1. Two populations of neurons in the visceroparietal ganglion of the surf clam have been identified as efferent to the posterior adductor muscle. Both populations are driven by input from the cerebrovisceral connectives and the posterior pallial nerves. The latter pathways originate in tactile receptors on the siphons and posterior mantle.

2. Cells designated as type I respond to input over the four major nerve roots by compound postsynaptic potentials, and to input over the adductor muscle nerves by facilitating strictly excitatory potentials. Type II neurons respond in a purely excitatory manner to input over all pathways tested. It is suggested that pharmacological analysis of the input excitatory synaptic contacts on type I cells would have implications for the structural organization of molluscan neuropile.

3. Neither efferent pathway has yet been identified in terms of functional connexions made at the periphery. It is assumed from behavioural observations that type I cells are motoneurons controlling phasic contraction of the adductor, but electro-physiological confirmation of this is still wanting.

Supported by research grants from the National Science Foundation (no. GB-3625) and the National Institute of Neurological Diseases and Blindness (no. NB-04989), United States Public Health Service.