190 



THE SEAS 



highly developed powers of light production, definite light 

 organs, some of them very simple but others of great com- 

 plexity, being found in many of these animals. In some 

 of the simplest cases, luminous slime is produced by little 



glands above the mouth, the 

 substance discharged glowing 

 with a yellow light. A few 

 of the ubiquitous Copepods 

 of the plankton are lumines- 

 cent, when disturbed they 

 throw off a cloud of lumines- 

 cence produced by glands 

 spread over the surface of 

 the body. Some prawns 

 and the shrimp-like Euphau- 

 siids — comparatively large 

 planktonic crustaceans which 

 furnish valuable food for the 

 herring — have very complex 

 light organs, so intricate 

 indeed that they were 

 originally thought to be 

 additional eyes ! There are 

 usually ten light organs, a 

 pair behind the eyes, two 

 pairs on the side of the body 

 and four on the under side 

 of the tail, and each of these 

 consists of a layer of light- 

 producing cells, behind which 

 is a reflector backed by a 

 layer of pigment which prevents any of the light from being 

 wasted by passing into the body, while in front is a lens 

 which focuses the light. Into the light -producing area 



Fig. 39. — Chcetopterus variopedatus, 



a phosphorescent marine worm as seen 



in the dark (Nat. Size). 



