354 
Proceedings of Royal Society of Edinburgh. [july 4, 
the surrounding tissues in between the anterior and posterior halves 
of the organ, but we have not satisfied ourselves that these strands 
are either muscular or nervous. The relations of the various layers 
are for the most part correctly described by Claus and Sars ; but the 
former described a cuticle enveloping the whole organ, which does 
not exist, and the latter did not correctly describe the relation of the 
external pigmented epithelium to the stratified layer. 
With regard to the emission of light, our experiments confirm 
the evidence of previous observations, that the luminosity is inter- 
mittent, and, as far as can be judged, closely dependent on stimula- 
tion. The following experiments were made : — 
1. Mechanical Stimulation . — In total darkness the hand was 
inserted into a vessel containing sea-water in which some of the 
animals were swimming, and moved about. When an animal was 
touched it instantly emitted light. When an animal was taken 
between the fingers and removed from the water, all the organs 
shone brilliantly for 5 to 10 secs., while the animal was flapping 
its abdomen and trying to escape; then followed a series of separate 
flashes, and after 10 secs, more the emission of light ceased 
altogether, until fresh stimulation was applied by means of a squeeze 
between the finger and thumb, when all the organs immediately 
flashed. 
When a fresh animal was crushed between the two hands, certain 
particles of the tissue became luminous, and remained so till they 
were dried up. 
When one of the organs was crushed on the stage of the micro, 
scope, the field became immediately illuminated, and remained so 
for some time. 
2. Chemical Stimulation . — When a living animal was placed into 
saturated solution of bichloride of mercury, all the organs shone 
most brilliantly during the energetic struggles preceding death : the 
luminosity lasted usually 5 to 7 secs., but in one case for 30 secs. 
When a specimen was placed in nitric acid per cent., the same 
result occurred. 
In both cases the posterior organs ceased to shine first, and the pair 
in the eye peduncles were the last to cease shining. One of us spent a 
whole morning at Millport in examination of the organs in the fresh 
state, by means of the microscope, with the object of ascertaining 
