The utriculi majores, the largest tubules of the mushroom-shaped sex gland complex of P. americana are lined by a single layer of secretory cells with basal plasmalemmal infoldings and apical microvilli, and enveloped by a thin layer of slow-acting muscle. The latter is predominantly monomyofibrillar, with an ill-developed T-system, sarcoplasmic reticulum and striations. The secretory epithelium consists of typical protein-synthesizing and-exporting cells with an extensive rough endoplasmic reticulum and active Golgi elements. Secretory granules, budded off from layered cisternae of the Golgi complex, are more numerous in the vicinity of the luminal surface. The luminal contents are rich in proteins, neutral lipids and phospholipids; there are also some sulphated and nonsulphated acid mucopolysaccharides.