42 LIVING LIGHTS. 



in -warin water. Brandy extinguishes the light, and Galeatus 

 and Montius found that vinegar and wine produced the same 

 result. If the body of Pholas is heated slowly, the light 

 gradually becomes more and more intense, until, finally, at 

 45° Reaumur, or 56° Centigrade, it disappears, and cannot 

 be restored. 



The secure position of the Pholas in its impregnable 

 fortress would hardly seem to require a warning or attractive 

 light ; and its use must remain a mystery, though theory 

 could, of course, suggest explanations. 



While the Pholas conceals its luminosity in its dungeon, 

 there are other moUuscan light-givers which float about like 

 light-ships astray. These are Pteropods, or wing-footed mol- 

 lusks ; delicate fairy ships of marvellous beauty. By some 

 authorities they are said to represent the higher forms of the 

 Cephalophora, while others consider them as degenerate or 

 backsliding Cephalopods, of which the squids and octopi are 

 representatives. They are pelagic, free-swimming mollusks, 

 in which portions of the foot are modified into seeming 

 wings, so that the little creature seems to fly through the 

 water. They differ much in appearance. Some secrete a 

 glassy, horny, cartilaginous or limey shell, which in some 

 cases is only present in the larval forms, disappearing in the 

 adult ; while others, again, preserve it through their entire 

 lives. The body is of various shapes : it is protected by the 

 shell when present, and can be drawn into it. 



Though simple, helpless creatures, many have an arma- 

 ment which in a larger animal would be considered ex- 

 tremely effective. Thus in Clio each tentacle bears nearly 

 three thousand cylinders, each containing stalked suckers; 

 and, as there are six tentacles, the little animal can grasp its 



