Abstract
In euopisthobranchs, the gill is external on the right side of the animal or is located within the mantle cavity. The term plicatidium designates the gills of euopisthobranchs which have a highly folded structure, being distinct from the lamellar ctenidial gills of other mollusks. In
Aplysia
, the gill is formed by wedge‐shaped pinnules in which the respiratory tissue is highly folded. On both sides of these folds, the epithelium comprises ciliated, unciliated, and secretory cells. Beneath each epithelium, a layer of connective tissue with muscle cells lines the central blood space. In
Aplysia depilans
, the ultrastructure of gill epithelial cells indicates an absorptive function, endocytosis, and intracellular digestion in lysosomes, as well as active transport of ions in addition to the respiratory function. Mucous cells are the most abundant secretory cells in the gill of
A. depilans
, but serous epithelial and subepithelial cells are also present. The gill is reduced or entirely missing in several small‐size species.