1. The proprioceptive organ of the shore crab Carcinus maenas, which signals all proprioceptive sensation from the joint between propodite and dactylopodite, has been shown to contain sense cells receptive to position as well as to movement.

2. Certain cells of the organ increase their discharge frequency in response to greater degrees of flexion, while other endings respond to greater degrees of extension. Movement has little effect on the frequency of the position fibres.

3. Cells responding to movement in one direction only are present, certain ones signalling fiexion and others extension. The most sensitive undirectional movement fibres are almost completely independent of position and velocity. They provide the crab with an extremely sensitive movement sense not as yet reported in other proprioceptive mechanisms.

4. Both position-sensitive and movement-sensitive cells show a wide range of thresholds.

5. Cells which cannot be strictly classified as either position or movement sensitive have also been found.

6. By tracing the origin of the signals to the cell bodies, it appears that movement fibres have on the whole larger cell bodies than position fibres and are more proximally located.

7. Flexor and extensor movement cells appear to lie on different sides of elastic strand.

8. The question of how unidirectional sensitivity may be achieved is discussed.

John Simon Guggenheim Memorial Fellow.