The pattern of growth of Clytia johnstoni.

1. A quantitative study of the growth of Clytia johnstoni is described. 2. The distance between hydranths is most commonly between 3 and 4 mm. The average internode lengths and their variability frequently differs not only between colonies but also between stolons of a single colony. The differences are therefore unlikely to be genetic ones. Occasionally much longer internodes occur (up to 8 mm). 3. Hydranths and their stalks differ widely in size but not in morphology. The differences can be caused by the amount of food given. 4. Very vigorous and very poorly growing colonies tend to have slightly longer internodes than the more common colonies of average vigour. 5. Most colonies produce both additional (secondary) hydranths and stolon branches. Hydranth branches grow spaced from existing hydranths; in normal colonies the spacing appears to originate from the hydranth away from the growing tip in an internode. In vigorous growth the hydranth towards the growing stolon tip is the place of reference. 6. Stolon branches occur mostly near to hydranths, either primary or secondary, more especially near to the hydranth farther from the growing tip of the main stolon. 7. There appears to be a limit to the number of branches to an internode. In vigorous growth an optimum of two and a maximum of three is found, either secondary hydranths, or stolons, or both. 8. Stolons vary in their rate of growth. A stolon slowly increases its rate of growth as it gets longer. In a colony of little vigour, the rate is low and variable and may cease and restart; the coenosarc sometimes breaks, dividing the colony. 9. Hydranths take 1-2 days to grow, sometimes 3 days in colonies of little vigour. They remain open for a limited period: normally about 5-7 days; sometimes longerup to 9 or 10 days; and significantly shorter, 1-3 days, in a colony of little vigour. 10. At the end of their period of existence, hydranths regress. They normally start to regenerate a new hydranth within 1-2 days but some do not do so for longer or very long periods. 11. The whole cycle of hydranth growth, maturity and regression falls into either a 6to 7-day cycle or a 10to 1.1-day cycle. 12. Evidence is presented to support the suggestion that there is a situation of competitive interaction between growth of the various parts of the colony. A priority series for the various parts of the colony is constructed.