It is often desired to grow animals and plants of the intertidal zone in the laboratory, under conditions as far as possible simulating those in nature and with the least trouble and expense. This need was felt by the writer during an ecological study of the diatoms inhabiting the mud-flats at Whitstable, Kent. It was found necessary to maintain such diatoms in the laboratory in London in a healthy condition for several days so as to study the effect of light and tides on their movements. The apparatus described in this paper was therefore devised. It has an advantage over other tidal systems (Bracher, 1919; Martin & Reid, 1935) in that the substratum could be automatically alternately flooded and drained at regular intervals by a much simpler mechanism. Besides, it is more handy and can be operated with only a small supply of sea water, a matter of some importance in localities remote from the sea.

It consists of a tidal chamber, T, and a reservoir, R (Fig. I). The former is an inverted bell-jar fitted with a rubber bung through which passes a long glass tube, g, which extends to the base of the reservoir. At the upper end of the tube is a small funnel by which sea water enters the apparatus. A ⋯tube, t, serves to connect the tidal chamber and reservoir, its horizontal arm being connected to the siphon S.

The wide-mouthed bottle constituting the reservoir is fitted with a stopcock, v, which is useful for expelling air when setting up the apparatus.