1406 CRUSTACEA—EUCARIDA CHAP. 
as in the type figured, upon the outer margins of the stalked 
eyes, on the bases of the second and seventh thoracic limbs, and 
on the ventral median line on the first four abdominal segments. 
These organs are lantern-like structures provided with a lens, a 
reflector, and a light-producing tissue, and they are under the 
control of the nervous system. Their exact use is not known, 
any more than is the use of phosphorescence in the majority of 
organisms which produce it; but in certain cases it appears that 
the Kuphausiids make use of their phosphorescent organs as 
bull’s eye lanterns for illuminating the dark regions into 
which they penetrate or in which some of them permanently 
Fra. 102.—Huphausia pellucida, female, x 5. G, Last gill; Z, luminous organ of first 
leg; Z’, luminous organ of 2nd abdominal segment; 7, biramous thoracic 
appendages. (After Sars.) 
dwell. At any rate, associated with the presence of these organs 
in some deep-sea Euphausiids are remarkable modifications of 
the eyes; and we may perhaps here fittingly introduce a short 
discussion of these visual modifications in deep-sea Crustacea, 
and the conditions which call them forth. 
The compound eyes of Crustacea resemble those of Insects 
in that they are composed of a very large number of similar 
elements or “ommatidia,” more or less isolated from one another 
by pigment. Each ommatidium consists typically of a corneal lens 
(Fig. 103, ¢), secreted by flat corneagen cells (c.g) below ; beneath 
the corneal lens is a transparent refractive body called the “ crystal- 
line cone” (e7), which is produced by a number of cells surround- 
ing it called the “vitrellae” (v). Below the crystalline cone 
comes the “rhabdom” (7/), produced and nourished by “ retinula- 
