PHOTOPHORES OF DECAPOD CRUSTACEA 323 



luminous organs and it appears very probable that the two pairs of large organs on the 

 carapace may have this function. Of the latter, one is situated at the base of the mandible, 

 beneath the carapace, and one pair postero-dorsally underlying both the posterior edge 

 of the carapace and the tegument of the first abdominal somite. In fresh material the 

 organs have the form of rounded cones, their sides shining red, and their bases, which 

 are directed downwards and backwards, dark purplish black. On dissecting out the 

 posterior organs they were found to be attached to the liver and on pulling this from 

 under the carapace it was found that the two mandibular organs were also attached to it 

 anteriorly. Specimens preserved, fixed in Duboscq. No trace of light was observed in 

 the specimens, some of which were alive when brought on board." 



Examination of these structures leads me to support the view expressed by Dr Kemp 

 that the liver organs are almost certainly photophores ; the pleopod organs are perhaps 

 so, but with less probability. 



I. THE LIVER ORGANS 



The structure and disposition of the liver of P. richardi was first studied in a series of 

 thick sagittal celloidin slices. The main mass of the liver occupies its customary position 

 in the cephalothorax, and is composed of closely packed blind tubules with a character- 

 istic structure. The walls of these tubules are made up of somewhat pyramid-shaped 

 cells, each containing at least one very large vacuole. Their nuclei are basally situated. 

 The bases of the cells are contiguous with each other, and form the outer wall of the 

 tubule, but the sides of the cells are not in contact. The tips of the cells therefore project 

 into the lumen of the tubule, which thus assumes an irregular stellate appearance as 

 seen in transverse section. It may be said at once that the structure of the modified 

 tubules differs profoundly from that shown by the normal tubules just described. 



The modified liver tubules do not occur in the main mass of the liver, but constitute 

 two pairs of well-defined outgrowths or horns. The anterior pair, the mandibular 

 organs, arise antero-laterally, and extend downwards and forwards so that their tips 

 come into close relation with the bases of the mandibles. The posterior pair of out- 

 growths occur dorsally and postero-laterally, and project upwards and backwards from 

 the main mass of the liver. In the borax carmine-stained sections these projecting horns 

 are seen to be of denser and more compact composition than the rest of the liver, and 

 furthermore show a slight but definite yellowish hue. 



With the specimens in my possession was a mandible with an adhering mass of liver 

 tubules — the mandibular organ. This is illustrated in Fig. 6, the arrow showing the 

 level of the transverse section seen in Fig. 7 and Plate XXIV, fig. 3. A study of the trans- 

 verse sections obtained from this structure clearly reveals the composition of the modified 

 tubules. Each tubule of the projecting horn as it leaves the main mass of the liver is 

 composed of the normal vacuolated cells just described. Soon, however, its walls are 

 formed by denser non-vacuolated cells (Figs. 7, 8, l.c.n.v.) which are responsible for 

 the denser structure of the organ as compared with the rest of the liver. These denser 

 cells extend almost to the tip of the tubule, which is made up of cells of a very different 



