LUMINESCENCE IN THE SEA 221 



was the naturalists of the " Challenger " expedition who 

 demonstrated that these were organs for the production, 

 not the reception, of Hght. 



The usual arrangement of these photospheres, as they 

 have been called, is — a pair on the head behind the true 

 eyes, two pairs on the sides of the thorax, and four 

 median ventral on the first four segments of the abdo- 

 men. 



In British seas, Meganyctiphanes norvegica (Plate XVI, 

 Fig. 2), is abundant in deep water off the western coasts, and 

 frequently comes to the surface in swarms at night. On 

 several occasions in the Hebrides, when we brought some 

 up in the deep tow-net, I have taken a few in a large 

 jar of sea- water into a darkened cabin and watched how, 

 on stimulation, they have lit up their little lamps and 

 sailed round and round the jar — a beautiful sight. Two 

 or three such, freshly caught, gave sufficient light to 

 enable one to read for a few seconds the newspaper on 

 which the jar was placed. In the case of aU these photo- 

 spheres of Meganyctiphanes and some alUed Crustacea, the 

 light is internal, and is produced in a closed organ in 

 which the oxygen necessary for the luminescence must be 

 obtained from the blood. The photosphere is always 

 well suppUed with blood sinuses and with nerves. It 

 has been suggested that the light may be of use to these 

 animals in enabling them to see their prey, or whatever lies 

 in front or below the head. 



MoLLUSCA. — I select two examples of luminescence from 

 this group of animals — first, the classic case of Pholas, the 

 bivalve that bores deep holes in stiff clay or in soft rocks 

 on the seashore, and in which Dubois first demonstrated 

 the presence of luciferine and lucif erase as the essential sub- 

 stances concerned in the production of light ; and secondly, 

 the Cephalopoda, or cuttle-fishes, in some of which compli- 

 cated closed light-organs are present on various parts of the 

 body. 



