340 DISCOVERY REPORTS 



Examination of a specimen of H. typiis, preserved in formalin, from the Dana 

 Expedition, has also revealed the presence there of similar photophores, arranged in a 

 curved row of five near the ventral margin of the carapace. Presumably the photophores 

 have in life a similar coloration to those described by Dr Kemp. 



The carapace organs of the above-mentioned three species are all very closely similar 

 in structure, and discussion of the significance of their pigmentation is postponed until 

 this structure has been described. The photophores have been studied both from whole 

 mounts of the carapace and from serial sections, and their similarity makes it possible 

 to give a basic description of their composition, the slight differences between them 

 being pointed out in proceeding. 



The organization of the photophores is best appreciated from the figures of the 

 transverse section of the branchiostegite of//, novae-zealandiae (Fig. 17 and Plate XXV, 

 fig. 2). The robust chitin of the outer surface of the branchiostegite is overlain by a 

 well-developed cuticle (Fig. 17, cut.), and in the neighbourhood of the photophore it is 

 thickened to form a distinct lens (Fig. 17, /.) which is double convex in section. In 

 surface view the lens is semilunar (Fig. 18, /.). It is not of uniform constitution, for a 

 middle layer (on which the guide-line /. in Fig. 17 terminates) is more heavily striated 

 and deeply stained than those adjacent to it, and presumably it has different optical 

 qualities. The position of the middle layer within the thickness of the chitin is somewhat 

 variable ; in some of the photophores examined it was centrally situated, while in others 

 it occupied a position between the centre and the surface of the chitin. But whatever 

 the precise disposition of the middle layer of the lens its presence must exert some 

 influence on the optical qualities of the lens as a whole. 



Underlying the integument of the outer surface of the branchiostegite is seen the 

 chitogenous epithelium, from localized areas of which the photophores appear to be 

 differentiated. Over the greater part of the branchiostegite the epithelium consists of 

 flattened densely staining nuclei (Fig. 17 and Plate XXV, fig. 2, 7i.chit.ep.) surrounded 

 by ill-defined cytoplasm, resting on a basement membrane. 



On the inner side of the area occupied by the lens, however, well-differentiated 

 structures appear. The most conspicuous of these are the pyriform bodies noted in 

 surface view by Dr Kemp. They are with little doubt the photogenic cells, and show 

 strong similarities to the photogenic cells of the pleopods (Fig. i o and Plate XXV, fig. i ), 

 having a portion of the cell entirely devoid of cytoplasm (Fig. 17 and Plate XXV, fig. 2, 

 phot.c.cl.a.). The dense distally disposed nucleus lies close against the chitin of the 

 branchiostegal wall, and its inner surface is smoothly concave. Owing to its breadth 

 being somewhat greater than that of the clear area of the cell the nucleus has the 

 appearance of constituting a massive distal cap to the cell. The distal position of the 

 nucleus corresponds with that seen in the pleopod photogenic cells, but unlike these 

 the carapace photophores show no distal cytoplasm in their photogenic cells. It will be 

 recalled that the pleopod photogenic cells terminate proximally on the base of a conical 

 granular zone (Fig. 10, gr.z.) which is segmented in conformity with the number of 

 photogenic cells and is intimately concerned with the innervation of the photophore. 



