The anatomy of the hook apparatus that attaches egg strings to the fish parasites Haemobaphes cyclopterina, Lernaeocera branchialis, Lernaeocera lusci, Lernaeenicus sprattae, Sarcotretes scopeli and Pennella balaenoptera (Copepoda: Pennellida) is described and illustrated. The hook rises from a cupulate base, extending posteriorly and anteriorly in the body cavity. The suspension of the apparatus in the trunk of the different species differs, but the function is similar. The hook tip enters the genital antrum, nearly penetrates the proximal end of the egg string, and continues into a notch on the antrum wall. The apex of the egg string acquires a concave depression like the finger end of a glove. In this way the string is mechanically attached inside the female genital segment. The mobile ectoparasites Lepeophtheirus salmonis and Hatschekia hippoglossi have hooks which function similarly, but perforate the strings.