Distribution et importance quantitative de la chitine dans les coquilles de mollusques

1. The wide distribution of chitin in the shells of Molluscs was confirmed thanks to the application of a quantitative and specific enzymatic method. Chitin was found not only in Cephalopod shells, but also in mother-of-pearl, pseudonacreous layers, and in many cases, in periostracum and prismatic layers of Gastropod and Bivalve shells. 2. The presence and the proportion of chitin are highly variable in the periostracum of the Bivalve species studied so far. On the Contrary, its permanent presence in the organic matrix, associated with mother-of-pearl, emphasizes the homology of this structure in the whole phylum nof Mollusca. 3. The more or less important participation of chitin in the vbuilding of the organic matrix of the different shell structures may be related to taxonomic and/or ecological characteristics. From a taxonomical point of view, on the one hand, chitin distribution is in good agreement with the classification grouping Arcidae and Glycimeridae among the Filibranchia Taxodonta and isolating Nuculidae in the Protobranchia. On the other hand, the lower rate of chitin observed in the inner calcified shell layers of seven species of Anisomyaria and the high chitin rate in those of four species of Adapedonta may be a further indication of the homogeneity of both taxa. From an ecological point of view, chitin constitutes an important fraction of the shell organic matter in burrowing species, while this polysaccharide is practically lacking or in very low proportions in the shells of fixed or free species.