The phylum Echinodermata contains forms that have been shown by ordinary analytical methods to contain both of the phosphagens, arginine phosphate (AP) and creatine phosphate (CP) occurring either singly or together. Members of the classes Crinoidea [Arztedofi bifida (Baldwin and Yudkin, '5O), A. mediterranea (Baldwin, '37) ] , Asteroidea [Marthastcrias glacialis (Needham, et al., '32)] , and Holothuroidea
[ Holothuria tzcbzclosa, Stidtopus sp. (Meyerhof, 'as), Synapta inhaexens (Needham, et al., '32), Thyorze briareus (Baldwin and Yudkin, '50)] which have been examined have been shown to contain only AP with one report questionable. Of the Ecliinoidea both A P and C P have been found together in the jaw muscles of E'chinus esculentus (Baldwin and Yudkin, '50) and Paracentrotus lividus (Needham et al., '32), and A P alone in Arbacia pur~lctulata (Baldwin and Yudkin, '50).
Of the Ophiuroidea [ Ophiothrix fragilis, Ophioderma brevispigza (Baldwin and Yudkin, '50), 0. Zongicauda (Baldwin, ' 3 7 ) ] examined, all assayed for C P alone. I n all these cases the characteristic hydrolysis rate in acid in the presence of molybdate was used as a differentiating assay. Meyerhof ('28) in addition, estimated arginine with arginase.
The final step in tlie synthesis of the phosphagens is the phosphorylation of the appropriate guariidine base by adcnosine triphosphatc ( ATP) (Lolmann, ' 3 5 ) . This equilibrium Contribution number 220 from Bermuda Biological Station for Research, h e .