192 



THE SEAS 



on to the most highly evolved of the molluscs, the group 

 which includes the octopus and cuttlefish, we find that 

 luminous organs are both common and in the highest degree 

 elaborate, especially in the squids which swim near the 



surface and sometimes in 

 those from deeper water 

 (Plate 74) . The octopus and 

 its immediate allies are never 

 luminescent. Though occa- 

 sionally the light organs are 

 simple, they usually consist 

 of a light-producing area 

 backed by reflectors and a 

 protecting coat of pigment,- 

 very similar to those we have 

 already described for the 

 crustaceans, the light being 

 focused by a lens and the 

 whole protected by a cornea, 

 just as in the eye. Some 

 of these cuttlefish from the 

 deep sea have over twenty 

 light organs in various parts 

 of the body ; most of these 

 throw a white light but a 

 few are deep blue, two near 

 the eyes are usually sky- 

 blue while two organs on 

 the under end of the body — 

 like rear lights — are most ap- 

 propriately red. The different 

 colours are probably the result of coloured screens in front 

 of the light organs. 



The interesting Tunicate named Pyrosoma, a glass-like 



Fig. 41. — The Piddock, Pholas, a rock- 

 boring bivalve, showing phosphorescent 

 regions as seen in the dark (Nat. Size). 



