Clytia hemisphaerica, a member of the early-branching animal phylum Cnidaria, is emerging rapidly as an experimental model for studies in developmental biology and evolution. Unlike the two existing genome-sequenced cnidarian models Nematostella and Hydra, Clytia has a free-swimming jellyfish form, which like "higher" animals (the Bilateria) has a complex organization including striated musculature, specialized nervous system and structured sensory and reproductive organs. Clytia has proved well suited to laboratory culture and to gene function analysis during early development. Initial studies have shed light on the origins of embryonic polarity and of the nematocyte as a specialized neurosensory cell, and on the regulation of oocyte maturation. With a full genome sequence soon to become available, and a clear potential for genetic approaches, Clytia is well placed to provide invaluable information on core mechanisms in cell and developmental biology, and on the evolution of key features of animal body plans.
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