GILL ULTRASTRUCTURE AND SYMBIOTIC BACTERIA IN CODAKIA-ORBICULARIS (BIVALVIA, LUCINIDAE)

The cellular organization of the gill, which harbors symbiotic bacteri a, is described in juveniles and adults of Codakia orbicularis, a larg e tropical Lucinidae. The ciliary zone is similar in every species of Lucinidae described and includes the large clear cell which has been p reviously described as an intermediary cell. The intermediary zone is composed of a few narrow unciliated cells, which bind adjacent filamen ts together and constitute channels through which sea water circulates along the abfrontal part of the filaments. The lateral zone is more c omplex in C. orbicularis than in other Lucinidae, being composed of fo ur cell types and differentiated into two distinct regions. The bacter iocytes and intercalary cells occupy the outermost bacteriocyte zone, while mucocytes and numerous cells crowded with proteinic, cystine-ric h granules constitute the innermost secretory zone which has not been described in other species. The newly described granule cells are cons idered to be a key factor in the storage and metabolic conversion of s ulfur compounds.