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cnidosac In certain Nudibranchia, one of the internal sacs located near the tips of the dorsal projections from the body surface (cerata), each of which contains a nematocyst derived from coelenterate prey. In the alimentary canal, nematocysts are separated from the nutritive parts of the meal and passed to the cnidosacs; should a predator bite through the cerata, nematocysts are discharged, with the probable consequence that the predator abandons its attack.