-8Results Exoskeleton The carapace was taken &s the sample of exoskeleton. It is easily removed, separated from other tissues and washed free of possible external contamination. The radioactivity in the carapace due to long-lived isotopes remained approximately constant throughout the period of 537 days during which collections were made. This was determined by re--- - counting all of the samples approximately 600 days after the Nectar test (Figs. 2 and 3). Radiochemical analysis of 18 samples taken at various times during the collecting period (Table 1 and Appendix Table 2), and three samples taken 35 days before Nectar demonstrated that virtually all of the long-lived activity was 20-year Sr9° and its y90 daughter. The nearly constant level in the carapace (ecological half life®™physical decay) indicates that this tissue quickly reaches and maintains equilibrium with the available strontium. Gross, Taylor and Watson (1954) report a plateau of retention of sr20 in rats during continuous feeding at the same rate, and apparent shifting of the pleteau with change in daily dose.t© DOE ARCHIVES It would be expected that this relationship also applies to available calcium which is metabolically similar to strontiun, and to 54-day sr89, and possibly Balto , which at the time the radiochemical analyses were made was present in amounts too small (<0.2% of total activity 7’) to be determined by the method used. The amount of gr89 present in the carapace immediately after Nectar was calculated from the yields given by Sullivan 18 on the ¢