1903.] IN THE ‘ CHALLENGER’ COLLECTION. 75 
composed of crown-glass and flint-glass. Behind the inner lens 
is found a thick layer of glandular cells (d), which are light 
greyish, very large, and most of them elongate, radiating towards 
the centre of the outer lens. The diameter of this layer is some- 
what larger than that of the inner lens; and when the luminous 
organs are examined in their natural position with a strong 
pocket- lens, this layer can often be seen through the skin as a 
whitish ring around the lens. Between the laye er of large cells 
and the inner lens a thin layer (c) seems to exist, but its quality 
could not be made out, and I do not venture to propose any 
hypothetical explanation. Behind the glandular layer is seen 
another (e), which is yellowish, with numerous transverse fine 
stripes, and without trace of cellular structure; it is rather thick 
in the middle, but thin around the sides of the glandular layer. 
The internal surface of the organ seems to be covered by a thin 
layer (f) of connective tissue. That the posterior layer with the 
transverse stripes is—as in the Euphausiidee—a reflector can be 
taken for granted. But it is impossible to decide whether the 
light is produced by the glandular layer or by the inner lens. 
Whether the thin layer enveloping the whole organ is pigmented 
or not cannot be seen on this old material, which has been 
preserved twenty-eight years in spit; that the organs are 
immovable scarcely needs to be stated. Future investigation of 
living animals and of sections of fresh material must elucidate 
whether the organs are especi ially innervated or not, and, besides, 
fill up the other gaps in the interpretation of the function and 
structure of the layers in these compound structures. 
A brief comparison of the luminous organs in Sergestes 
challengert with those in other Crustacea is not without interest. 
Of animals belonging to that class, luminosity has been observed 
in some Copepods, a few Ostracods, Euphausiide, and one 
macrurous Decapod. Giesbrecht has published a thorough and 
interesting study: ‘ Ueber das Leuchten der pelagischen 
Copepoden und das thierische Leuchten im Allgemeinen” (Mitth. 
zool. Stat. Neapel, 11 B., 1895, pp. 648-689). He has examined 
a small number of pelagic Copepoda—necessarily restricting 
himself to all the luminous forms which he could procure in a 
living state in the Gulf of Naples—showing that these animals 
possess a number of small dermal glands, the secretion of which 
produces the luminosity when it, by exhaustion from the glands, 
comes in contact with the surrounding water. In a few Ostracoda 
a brilliant luminosity is produced in a similar way from glands in 
the labrum ; it was already suggested by G. W. Miiller in 1890, 
and has since been observed and published by another author. 
Furthermore, 1 can mention that during a voyage in the Indian 
Ocean, Dr. Th. Mortensen met with a vast number of a pelagic 
Ostracod which showed a most brilliant light, and he observed 
how this was produced. Finally, the present writer has observed 
nearly the same in a number of a large Ostracod which had 
been procured in Davis Strait. Late in the evening I observed 
