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



as Ussow found in different species certainly do not exist between these organs of the 

 various species examined by myself. The differences in Ussow's figures (pi. iii. figs. 

 11, 12, 13), might be accounted for by his having studied sections made in diflferent 

 directions. 



The only form of this organ seen by me is the following : — 



A sac is attached to the cuticle, which is about as deep as wide, cylindrical, rounded 

 below, and opening outwards by a circular aperture, which is covered by a continuation 

 of the cuticle as in the case of the simple organs without a pigment coat described above 

 (PL LXIX. figs. 2, h, 5, 10 ; PI. LXXII. fig. 38). 



These sac-shaped organs measure 0"3 to 0*5 mm. in diameter. The sac (PL- LXIX. 

 fig. 5 ; PI. LXXII. fig. 38) is formed of a dense layer of pigment, similar to the pigment 

 found in the skin in other locaHties, and to the naked eye these organs therefore appear 

 as darlc spots scattered over the skin. 



These pigment-sacs, which appear very striking and prominent in sections, are 

 generally slightly expanded at the base and contracted near the mouth (PL LXIX. fig. 5), 

 with their axis sometimes perpendicular to the surface of the skin (PL LXIX. fig. 5) as 

 m Astro7iestlies, and sometimes oblique (PL LXXII. fig. 38), as in Echiostoma barbatum. 

 The thickness of the pigment-layer forming this sac is 0"02 mm. The sac is surrounded 

 by a loose plexus of bloodvessels ' and nerves, from both of which branches arise and 

 penetrate the pigment-layer. The cornea-like continuation of the cuticle which closes 

 the sac is convex, watch-glass-shaped (see figures). 



The internal structure appears to be very similar to that of the simple organs without 

 a pigment coat. The lower, proximal part of the sac is occupied by radially placed 

 pyramidal gland-tubes, closely packed, and, therefore, flattened against each other. 

 Their wide distal ends are rounded, and nerves and bloodvessels radiate upwards 

 between the tubes. The membranes forming the tubes are very fine, and the tubes have 

 an average width of 0'04 mm. at the base and 0'02 mm. at the distal, open end. 



These tubes are Jilled with spherical or slightly irregular granular cells, which are 

 very similar to those attached to the wall of the gland-tube. The cells have an average 

 diameter of "0 1 mm. ; Ussow ^ has given a good representation of them. In the centre 

 of the organ, within the terminations of the gland-tubes, there is a space 0"06 mm 

 wide, which is filled with a granular secretion. 



The portion of the organ underlying the cuticle is also granular, but it is easy to 

 perceive that this portion of the gland is occupied by cells. In Ussow's^ figure 

 (pi. iii. fig. 13), these cells are represented (Scopelus rissoi), but I have never observed 

 them nearly so distinct. The spherical nuclei in the outermost layer, with a diameter of 

 0"002 mm. (which are not so large as represented in Ussow's figure, loc. cit.), become 



' Compare F. Leydig, Die augenalinliclien Organe der Fische, pi. i. fig. 6. 

 ^ M. Ussow, loc. cit. 3 -^i Ussow, loc. cit. 



