Intramarsupial glochidia of Anodonta anatina (L.) and Pseudanodonta complanata (Rossmässler) were studied in southern Finland. Material staining positively with periodic acid – Schiff's reagent (PAS), neutral lipid reserves, and acid phosphatase activity have different distributions in the mantle of the two species. Moreover, the mucous covering of the mantle of the two glochidia behaves differently on critical-point drying. The presence of microvilli with alkaline phosphatase activity on the mantle surface and acid phosphatase activity in the mantle cells in both glochidia suggest that the mantle plays a role in nutrient uptake and digestion and possibly also in electrolyte uptake. The primordia of the stomach, digestive diverticula, and intestine, at least in A. anatina glochidia, contain neutral lipids and exhibit acid phosphatase activity: In A. anatina glochidia, a microvillous layer with alkaline phosphatase activity continues from the ventral walls of the lateral pits to the suspected kidney diverticula. In both glochidia, there may be three pairs of rudimentary ganglia, which do not stain with methylene blue. The eight ciliated sense organs of the glochidia are methylene blue- and PAS-positive and they exhibit succinate dehydrogenase and acid phosphatase activity. In each mantle lobe, the enveloping cell of the dorsal ciliary organ is interconnected with those of the ventral triad via a cellular fold or "tract," and the ciliated central cells of the organs send axons towards each other.
