If it finds a small victim Glaucus simply envelops it with its large mouth , but if the prey is a larger siphonophore the mollusc nibbles off its fishing tentacles , the ones carrying the most potent nematocysts . If it finds a small victim Glaucus simply envelops it with its capacious mouth , but if the prey is a larger siphonophore the mollusk nibbles off its fishing tentacles , the ones carrying the most potent nematocysts .