Calcium phosphate granules in muscle cells of Nephtys (Annelida, Polychaeta)--a novel skeleton?

Analyses of various marine polychaete worms (phylum Annelida) reveal that members of the family Nephtyidae are exceptional in containing very high concentrations of calcium and phosphorus. We report here that in the genus Nephtys, the two elements form rounded granules packing the sarcoplasmic core of obliquely striated muscle cells. Intracellular calcium granules rich in phosphorus are found throughout the animal kingdom and occur in a wide range of tissues. Although some examples of granules in muscle tissue have been recorded (as in Syllis proventriculus), as far as we are aware, none appears in the abundance found in Nephtys.