Many [female] gonosomes of unusually large size (22 mm. in diam.) studied in lake and laboratory were killed by 30[degree] C but readily tolerated 24[degree] C and while partially inactivated by 17[degree] C, were not killed by several hours at 9[degree] C. Locomotory contractions one per second yielding upward movement averaging 36 m. per hour were measured. Swimming movements were analysed and the importance of velar coordination stressed. Reactions to light, to gravity and to moderate changes in temp. were found irregular or lacking. Ingestion of particles in an eddy at the manubrium tip was suggested as a probable mode of feeding. Water ejected from the bell cavity in swimming caused the eddy concerned, and simultaneously provided for replacement of respiratory water.