PHOTOPHORES OF DECAPOD CRUSTACEA 331 



Behind the lens occur the radially arranged photogenic cells, which are lacking in 

 cytoplasm at their inner or proximal ends (Fig. 10 and Plate XXV, fig. i, phot.c.cl.a.). 

 The distal cytoplasmic portions (Fig. 10, cyt.phot.c.) never occupy more than a third 

 of the length of the cell, and are most extensive in those cells near the periphery of 

 the organ. The cytoplasm is regularly and finely granular, and shows only a slight 

 increase in density at the edges of the cell. No vacuoles or other structures have been 

 observed in the cytoplasm. The nuclei of the photogenic cells (Fig. 10, n.phot.c.) lie 

 at the proximal ends of the cytoplasmic portions of the cells, so that a cytoplasmic 

 zone lies immediately inside the lens between it and the zone occupied by the nuclei. 

 The nuclei show to some extent the gradation in size described by Kemp (iqioZj, 

 p. 644) in Systellaspis debilis, being regularly larger towards the periphery of the lens 

 than at its centre, and this, together with and mainly due to the variation in extent 

 of the cytoplasmic portions of the cells, results in a concave inner surface being 

 presented towards the vacuous zone of the photopore caused by the absence of cytoplasm 

 proximally. This arrangement of the nuclei is not so well defined here as in S. debilis, 

 for the nuclei are merely oval and the difl^erence in size is not great, whereas in 

 S. debilis the nuclei are squarely and abruptly truncated and show a marked difference in 

 size, but it must nevertheless exert some optical effect. That the characteristic shape of 

 the nuclei of S. debilis has some significance is indicated by the fact that it is again 

 encountered in the carapace organs of Hoplophorm (p. 339) and in photophores in other 

 parts of the body in Systellaspis affinis (p. 355) and S. debilis (p. 365). The nuclei of the 

 photogenic cells of the pleopod photophores of Hoplophorus riovae-zealandiae possess 

 one or two dense chromatin masses, the remainder of the nuclear contents being uni- 

 formly vacuolated (Fig. 10, n.phot.c). 



The slender proximal ends of the photogenic cells are in close association with a 

 conical mass of granular material (Fig. 10 and Plate XXV, fig. i, gr.z.), which has a 

 characteristic staining reaction. Unlike the granules which occupy the same position in 

 Systellaspis debilis (Kemp, 1910&, p. 644) they are readily stained with eosin. The apex 

 of the cone is directed towards the entrant nerve bundle (Fig. 10 and Plate XXV, fig. i, 

 phot.nv.), and the cone itself is seen to be made up of individual columns varying in 

 length and of a number corresponding with the number of photogenic cells. The distal 

 end of each column is markedly concave and receives the proximal end of the corre- 

 sponding photogenic cell, so that the column forms a proximal cap to the cell. The 

 whole cone is in close association with the entrant nerve fibres through an underlying 

 striated zone. 



The photophore nerve (Fig. 10 and Plate XXV, fig. i, phot.nv.) runs down the base of 

 the pleopod and expands somewhat on reaching the centre of the proximal surface of the 

 organ. Within this expansion lies a clearly defined striated zone (Fig. 10, stri.s.) which 

 stains more lightly than the nerve fibres, and is composed of very delicate fibrils or 

 striae regularly arranged. The central bundle of fibres in the photophore nerve comes 

 into immediate and direct communication with the apex of this cone of striae, but 

 the peripheral fibres on reaching the neighbourhood of the striated zone become 



