Ultrastructure of muscle cells in the adductor of the boring clam Tridacna crocea

The ultrastructure of the adductor muscle of the boring clam (Tridacna crocea) was investigated. The adductor was composed of opaque and translucent portions. The opaque portion contained smooth muscle cells; the translucent portion contained obliquely striated cells. Smooth muscle cells were classified, according to the statistically analyzed diameters of their thick myofilaments, into two types, S-1 and S-2. S-1 cells had thick myofilaments, 50-60 nm in diameter. S-2 cells had thick myofilaments of two sizes, about 55-65 nm and 85-100 nm in diameter, respectively. Obliquely striated muscle cells in the translucent portion were also classified into two types: O-1 cells, with thick myofilaments 30-35 nm in diameter, and O-2 cells, with myofilaments of 50-60 nm.