Many noteworthy features in Euphausiid organisation are brought out in Fig. 102. The shrimp-like appearance of the carapace and antennae indicate the special Decapodan affinities of the family; noteworthy, also, are the single series of gills and the biramous thoracic and abdominal limbs, similar to those of the Mysidacea. The Euphausiidae also possess phosphorescent organs of a highly developed kind, and these are usually situated, 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 Euphausiids 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 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. [Illustration: FIG. 102.—_Euphausia pellucida_, female, × 5. _G_, Last gill; _L_, luminous organ of first leg; _L′_, luminous organ of 2nd abdominal segment; _T_, biramous thoracic appendages. (After Sars.) ] [Illustration: FIG. 103.—=A=, Sections (diagrammatic) of Crustacean compound eye, =A=, with pigment in light-position for mosaic vision; =B=, with pigment in dark-position for refractive vision. _c_, Corneal lens; _c.g_, corneagen cells; _cr_, crystalline cone; _f_, basal membrane, or membrana fenestrata; _ip_, irido-pigment; _n_, nerve; _r_, retinula; _rh_, rhabdom; _rp_, retino-pigment; _v_, vitrella. ]