Isogenic clones of H. luciae were raised at each of two developmental temperatures, 18° and 28° C. Despite prolonged acclimation to common thermal conditions, oxygen uptake rates differ according to the temperature of reproduction, regeneration and development. 2. The effects of developmental temperature, however, are masked by body size differences. Only when this variable is eliminated can the underlying effect of developmental temperature be detected. 3. The irreversible change is not due to an increase in the gas exchange surface area at the primary site of O uptake, the tentacles. 4. No qualitative changes in banding patterns for five enzymes (HK, PGI, IDH, MDH, G-6-PDH) were found.