1. The histological characteristics of the cement gland cells of barnacles have been compared in B. nubilis, B. psittacus, B. eburneus, B. balanoides and B. amphitrite. 2. In B. balanoides, the cement gland cells show a very simple composition; they are situated at the base of the animal and the secretion appears scattered throughout the cytoplasm. The cement apparatus of B. amphitrite and B. eburneus looks like that of B. tintinnabulum, but in B. psittacus and B. nubilis the cement gland cells appear more complex. 3. The extrusion of the cement secretion is brought about by muscle fibers that pass through the connective tissue, and in B. psittacus and B. nubilis by elastic fibers around the secondary and principal canal systems.