408 RAMBLES OP A NATUEALIST. [Ch. XXIII. 



pyris (the glow-worm), in which nervous centres exist, there 

 is a special organ for the development of light, doubtless 

 regulated by some part of the nervous system. In others, 

 the contractility of muscular tissue or of sarcode substance, 

 which contractility is itself a vital act, seems sufficient to 

 produce the phenomenon in question. In animals which 

 have no definite nervous system, we can scarcely predicate 

 the existence of a wiU ; and, therefore, while the glow-worm 

 and many of the higher marine luminous animals probably 

 exercise a control over the functions of their light- giving 

 organs, — ia the Noctilucse, and such lowly organisms, any 

 external excitement which produces a temporary contrac- 

 tion, is at the same time sufficient to exhibit its correlative 

 accompaniment, light. 



There does certainly appear to be a phase of luminous- 

 ness which is scarcely of this character, and which takes 

 place both on land and in water. I refer to a luminous 

 coat external to the animal, as in the case of the trail of 

 light left by the little Scolopendra electrica on the ground, 

 and the luminous mucus which exists in certain Medusae, 

 and, it is said, Pholads, or boring Mollusks, and which re- 

 tains its properties apart from the animals. But whatever 

 may be the explanation of these phenomena, there can be 

 no doubt that they are no more due to combustion or to 

 phosphorus than the appearances exhibited by the spe- 

 cialised organs of the glow-worm, but may more probably 

 belong to another group of facts, that, namely, which in- 

 cludes the luminous appearances presented by certain plants, 

 in which we cannot caU to our assistance either nerve force 

 or vital contractility. 



Organic lumiaousness exhibits itseK with a wonderful 

 range over the animal kingdom ; and if we were called 



