is usually colonial giving rise to the medusoid phase by budding: hydras and jellyfishes