PHOTOPHORES OF DECAPOD CRUSTACEA 351 



nerve cord immediately anterior to and independently of the nerves supplying the 

 uropods. 



The lens of the organ is only relatively feebly developed, and is in fact thinner than 

 many parts of the neighbouring chitin, from which it is mainly distinguished by its 

 possession of an inner and outer layer. The inner layer (Fig. 24, /./.) is more compact 

 and densely striated than the outer (Fig. 24, l.o.) and stains more deeply. As a whole 

 the lens is not sharply delimited from the surrounding integument. The reduction of 

 the lens is, I believe, closely correlated with the peculiar condition of the photogenic 

 cells, each of which possesses its own lens. 



The photogenic "cells" exhibit no structure which can be interpreted as a nucleus, 

 and for this reason I have preferred to refer to them as "photogenic units". They do 

 not show the pronounced radial disposition of the pleopod organs already discussed, 

 but lie almost parallel with each other beneath and perpendicular to the lens. Like the 

 photogenic cells of the pleopod organs the greater part of their volume is entirely 

 unoccupied by any cytoplasmic content (Figs. 23, 24, phot.u.cl.a.). Distally the nucleus 

 is replaced by an aggregation of structures, the most obvious of which is a lens-like 

 septum (Fig. 23, l.s.) extending across the photogenic unit at some distance from its 

 distal tip. This lens septum is plano-concave, its concavity facing the clear area of the 

 photogenic unit. It appears to be composed of a number of laminae, and stains an 

 intense red with Mallory's triple stain. That it is a relatively rigid and definite structure 

 is shown by the fact that in the process of sectioning it may be displaced bodily as seen 

 in Plate XXVI, fig. i. 



Immediately distal to the lens occurs a structureless zone (Fig. 23, st.z. i) staining a 

 uniform light Cambridge blue. The distal surface of this zone merges with the next 

 zone, which is highly vacuolated (Fig. 23, vac.z.). Its vacuoles are minute proximally, 

 but progressively increase in size towards the distal surface of the zone, where they 

 cease abruptly. Succeeding the vacuolated zone occurs a second structureless zone 

 (Fig. 23, st.z. 2), again assuming a blue coloration with Mallory, but not uniformly. 

 Staining is most intense at the proximal boundary, becoming progressively lighter 

 towards the distal tip of the photogenic unit, which is in addition finely granular 

 (Fig. 23, gr.t.). There are thus recognizable five zones, including the lens, at the distal 

 end of the photogenic unit. 



Proximally the units are in close association with a dense granular mass (Fig. 23, 

 pr.gr.m.), which is indifferent to any of the constituents of Mallory's triple stain, and 

 which appears golden brown by transmitted light but a pure opaque white by reflected 

 light. Similar granular substance appears in photophores in other parts of the body of 

 S. ajfinis (p. 356), although laid down according to a diff'erent pattern, and it is clearly 

 the presence of this substance which causes the organs to remain visible as white marks 

 after the loss of the original pigmentation. 



In the uropod photophores the proximal granular mass occupies about one-quarter 

 of the total length of the photogenic unit, and is densest at its proximal end. Fine striae 

 are discernible in some instances near its distal border (Fig. 23, str.l.). At first sight 



