Contractile vacuoles in cells of a fresh water sponge, Spongilla lacustris.

Forty or more independently functioning contractile vacuoles (CVs) occupy the central region of fresh water sponge pinacocytes. Eash CV undergoes a cycle of enlargement by fusion, movement, shape change, rounding up, and emptying over the course of 5--30 min. Diameter at discharge varies between 1 and 13 micron. CVs in all cell types are associated with submicroscopic coated vesicles. Filled CVs are bounded by an unmodified trilaminar membrane, but vacuoles with excess membrane frequently show coated evaginations. These evaginations are thought to pinch off as coated vesicles, providing an avenue for membrane recycling in the CV system.