The cruciform muscle complex in Egeria radiata L. (Bivalvia, Tellinacea, Donacidae)

The cruciform muscle complex in the freshwater bivalve Egeria radiata L. is highly developed and probably more complicated than in any previously described species. The muscle strands divide at the chiasma into long flat strips which are loosely interlocking. The sense organ consists of a blind-ending, thin-walled and wide ventral sac, becoming progressively narrower dorsally as it leads to a ciliated pit or groove and a semi-circular intramuscular slit. The posterior ends of the cruciform muscle consist of muscle bundles similarly arranged in a semi-circular manner so that the large sac, ciliated pit and sensory ganglion are enclosed between the muscle bundles. Dendritic or sensory nerve endings from the ganglion cells penetrate the epithelium of the pit as in Donax. The sense organ is of the closed type as in Donax, Tagelus and Sanguinolaria.