286 THE VOYAGE OF H.M.S. CHALLENGER. 



take the shape of transverse bands, which are about 3 mm. broad, and divided from 

 each other by darker zones in which the white organs are scarce. Within the bands the 

 phosphorescent organs are on an average 07 mm. apart (from centre to centre). 



Towards the ventral side these organs diminish in number and stand further and 

 further apart, until they disappear altogether below the line formed by the upper row of 

 composite phosphorescent organs on the side. 



h. Structtire. 



Surface views and section-series reveal the structure of these organs (PI. LXXII. 

 figs. 34, 35, 37). They are mostly lens-shaped, flat and extended, and sunk into the 

 skin of the fish for about half their height (figs. 34, 35). In other cases (fig. 37) they 

 appear nearly hemispherical, are not sunk into the skin at all, their hemispherical 

 surface projecting beyond the surrounding surface. The lower lens-shaped forms 

 likewise project beyond the surface, but not so much (fig. 34). They are from 0"! to 

 0"2 mm. high, and generally project about 0"07 mm. The flat lens-shaped form is pre- 

 dominant ; the high forms being rare and intermediate shapes by no means common. 



The circumference is usually spherical, sometimes slightly irregular, never elliptical. 



These organs do not seem to possess any special membrane, the common cuticle 

 passing over them, and the capillaries of the skin appear aggregated at the base of the 

 organ, whence bloodvessels extend up into the organ itself. These are on an average 

 0'02 mm. apart, and converge towards the centre of the organ at right angles to the 

 convex base. Between them radial tubes, rounded and closed at the distal ends, ar(- 

 situated, and these are filled with small granular spherical cells similar to those described 

 by UssowMn the gland tubes of the simple ocellar organs of Gonostoma denudatnm. 

 There is no difference between these cells and the homologous ones in the simple ocelhi.i- 

 organs with pigment coat. 



The tubes in which these granular elements are contained are polygonal pyramids, with 

 their points cut ofi", and consequently their narrow converging ends, which are situated 

 near the centre, are open. The broader rounded distal ends are attached to the tissue 

 which is situated below the organ. 



The walls of the tubes are thin membranes of connective tissue into which capillaries 

 and nerves extend. The nerves are medullated, the sheath disappearing apparently at 

 the points where the branches of these nerves enter the mass of granular cells. 



In the material at my disposal the extreme nerve ends cannot be traced. Near tlie 

 centre of the organ, above the terminations of these pyramidal tubes, there is an empty 

 space extending right across the organ ; it has a shape somewhat similar to that of the 



1 M. Ussow, Ucber deii Bau der sogenamiteii augenivhnlichen Flecken einiger Knochenfische, Ball. Hoc. imp. des 

 Nat. Moscou, t. liv. p. 98, pi, iii. fig. 11. 



