With this apparatus, 1 ml of diet will provide about 7 cm2 of feeding surface. The use of such small volume of diet reduces the expense and the hazards associated with the feeding of radioactive isotopes. The diet is held between two membranes, and so it can easily be removed and discarded after use, a feature which minimizes the chance of radioactive contamination. Temperatures are controlled to + 0.1C through the use of a proportional controller which supplies power to micro-immersion heaters in proportion to the difference between the actual and the desired temperature.