AbstractActinopygid Cuvierian tubules are few in number. They are made of a basal trunk from which arises 2–3 branches. The trunk is smooth and hollow (proximally) or slightly swollen and solid (distally) and the branches consist of a central rachis to which attach many peripheral spherules. The fine structure of the tubules is similar in the three investigated species ofActinopygabut differs considerably from that of non‐actinopygid tubules. Basic behavioural differences occur also as actinopygid Cuvierian tubules cannot elongate nor become sticky, and are not expelled by the individuals. It is concluded that actinopygid Cuvierian tubules do not fulfil a defensive funct