Adult individuals of Janolus species can be between 2.5cm to 8cm long, depending on the species. They are semi-translucent and the body is covered in short cerata.
In California, Navanax is a known predator of Janolus. Navanax tracks the slime of Janolus by using chemoreceptors. When Janolus is about to be caught, it rolls into a ball, leaving its cerata exposed.
↑Bergh, L.S.R., 1884. Report on the Nudibranchiata dredged by H.M.S. Challenger during the years 1873-1876. Report of the Scientific Results of the Voyage of H.M.S. Challenger during the years 1873-76, under the command of Captain George S. Nares, R.N., F.R.S. and Captain Frank Tourle Thomson, R.N. prepared under the superintendence of the late Sir C. Wyville Thomson, Knt., F.R.S., &c. Regius Professor of Natural History in the University of Edinburgh Director of the Civilian Scientific Staff on board, and now of John Murray one of the naturalists of the expedition, Zoology 10(26):1-154, pls. 1-14.