The relationship between the calcite skeleton and associated cells in the spines of Echinus esculentus is described. The cell bodies lie in channels within the skeleton and cytoplasmic processes extend from them toward the calcite. Swollen regions of the cytoplasmic processes contain organelles such as mitochondria and various vesicles. The plasma membranes of the cytoplasmic processes appear to spread over the calcite surface which is thus topologically intracellular. A considerable portion of the stereom space is extracellular and contains an extracellular fluid and collagen fibres.

SUMMARY1. The passive electrotonic properties of neurones VD1 and RPD2 in the brain of the snail Lymnaea can be represented by a soma-finite cable model with closed-circuit axon termination.2. There is a considerable individual variation in input resistance, membrane time constant, electrotonic length and axon-soma conductance ratio, but the average values for these parameters are similar in the two neurones.3. The cells are tightly coupled by an electrotonic synapse giving an average steady-state coupling coefficient of 0-68 and an average resistance measured between recording sites in the cell bodies of 20 MC.4. Calculations using a model consisting of a symmetrical pair of cells with standard values for the electrotonic parameters show that in this system, for a soma-soma resistance of 20 MCI, the junction cannot be more than 0'16 length constants from the cell bodies.5. Reduction in coupling due to membrane current losses in such short proximal axon segments is insignificant.6. Intra-axonal recordings indicate that most of the coupling resistance is located at the junction between VD1 and RPD2, which must therefore be closer to the cell bodies than the limiting value of 0-16 length constants assuming an electrical equivalent model which includes the standard electrotonic parameters.7. If all the soma-soma resistance is located at the junction, then it could be physically a single array of gap-junction particles.8. Despite its low conductance (1/20 MC = 50 nS) and possibly small physical dimensions, the electrotonic synapse is more than sufficient to ensure spike synchrony in the two cells.

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The second maxilla is innervated typically by 35 motor neurones in Nectocarcinus antarcticus and about 39 in Cancer novaezelandiae. Muscles are multiply innervated. Within one species variations in the number of motor neurones innervating any muscle do occur but are not common.