Nematocysts or cnidocells or "stinging cells" as they have been variously called, are found in many Coelenterates and some Planarians.
Proceedings of the Iowa Academy of Sciences
©1889 Iowa Academy of Science, Inc.
Nutting, C. C.
Proceedings of the Iowa Academy of Science, 1(Pt. 1), 95-96.
Available at: https://scholarworks.uni.edu/pias/vol1/iss1/43