Abstract
The integument comprises the epidermis formed by a layer of cells with apical microvilli and cilia, underlined by connective tissue with some muscle fibers, nerves, and different types of glands. Mucus is the main secretory product of the integument, but glands that release bioactive compounds are crucial for the chemical defense strategies of these animals. Blochmann's glands are one type of gland that occurs in cephalaspideans and anaspideans consisting of a very large single subepithelial cell surrounded by muscle fibers, with a duct that opens in the epidermis. The ink gland of anaspideans is formed by a cluster of Blochmann's glands located on the roof of the mantle cavity. The opaline gland is another kind of gland present in anaspideans, consisting of a cluster of very large secretory cells positioned beneath the floor of the mantle cavity. The secretions of the ink and opaline glands mix in the mantle cavity before being released to defend the sea hares from predators. Pigments contained in integument cells are responsible for the color of these animals, rendering them cryptic or conspicuous to advertise their toxicity or distastefulness, which is another important element of their defense.