PRESIDENTIAL ADDRESS. 535 
and above it is a rounded gelatinous mass, fibrous in structure, transparent during 
life, covered with a delicate muscular layer. Dr. Meyer hesitates as to the function 
of this mass; but I think, in view of the structure of the luminous organs in other , 
species, we may hazard the suggestion that it is some kind of lens, This organ 
is of particular interest, because it is the only instance yet recorded of a luminous 
organ among the Myopsida and the only glandular luminous organ in the 
Cephalopoda. Glandular luminous organs are, however, known in many species 
of fish, and in Pholas among the Mollusca. 
B. Non-glandular Organs.—These may perhaps be divided into 
(i) Simple, without special optical apparatus. 
(ii) Complex, with more or fewer of the following structures: pigment 
layer, reflector, lens, diaphragm. 
(i) As a type of the simpler kind we may take the branchial organ of 
Pterygioteuthis yiardt (15), in which we have a central mass of parenchymatous 
tissue, with a delicate superficial membrane (consisting of two thin layers), and 
resting upon a rather thick layer of close, compact tissue, which stains very deeply; 
beneath this organ is a single layer of cells containing a reddish-brown pigment. 
The corresponding organ in the nearly allied Pyroteuthis (or Pterygroteuthis) 
margaritifera (14) is a degree more complex, for underneath the central cell mass 
is a thick layer of scale-like bodies, similar in structure to that regarded in other 
cases as a reflector (‘tapetum’ of Chun). In both these cases it seems necessary 
to regard the central cells as the source of light (see fig. a). 
Another organ, almost equally simple, is that found in the tentacles of 
Thaumatolampas (5), where the central portion of the stem of the tentacle for about 
2 mm. of its length is occupied by a large rounded cell-mass whose diameter is 
more than half that of the tentacle. The nerve which usually occupies this 
position is pushed to one side and flattened out like a ribband. Most curious is 
the fact that on the side opposite to the nerve a second organ is superposed on the 
first, which is of more complex structure, inasmuch as it has in its centre a mass 
of photogenic cells surrounded by a system of radiating fibrils with a pigment 
layer and tapetum at one side (see fig. B). 
(ii) As an example of the complex organs we may conveniently take those of 
Histioteuthis riippelli (22), where they are scattered over the ventral-surface of the 
mantle, siphon, head, and arms, forming in particular a definite ring round the 
ventral half of the margin of the ocular aperture. The organ itself is an ovoid 
body, about 1 mm. in length and somewhat less in diameter. The deeper three- 
fourths of this cup are covered with a thin layer of pigment, which is lined witha 
thick coating made up of small lenticular bodies packed closely together and 
forming a kind of mirror. The space within this, equal in diameter to about half 
the diameter of the organ, is filled with a mass of large deeply staining cells with 
large distinct nuclei. The more superficial portion of the organ is made up of 
what seem to be refractive structures. The deeper portion is conical, fitting into 
a hollow in the photogenic mass, whilst the upper part is bounded by a definite 
convex surface the function of which is obviously lenticular. Nerves have been 
traced passing through the mirror to the light-producing cells in the centre. This 
ovoid body is situated at the posterior end of a somewhat hollowed patch of an 
elongated oval shape, which may measure as much as 10-12 mm. in its antero- 
posterior diameter. A consideration of the form and position of this hollowed 
patch and of its relation to the axis of the organ shows pretty clearly that it is an 
sah mirror, destined to throw the rays of light downwards and forwards 
(see fig. D). 
One of the most complicated organs known is that found in the mantle of Abra- 
liopsis (15). Here the whole apparatus is spheroidal in form and surrounded bya 
black coating, derived apparently from a number of confluent chromatophores. The 
photogenic cells lie rather in front of the centre, and before them again a ring of 
black cells seems to discharge the functions of an iris diaphragm. Behind the 
source of light is a reflector consisting of two parts: the deeper is concave, 
spheroidal, and made up of numerous concentric layers; the more superficial 
