-T- of the isotope. In the converse situation where the ecological half life is less than’ the physical half life, a net loss of the 4sotope is indicated. This condition coyld result from loss of the isotope by the environment, or eco-system, or from a physio- logical change in the organism or its primary food source. Such physiological changes may be transitory or seasonal. The increase in radioactivity over preshot levels during the first few days after the Nectar test was less in muscle and carapace than in the four other tissues by a factor of 5 to 10. Max imum post-Nectar levels of activity were 100 to 250 times greater than pre-Nectar levels in gut, liver, and gill, but only respectively. 22 and 26 times greater in muscle and carapace The lower rate of accumulation in muscle and carapace would be expected since the material must be absorbed from the hepatopancreas where some selection takes. place. gut and The specific patterns of changing radioactive content of the tissues with.time, rate of decline, will be presented individually for each tissue. The amounts of radioisotopes involved are so small that they probably do not constitute a significant proportion of the natu- rally occurring isotopes. If, for example, a tissue contained 107 a/m/g wet of Sr929, or 5,000 times the maximum level found in the hermit crab, this would represent only 0.02 mg of strontium, or about 1079 per cent of the ash weight. The presence of Strontium has been reported qualitatively in crustacea and a quantitative estimate of about one per cent strontium has been given for the ash of Eupagurus bernhardus.?°