A swollen glandular saddlelike region in the epidermis of certain annelidworms such as the earthworm that secretes a viscous fluid to form a cocoon for their eggs.
A survey has revealed 168 species of finfish, 60 species of cnidaria, including corals, eight molluscs, 14 sponges, 11 echinoderms, 15 arthropods and eight annelidworms.