PHOLAS. 107 



nic acid set free during the respiratory process. Evidence 

 of a secreted solvent there is none. 



PJiolades are often described as occurring in sand, in in- 

 stances where their true habitat is in clays or soft rock, as 

 lias or pleistocene marl, immediately beneath the sand. 

 This was first noticed, in the case of Pholas Candida and 

 P. dactylus^ by Audouin and Milne Edwards.* The wood 

 and peat in which they occur not unfrequently on the 

 British shores, are usually the remains of submerged 

 forests. 



A remarkable property of the animals of this genus, and 

 one which has long attracted notice, is their phosphorescence 

 when placed in the dark. This phenomenon is exhibited 

 by some other acephalous mollusks, and by the compound 

 tunicated genus Pyrosonia. The light is of a bluish-white 

 ime, and is regarded by Mayen to proceed from a luminous 

 mucous, like that given off by the Medusa. This mucus 

 is thrown off into the surrounding water, so that the cur- 

 rents proceeding from the animal are luminous. Dr. Cold- 

 stream states -f- that the light is given out most strongly by 

 the internal surfaces of the respiratory tubes, and that it is 

 strongest in summer. Professor John M tiller has observed 

 that when PJiolades are placed in a vacuum the light dis- 

 appears, but re-appears on the admission of air ; also, that 

 when dried, they recover their luminous property on being 

 rubbed or moistened. 



All our PJiolades might be used as articles of food, 

 though we are not aware of their being eaten in this 

 country. A very large West Indian species, the PJiolas 

 costata, is much prized as such, and is regularly sold in the 

 markets of Havannah. 



* Hist. Nat. du Littoral de France, t. i. p, 233. 



t Cyclopaedia of Anatomy and Physiology. — Art. Luminous Animals. 



