346 



DISCOVERY REPORTS 



OC. 



p. phot 



organs are not very clearly defined it is unwise to state a precise number until more 

 specimens have been carefully examined. On the other hand, however, some of the 

 organs, particularly those on the ocular 

 peduncles, the transverse streak behind 

 the bases of the last thoracic limbs, and 

 those on the inferior margin of the cara- 

 pace, stand out prominently as white 

 streaks and dots against the now dull 

 yellow of the integument. The ocular 

 peduncle photophores (Fig. 19, oc.p.phol.) 

 appear as an irregular, somewhat raised 

 white bar, placed on the inner ventral 

 face of the peduncle, and a pear-shaped 

 similar mass borne on a small ventro- 

 lateral tubercle. Any light produced by 

 these photophores would be shed mainly 

 downwards from the inner side of the 

 peduncle. 



The transverse streak at the bases of the 

 last pair of thoracic limbs (Plate XXVI, 



fig. 2, phot.t.s.) is also raised from the surrounding integument, and extends laterally, 

 curving upwards and somewhat forwards on the ventral side of the body. The right 

 and left streaks almost meet in the middle line. They thus occupy precisely the same 

 position as those in the species of Hoplophoriis (p. 337). 



The branchiostegal photophores (Plate XXVI, fig. 2, phot. cm.) consist, in this specimen 

 at any rate, of a long posterior streak, with fourteen short bars of varying length 

 extending from its anterior end to the anterior end of the lower margin of the carapace, 

 extending in fact to the base of the prominent branchiostegal spine. Although only 

 fourteen streaks are visible superficially, a cleared and mounted portion of the branchio- 

 stegite shows that between them occur numerous very small patches of similar tissue, 

 so that the inferior branchiostegal organs may almost be regarded as constituting a 

 potentially continuous band of photogenic tissue, strongly developed in localized 

 areas. 



The nature of the deposits of white opaque pigment in the photophores of S. affinis 

 will be considered in detail in the discussion at the end of this paper (p. 374). 



Fig. 19. The left ocular peduncle of Systellaspis affinis, 

 viewed from the side and slightly below, showing the 

 photophores in the form of somewhat raised opaque 

 white masses, x 140. oc.p.phot. ocular peduncle 

 photophores. 



(a) The pleopod photophores 



The pleopod photophores of Systellaspis affinis (Fig. 20, e.g. phot.pl. 5) are borne in 

 the manner which appears typical of the Hoplophoridae and which has already been 

 described in Hoplophoriis (p. 329). Here too they are overlapped by the pleura of the 

 abdominal somites, in which there is little doubt that a window-like zone free from 

 pigmentation occurs. (It must be admitted, however, that in a water-colour sketch of 



