1. Luminescence in Ptychodera and Balanoglossus is normally neurally mediated.

2. The main luminescence appears to be intracellular; in addition a luminous slime is secreted.

3. The response of individual photocytes is all-or-none, but requires neuroeffector facilitation which grades intensity by recruitment (in contrast to crustacean muscle).

4. Repetitive discharge, both to mechanical and single electrical stimuli, is considered to be a prime factor together with interneural and neuro-effector facilitation, in determining spread of excitation in the general plexus.

5. Four types of spread of luminescence occur, differing in conduction velocity polarization of spread, area of spread, necessary conditions for elicitation and adequate stimulus. At least three different neural pathways are involved.

6. Prolonged autoexcitatory displays produce luminous waves arising repetitively from the same locus and from multiple loci; the general plexus is implicated.

7. There is a diurnal rhythm of responsiveness to stimuli. This is attributed to the photocyte rather than to the nervous system.

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