FACTS AND THOUGHTS ABOUT LIGHT-EMITTING ANIMALS. 237 
Ophiacanthce , only lately rid of their ‘pluteij shone very 
brightly.” 
The position of the luminosity is removed from the nervous 
cords, and in decalcified specimens I have failed to trace nervous 
filaments on the top of the disc and in the substance, or near the 
upper arm-plates of the rays. But in a specimen from the 
Icy sea of North Smith’s Sound, collected during Sir George 
Nares’ expedition, and sent to me for description, I traced 
a filmy mucous covering here and there, which seemed to be 
an exaggeration of the excessively thin epiderm which evidently, 
in the young forms, covers the plates and the bases of the spines. 
The disc is covered with a crowd of minute spicular projections, 
each terminating in a bunch of small thorny knobs, or in three, 
four, or more rather sharp spicules. These delicate appendages 
are developed within the skin, as are also the granular elements 
which constitute the plates of the arms. It is possible that 
the luminous property resides in this delicate epiderm ; and the 
probability is increased when it is noticed that the phenomenon 
is most decided in young individuals. It may be possible, how- 
ever, that the Ophiuran has no photogenic structures, and that 
the light is the product of foreign animal substances which have 
become entangled by it as it moved over the mud of the sea floor 
on which it feeds. 
Many years since, Quatrefages, in a very exhaustive memoir on 
the phosphorescence of marine animals,* attributed the light 
of the Ophiuran he examined to muscular contraction, and he 
found it arising between the plates of the arms. He did not see 
any luminous condition of the disc. But that this occurs is un- 
doubted, and there are no muscular fibres there. 
A considerable number of Crustacea are luminous under cer- 
tain conditions, and the light- emission is sufficiently remarkable. 
In very transparent ten-footed kinds, and indeed in the small 
Entomostraca, as well as in many of the Sand-hopper group, a 
vivid short-lived light is emitted. Its colour is often redder 
than that of any other animals, and it is localized at first, for it 
starts from the junction of the legs with the body, and extends 
rapidly beneath the skin ; and then it becomes diffused, the 
whole body glowing for a while. Some of the host of marine 
worms are luminous occasionally, and especially some of the 
genus Nereis and of the tube-making Chcetopterus. 
They emit a greenish light, and Quatrefages noticed that the 
phenomenon consists of a quick series of scintillations, which pass 
along several segments of the body, lasting but an instant. 
The flashes can be produced by irritating the worm, and they 
appear to accompany muscular contraction. Finally, as regards 
* See the literature of the subject at the end of my article “Photogenic 
Structures ” — “ Micrographic Dictionary,” third edition. 
