AbstractTwo types of refractile inclusions in the integument of nemertean worms have been studied by X‐ray microanalysis and electron microscopy. The body wall ofTetrastemma bilineatumcontains ovoid concretions that are 10–20 μm long. Qualitative X‐ray microanalyses indicate that the concretions are composed of calcium, phosphorus, barium and perhaps magnesium and sodium. InZygonemertessp. andZ. virescensminute hook‐like structures occur throughout the epidermis. The hooks develop by one week post‐fertilization and appear to be non‐calcified in both larvae and adults. Each hook is formed within a large vacuole and consists of several fused subunits. The subunits lack well defined cortical and medullary regions and seem to be produced by the rough endoplasmic reticulum of the epidermal secretory cell. The possible relationships between the calcareous concretions and proboscidial stylets of nemerteans are discussed and the structure of the integumentary hooks is compared to that of rhabdite‐like bodies secreted by other