24 Thomas G. Thompson and Tsaihwa J, Chow 



of calcite structure. Besides the Cirripedia, the Decapoda also contain calcite as well 

 as an appreciable amount of phosphorite. Vinogradov and Borovik-Romanova 

 (1945) found 0-2% of strontium in the ash of Balanus balanoides. Webb (1937) also 

 reported 0-1% of strontium in the ash of the hermit crab Eupagums {=Pagurus) 

 bernhardus. 



The results of analysis of calcareous portions of Arthropoda are given in Table I (F). 

 The strontium-calcium atom ratios of Cirripedia averaged 4-45 x 10 -^ which is 

 the highest value for the organisms containing calcite. 



The canapace of Decapoda was found to contain more organic matter than Cirri- 

 pedia, but their strontium contents were of the same order of magnitude, although the 

 calcium content of Decapoda was much lower. Appreciable amounts of magnesium 

 and phosphate were detected qualitatively. The strontium-calcium atom ratio was 

 rather uniform with an average of 6-17 X 10-^ The claws of Cancer antennarius 

 were also analyzed, and showed no difference in chemical composition as compared 

 to the carapace. The soft carapace of C. productus in its moulting stage was found to 

 consist mainly of organic matter and only 0-44% of calcium. This may be cited as 

 an illustration of the change in chemical composition of the exoskeletons during 

 moulting. 



8. Phylum Mollusca: The molluscs are widely distributed in the ocean and con- 

 stitute the largest invertebrate group which possesses calcareous protective shells. 

 The chemical composition, mainly calcium, of Pelecypoda and Gastropoda was 

 studied by Clarke and Wheeler (1922), Fox and Ramage (1931), Noll (1934), 

 McCance and Masters (1937), Webb (1937), Asari (1951), Odum (1951 b), Kulp, 

 et al. (1952) and Vinogradov (1953). The majority of Pelecypoda and Gastropoda 

 possesses shells consisting chiefly of a calcite-aragonite mixture. Only three families 

 (Anomiidae, Ostreidae and Pectinidae) were reported to have calcite shells. Aragonite 

 was reported in calcareous portions of Amphineura, Scaphopoda, Cephalopoda and 

 Nudibranchia. 



Class Amphineura: The chitons are considered morphologically to constitute the 

 most primitive class of living shell-bearing molluscs. Only a few calcium analyses on 

 these organisms were reported and none on strontium. 



The results of analysis of chiton plates are given in Table I (G). They were composed 

 mainly of calcium carbonate with relatively high percentage of strontium. The 

 strontium-calcium atom ratios averaged 8-06 X 10-^ which was much higher than 

 those in other classes of molluscs. 



Class Pelecypoda: The results of analysis of 44 species of Pelecypoda are shown 

 in Table I (H). Nine families (Myidae, Clinocardium, Saxicavidae, Tellinidae, 

 Lyonsiidae, Mactridae, Periplomatidae, Pholadidae and Solenidae) had strontium- 

 calcium atom ratios greater than 2-0 X 10-^ The lowest strontium-calcium atom 

 ratio was found in the families which were reported as having calcite shells, and that 

 of the highest was Myidae. 



Class Gastropoda: Forty-six species of subclass Prosobranchia shown in Table I (I) 

 were analyzed. The strontium-calcium atom ratios among all families ranged from 

 1-31 to 2-14 X 10-^ and were less than that for Pelecypoda. 



The Nudibranchia of subclass Opisthobranchia do not possess any calcareous 

 protective shells. The body wall of Anisodoris and Archidoris, which contains cal- 

 careous materials, was analyzed (Table I (I)). The organisms were high in organic 



