1. Stimulation of single giant nerve fibres in the stellar nerves of the squid (Loligo pealii) shows them to be motor axons which produce contraction of the circular fibres of the mantle muscles.

2. When a stellar nerve is stimulated with condenser discharges a maximal response is obtained at threshold voltage. No increase of response is obtained by further increase in the strength of stimulation except for an occasional slight increase at about ten times threshold voltage probably due to repetitive firing. It therefore appears that the stimulus produces a single impulse in the giant fibre, and that this is capable of exciting contraction in all the muscle fibres which it reaches. This confirms the conclusion reached on histological grounds that in spite of their syncytial nature each of the giant nerve fibres is a single functional unit.

3. Since there are about ten giant fibres on each side the mantle is divided into 20 neuromotor units, each nerve fibre innervating an enormous number of muscle fibres. The existence of these units can also very readily be demonstrated by the fact that threshold electrical stimulation at any point within the territory innervated by each single giant fibre sets up a contraction of the muscle fibres of all parts of the territory with which the stimulated area is in connexion through the nerve.

4. Stimulation of the smaller fibres in a stellar nerve after destruction of the giant fibre also causes contraction of the circular muscles of the mantle. The amount of this contraction increases progressively with increased voltage, presumably on account of the stimulation of more and more nerve fibres. The maximum tension developed in this way is always very much less than that produced by stimulation of the giant fibres.

5. The mantle is therefore provided with a double mechanism of expiratory contraction, maximal contractions being produced by single impulses in the giant fibres and graded contractions by those in the smaller fibres of the nerve. Presumably the former contractions are those involved in rapid movement, the latter in respiration.

6. There are also radial muscles, running through the thickness of the mantle, whose contractions effect the inspiration by making the cavity larger.

This investigation was made at Wood's Hole, Massachusetts, with the help of a Fellowship of the Rockefeller Foundation. I wish to thank the officers of the Foundation and of the Marine Biological Laboratory for their hospitality and assistance. I am also indebted to Prof. E. S. Goodrich, F.R.S., for criticism of the MS.