This is the third and concluding part of an investigation into defensive mechanisms in some British gastropods. The first part (Thompson & Slinn, 1959) dealt with a single species of opisthobranch, Pleurobranchus membranaceus, which proved able to secrete a strong acid if disturbed. The second part (Thompson, 1960 a) recorded further instances of acid secretion. The present paper describes the remainder of my work on opisthobranchs.