. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 204 EDWARD C. ROOSEN-RUNGE Several settling experiments were undertaken (see Results). The simplest method proved to be the most efficient. A dozen 3 X 1 inch microscopic slides were arranged on the bottom of a large bowl containing many hundreds of planulae. After 6-12 days, 50-66% of the slides were found to have at least one primary hydroid growing on them. Such slides were hung in plastic frames, each holding 5 slides into the tanks in which most of the hydroid colonies and many medusae were raised. More than 30 cultures
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 204 EDWARD C. ROOSEN-RUNGE Several settling experiments were undertaken (see Results). The simplest method proved to be the most efficient. A dozen 3 X 1 inch microscopic slides were arranged on the bottom of a large bowl containing many hundreds of planulae. After 6-12 days, 50-66% of the slides were found to have at least one primary hydroid growing on them. Such slides were hung in plastic frames, each holding 5 slides into the tanks in which most of the hydroid colonies and many medusae were raised. More than 30 cultures were observed in detail for nearly 2 months and many more were used occasionally for short-term observations. The tanks were modified after the "Plankton-Kreisel" described by Greve (1968). The principle of the "Kreisel" is a rotary circulation in which inflow into an outer compartment and outflow from an inner compartment are separated from each other by the sand on the bottom. The "airlift" of the inner compart- ment was omitted as unnecessary in an open seaw-ater system. The Kreisels (Fig. 1) were made out of carboys by removing the tops. They were 31-32 cm Inflow Outflow Inner compartment. FIGURE 1. Sketch of the modified "Kreisel" used in raising hydroid colonies and medusae. high and 26i cm in diameter. A self-syphoning outflow was arranged so that the volume of water in the tank oscillated between approximately 15 and 12| liters. Flow was kept between 5 and 20 liters per hour. Occasional plugging of the water lines caused irregularities and standstills which did not noticeably influence the experiments. The tanks were standing in 5-8 cm of seawater with continuous flow which assisted in maintaining a low temperature even when the circulation through the Kreisels stopped. At the inflow into the water tables, the temperature of the seawater varied from ° C depending on the outside temperature. At an average temperature of th
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