Fig. 3. Ca²⁺-ATPase activity in the swimming muscle of Polyorchis penicillatus. The reaction product (lead phosphate precipitate) of Ca²⁺-ATPase activity is located in (A) the nucleus; (B) Golgi stacks; (C) the outer leaflet of the lateral plasma membrane (arrowhead) and vesicles of the sarcoplasmic reticulum (arrow). The apical region, at the top of panel, shows little precipitation and there is an absence of reaction products in the mitochondria. (D) Precipitates were found along both sides of the sarcolemma surrounding myofibres and at their basal attachment to the mesoglea, but none were seen in the mitochondria. Arrows point to subsarcolemmal vesicles showing Ca²⁺-ATPase activity. The arrowhead indicates a tubular structure that does not have any precipitate. (E) Negative control for the Ca²⁺-ATPase activity in swimming muscle. A section through the nuclear region showed no lead phosphate precipitate when tissues were incubated in a medium without ATP. (F) Negative control for the Ca²⁺-ATPase activity in swimming muscle. Tangential section through myofibrils showing that there was no lead phosphate precipitation when tissues were incubated in a calcium-free medium with 10 mmol l^-1 EGTA. The arrow indicates one of the vesicles and the arrowhead the sarcolemmal membrane, both of which are free of precipitate. Mi, mitochondria; Me, mesoglea. Scale bars, 1 µm (A,E); 50 nm (B); 100 nm (C); 0.5 µm (D,F).