either Pyrex glass,silicone rubber, or nylon. The pump surface in contact with the medium is polyethylene, except for a titanium shaft. Polyethylene and polyvinyl chloride also are used in the system . None of these materials had any demonstrable effect on the copepods when tested separately. The 2 culture tanks have a capacity of 100 l each. Culture medium passes through removable screens into troughs extending the width of the tanks, and flows out troughs extending the width of the tanks, and flows out through a gravity drain.