Nematocysts: the nonliving output of certain types of cells. Simply put, these are stinging/poisonous mechanisms that "fire" into the prey or the attacker (for there are multiple ways these adaptataions are used) (Kass et al, 2002).This fascinating (and painful and deadly) adaptation is often accompanied by "warning" coloration. Nudibranches are some of the most colorful of the sea-bottom dwelling, and that coloration is in fact a warning to other creatures to beware their sting as well as to blend in with their colorful counterparts (Futuyma, 2009).
Source: Kass-Simon, G., & Scappaticci, A. A., J. (2002). The behavioral and developmental physiology of nematocysts. Canadian Journal of Zoology, Retrieved from http://search.proquest.com/docview/220508335?accountid=40659
Futuyma, F.J. 2009. Evolution. 2nd Ed. Chapter 11. Retrieved from http://ncse.com/files/pub/evolution/Evolution--Futuyma--chap11.pdf