FACTS AND THOUGHTS ABOUT LIGHT-EMITTING ANIMALS. 235 
over the surface of the colony as the stimulation is transmitted 
to the surrounding animals. I wrote my name with my finger 
on the great Pyrosome as it lay on deck in a tub at night, and 
my name came out in a few seconds in letters of fire.”* Sir 
Wyville Thomson, noticing the “blaze of phosphorescence” off 
the Cape Verd Islands, states that the track of the ship was an 
avenue of intense brightness. “ It was easy to read the smallest 
print sitting at the after-port in my cabin, and the bows shed on 
either side rapidly widening wedges of radiance, so vivid as to 
throw the sails and rigging into distinct lights and shadows. 
The first night or two after leaving San Iago the phosphor- 
escence seemed to be chiefly due to a large Pyrosoma, of which 
we took many specimens in the tow-net, and which glowed in 
the water with a white light like that from molten iron.” f 
All luminous animals are not illuminators of the surface 
water or deep sea, for some shine where their gift is not appre- 
ciated by others. The burrowing shell-fish, Pholas dactylus , 
lives hidden up ; but is nevertheless provided with photogenic 
structures and substances, and these are also nearly hidden in 
the enveloping tissues of the bivalve. The elongated cylin- 
drical shells are well-known objects in most cabinets, and it is 
only necessary to state that the animal has a large foot, and that 
the combined siphons are large, cylindrical, and furnished with 
fringed orifices. Now, the photogenic structures are two parallel 
cords, containing opaque white matter running down the an- 
terior siphon, and two small triangular spots at the entrance of 
it, and, lastly, an arched cord corresponding with the superior 
edge of the mantle, reaching to the middle near the valves. 
The cords and spots are convoluted lobes of the mucous mem- 
brane, The cords stand out in relief, and their white colour dis- 
tinguishes them, and although they are only elevations of the 
subcuticular tissue, they contain special cells, or rather epithe- 
lium, which produces the phosphorescent matter. The whole 
surface of the Pholas is covered with ciliated epithelium, which 
dips down into all the parts of the animal ; but the special 
epithelium differs from this. It is nucleated and crammed 
with granules, and the cells are very refractive. The cells 
are very fragile, and allow their contents, i.e. granular nuclei 
and refractive granules, to escape readily. These are soluble 
in ether and alcohol. Under ordinary circumstances this pho- 
togenic apparatus is hidden ; but violence readily displaces the 
special cells, which burst, and their contents are carried all over 
the surface by the water, assisted by the general ciliation. The 
white substance, fat-like, retains its luminosity when spread 
out on paper for hours, but the light does not appear to be accom- 
* Moseley, w Notes of a Naturalist on tlie Challenger.” 
t 11 Voyage of the Challenger,” ii. p. 85. 
