Cnidosac

A cnidosac is an anatomical feature that is found in the group of sea slugs known as aeolid nudibranchs, a clade of marine opisthobranch gastropod molluscs.

The sea slugs within the nudibranch clade Aeolidida have protruding cerata (singular "ceras") on their dorsal surface.

At the tip of each ceras is a small sac in which nematocysts (stinging cells) are stored.

These nematocysts originate in the cnidarians (such as sea anemones, hydroids, jellyfish, corals, siphonophores, etc.)

Individuals in this species can be dangerous for humans to handle; the cnidosacs of G. atlanticus often contain particularly powerful venomous stinging cells from one of its food species: the pelagic siphonophore known as the Portuguese Man o' War, Physalia physalis.

Detail of the aeolid nudibranch Flabellina aff. lineata showing the cnidosacs at the tips of the cerata .
Glaucus atlanticus is the blue sea slug shown here out of water on a beach, and thus collapsed; however, touching the animal directly with your skin can result in a painful sting, with symptoms similar to those caused by the Portuguese man o' war