XXVIU 



THE VOYAGE OF H.M.S. CHALLENGEK. 



fishes ; and it is a remarkable fact that they are almost entirely absent in those which 

 have the muciferous system most developed; thus, neither the Macruridse nor the 

 Ophidiidaj possess specialised luminous organs, probably because the mucus, which is so 

 abundantly secreted, supplies a sufficient amount of luminosity. In the Berycidse and 

 GadidiB we find only isolated instances, which remind us of the praeoeular or rostral 

 orwan of certain Scopdi. On the other hand, the Ealosauri ])ossess a wide lateral-line 

 system with well difi"erentiated luminous organs superimposed on it. In each of the 

 families of Carangidse and Alepoccphalidse one species only is provided with them. In 

 the Pediculati luminous organs are of common occurrence and serve as lures to attract 

 other fishes ; they are most common in the Sternoptychidse, Scopelidse, and Stomiatidse, 

 in which they have principally the function of enabling these fishes to illuminate their 

 surroundings, more rarely of alluring other fishes. In the MurgenidEe no luminous organs 

 have been found ; but in some of them the muciferous system is enlarged. 



The luminous organs present many modifications as regards their seat, appearance and 

 structure. 



' 1. In their most primitive condition they appear as innumerable minute tubercles 

 more or less raised above the surface of the skin, and covering the sides of the body ; 

 they are crowded together in transverse bands corresponding to the segments of the 

 muscular system. This form occurs in Echiostoma, Opostomias, Pachystomias, Photo- 

 nectes, Malacosteu's. I suppose that the pores scattered over the skin of some species of 

 Ceratias (Ceratias uranoscopus, Ceratias canmctdatiis) are apertures of follicles in which 

 luminous mucus is secreted. 



2. Larger in size, less numerous and more projecting beyond the surface, are the 

 small nodules in the skin of Xenodermichthys ; they are distributed over the head as 

 well as the body, and follow, on the former, the muciferous ducts, whilst they are 

 arranged in a quincuncial fashion on the Ijody, and are wanting along the tract of the 

 lateral line. 



3. More difierentiated are the eye-like spots, of a white colour in preserved specimens, 

 and red or green during life, which are arranged at regular intervals in two series on the 

 lower side of the body of the fish, and which occur also on the head, at the base of the 

 branchiostegals and on the gill-cover. Their arrangement is constant within the limits 

 of a species, and to judge from their external appearance they gradually pass into the 

 next form. They occur in numerous genera : — Gonostoma (some species), Chauliodus, 

 Astroncsthcs, Stoinms, Echiostoma, Opiostomias, Pachystomias, Photonectes, and 

 Idiacanthus. 



4. Still more difierentiated are large round fiat organs of a peculiar mother-of-pearl 

 brightness, arranged like the former in rows on the lower side of the body and head, with 

 isolated ones on the sides and on the opercles, and frequently with the addition of a 

 .short dorsal and ventral series on the peduncle of the tail. ^ They are found in the 



