I.—PHYSIOLOGY. 211 
depression just. under the eye and in front of the gills. The organ 
looks as if made for experimentation, as it is attached only at the dorso- 
anterior end, and can be cut out with the greatest ease, giving a piece 
of practically pure luminous tissue. The back of the organ is covered 
with a layer of black pigment, which serves to keep the light from 
shining into the tissues of the fish. In both fishes there is a mechanism 
for obscuring the light, but, curiously enough, the mechanism developed 
is totally different in the two species, notwithstanding the fact that in 
structure the organ is identical in the two, and in every detail except 
proportion the fishes are very similar. Im Anomalops the organ is hinged 
at the antero-dorsal edge, and can be turned downward until the light 
surface comes in contact with a fold of black pigmented tissue, forming 
a sort of pocket. The light is thus cut off. In Photoblepharon a fold 
of black tissue has been developed on the ventral edge of the organ 
socket, which can be drawn up over the light surface like an eyelid, 
thus extinguishing the light.’ The histological structure of this organ 
was worked out by Steche (1909). The organ is continuously 
luminous day and night, and independent of stimulation. According 
to Steche, Anomaiops constantly turns the light on and off (10'’ light, 
5’’ dark), the fish using it, he supposes, as a searchlight to attract 
and mislead its prey, The natives use the amputated organ as a bait 
in night fishing; it maintains its luminosity for about eight hours. 
The organ is described by Steche as composed of a great number of 
sets of parallel gland tubes (acinose), separated by connective tissue, 
and extending across the organ from the back pigmented surface to the 
front transparent surface, each set arranged in a ring about a vessel 
Which provides them with blood and oxygen. Near the surface a 
number of these tubes unite into a common reservoir opening outward 
through a minute pore which admits sea-water. A number of pores 
dot the surface of the organ. The luminous material fills the lumen 
of the tubes; it is extracellular but intraglandular, and is never voided 
from the gland. Harvey states that the luminous material filling the 
tubes consists of an emulsion containing many granules and rods: the 
latter move about with a corkscrew-like motion, and are undoubtedly 
bacteria. The luminosity of the organ is due to these symbiotic bac- 
teria. An emulsion containing the symbionts behaves exactly like an 
emulsion of luminous bacteria in being sensitive to lack oxygen, desic- 
cation, bacteriolytic agents, potassium cyanide, &c. The continuity of 
the light, independently of stimulation, is characteristic of luminous 
bacteria and fungi alone among organisms; this, and the circumstance 
that luciferin and luciferase could not be demonstrated, all go to confirm 
the correctness of Harvey’s conclusions regarding the cause of 
luminosity in these fish, notwithstanding that he has failed hitherto to 
cultivate the bacteria found in the luminous organs. 
In coneluding this section dealing with light production by animals 
it may be repeated that we have to distinguish between (a) luminescence 
due to symbiotic organisms, such luminescence being continuous in 
the presence of oxygen as in cultures of luminous bacteria (of which 
some thirty species are known), and (b) that due to animal cell-products 
known as luciferin and luciferase which are secreted and expelled 
