PHOTOPHORES OF DECAPOD CRUSTACEA 



355 



mainly by the thickening of the chitin above the photogenic cells. As seen in Fig. 25 

 the chitin {chit.) in this region is about 60// in thickness, whereas elsewhere it is only 

 about 15/i thick. In transverse section the thickened band has the form of a plano- 

 convex lens, and is seen to be made up of two layers. It is overlain by the non-striated 

 cuticle (Fig. 25, cut.). 



The photogenic cells lie immediately beneath the integument, and are disposed for 

 the most part in a double or a treble row. They exhibit the now familiar clear area, 

 devoid of cytoplasmic contents (Fig. 25, phot.c.cl.a.) and are as usual so orientated that 

 their nuclei are situated at their distal ends. The nuclei are extremely dense and their 

 chromatin appears to consist of closely packed granules. No trace of cytoplasm is 

 apparent either distal to the nuclei or anywhere else in the cells. The shape of the nuclei 

 strongly recalls that seen in the corresponding nuclei of the carapace photophores of 

 Hoplophoriis (p. 340), their distal surfaces being smoothly and convexly rounded, while 

 their proximal surfaces, facing the clear areas of the cells, are markedly concave. Each 

 nucleus has the form of a thick concavo-convex lens. 



Proximally prominent granular masses (Fig. 25, pr.gr. m.), of precisely the same 

 material and form as those seen in the uropod photophores, occupy the bases of the 

 cells, and so constitute a massive reflecting layer behind them. Again the layer is 



f 00 jj 





/idem. ^str. 



phof.cda. 

 prgr.m nphotc. 



Fig. 25. Part of a section passing vertically through the length of the transverse photogenic streak behind 

 the last thoracic limb of Systellaspis affinis. Mallory's triple stain, b.l. basal layer; b.c. blood channel; 

 chit, chitin; cut. cuticle; haem. haemocoele; m.str. muscle strand; n.b.l. nucleus of basal layer; n.phot.c. 

 nucleus of photogenic cell; phot.c.b.c. basal cap of photogenic cell; phot.c.cl.a. clear area of photogenic cell; 

 r.l. reflector hyer ; pr.gr.m. proximal granular mass. 



