326 DISCOVERY REPORTS 



the chromatin knot always being found on the median side of the nucleus. This is clearly 

 illustrated in Fig. 8 and Plate XXIV, fig. 5. In Fig. 7 the modified portion of the 

 liver lies behind and somewhat to the outer side of the mandible (md.), and the external 

 face of the liver organ lies to the left of the figure, so that any light which might be 

 emitted by the tubule (bim.l.) would be seen from the left. Fig. 8 shows the same 

 tubule in the same relative position, with the chromatin knots all lying on the right side 

 of the nuclei, away from the exterior. This very curious disposition of the nuclei 

 presumably has some bearing on the mode of functioning of this part of the organ, but 

 at present its meaning is entirely obscure. 



Surrounding the mandibular organ (and also the posterior organs, whose structure is 

 precisely the same as that just described) is a fibrous tunic (Fig. 8, fib.t.) apparently 

 composed of connective tissue. Its nuclei are most plentiful in the neighbourhood of the 

 tubule tips, where they are elongated and lenticular. Elsewhere they are rounded and 

 only sparsely distributed. The elongated nuclei are of two kinds. One type (Fig. 8, to 

 right and left of the guide line^Z>./.) shows a regular vacuolated appearance, while the 

 second type (Fig. 8, l.n.) has a central knot and radiating strands of chromatin. It 

 would appear possible that if the tubules are indeed engaged in the active production 

 of light these nuclei may function in some measure as minute lenses. The fibrous tunic 

 probably carries the red pigment mentioned by Dr Kemp in his account of the colour 

 of the organs and so forms a dorsal pigment cap like that of the organs of Pesta of 

 Sergestes. It will be realized that these liver organs, like the organs of Pesta, must, 

 if they are actually luminous, illuminate the branchial chamber. 



Other members of the family Pandalidae possess organs, probably luminous, which 

 occupy a similar position in the body to those of Parapandalus. In the genera Chloroto- 

 coides and Thalassocaris Kemp (1925, p. 272) mentions them as being conspicuous in 

 the living animal on account of the colour of the investing membrane, and in his 

 description of Chlorotocoides spinicauda (1925, p. 277) describes the position of the 

 organs as follows: "One of these organs is close to the base of the maxilla under the 

 branchiostegal wall and one is situated on a slight prominence behind the posterior 

 pleurobranch and above the coxa of the last leg. In shape each organ is nearly spherical 

 and rather less than 0-5 mm. in diameter. The upper part was completely invested with 

 bright red pigment, while the lower part, which in living specimens seemed to be 

 slightly flattened, was glistening white." The author then states that similar organs 

 are found in Thalassocaris crinita and T. lucida, quoting Dana as remarking that the 

 latter species is "very brilliantly phosphorescent". He gives a brief description of the 

 organs of T. crinita (p. 278), and leaves little doubt that the organs he describes are 

 groups of modified liver tubules. 



The occurrence of tracts of modified liver tubules, probably functioning as photo- 

 phores, in members of the Caridea presents an interesting and inexplicable parallel 

 with the organs of Pesta in some species of Sergestes. In each we have organs of endo- 

 dermal origin, derived from tubules of the digestive gland, and occupying similar 

 positions in the body. The colour in life is closely similar, purplish black, blue, or white 



