Dye coupling between the cone-shaped radial muscle fibres, which contr
ol the expansion and closing of a squid chromatophore organ, was inves
tigated in the squid Loligo vulgaris. Particular attention was paid to
the role of the myomuscular junctions located between the muscle fibr
es. Lucifer Yellow was injected ionophoretically into single muscle fi
bres under normal artificial sea water (ASW) and under various concent
rations of calcium in ASW, Under ASW, 44% of muscle fibres examined we
re dye-coupled, 82% were coupled under calcium-free sea water and 67%
were coupled under sea water containing high concentrations of calcium
. Dye transfer was blocked by octanol, Muscle fibres were never seen t
o link adjacent chromatophore organs, Results are discussed in terms o
f the role of the myomuscular junctions in the regulation of chromatop
hore expansion in the living animal.