Unusual aggregations of tubules associated with endoplasmic reticulum in digestive cells of Alderia modesta (Mollusca: Gastropoda: Sacoglossa).

Aggregations of cylindriform tubules closely packed in a hexagonal arrangement and associated with the rough-surfaced endoplasmic reticulum (RER) are described in cells of the digestive gland of a sacoglossan gastropod. Each tubule is approximately 61 nm in outer diameter and the central lumen is lined with a layer of finely granular material. The tubule wall is composed of approximately 22 dense subunits. Adjacent tubules are associated with a pair of indistinct granules. Membranes of the RER frequently join the aggregations. The lumen of a tubule may be continuous either with the intracisternal space of the RER or with the cytoplasmic matrix of the cell. It is suggested that the aggregations of tubules are elaborations of the endoplasmic reticulum.