Observations were made on muscular and ciliary activity and particle transport and retention in intact gills and gill fragments of the suspension-feeding, epifaunal bivalves Anomia achaeus Gray, Juxtamusium maldivense (E. A. Smith), Pteria macroptera (Lamarck), Pycnodonte hyotis (L.), P. numisma (Lamarck), Crassostrea cuccullata (von Born), Crassostrea lugubris (Sowerby), Tridacna maxima (Röding), and T. squamosa Lamarck. 2. Gill contractions were especially violent in Pteria and both species of Pycnodonte. 5-HT in concentrations of 10-10 M caused relaxation of contracted gills, and reduced or abolished the response to mechanical stimulation, except in Tridacna. 3. In Juxtamusium, the concertina-like movements of the gill plicae could be correlated with the functional state of the water-transporting lateral cilia. When all lateral cilia were active, and the plicae inflated, movements of the plicae were discontinued. At periods of arrest of the lateral cilia the plicae collapsed and resumed concertina-like movements. 5-HT had no clear effect on the concertina-like movements of the gill plicae, except in Juxtamusium in which addition of 5-HT to the ambient medium stimulated the movements. 4. Exposed gills or gill fragments of all the species examined continued to transport water, but only inefficiently retained particles, 10-20 µm Tetraselmis cells, added to the water. 5-HT enhanced the rate of water transport in most species by stimulating the activity of the lateral cilia, but reduced the ability of the gill to retain Tetraselmis cells. The cilio-excitatory nerve transmitter 5-HT of bivalve gill filaments thus did not restore normal feeding activity of the gill. 5. Latero-frontal cirri could not be distinguished on the gill filaments of Anomia, Juxtamusium, and Pteria. In Crassostrea lugubris they were small (ca. 13 µm, in length) and inconspicuous. They were about 15 µm long in the two species of Tridacna, and about 17 µm long in Crassostrea cuccullata and in the two species of Pycnodonte, spanning about half of the interfilamentar space in relaxed plicae. The latero-frontal cirri varied greatly in activity and in orientation of inactive cirri. Also the effects of 5-HT were variable, but the drug tended to arrest the cirri in an erect position. 6. 5-HT reduced the creeping rate of gill fragments of Pycnodonte numisma. 7. The gill fragments secreted mucus at low rates even in dense suspensions of Tetraselmis cells. The majority of Tetraselmis cells that were retained and transported by the gill fragments remained free of mucus, to be redispersed in the medium when they arrived at cut ends of the particle transporting ciliary tracts along the gill bases or the ventral margins of the demibranchs. 8. It is concluded that the feeding state of the bivalve gill, which is characterized by high rates of water transport through highly retentive gills, is restricted to undisturbed, intact animals. The mechanisms of feeding, therefore, cannot be finally understood from studies on exposed gills or gill fragments. The physiological significance of the nonretentive gill of disturbed animals and gill preparations is not clear.
