PHOLADID^. — PIDDOCK. 159 



saySj " Having procured several of these mollusks in 

 pieces of timber, I extracted one and placed it loose 

 in my aquarium, in the vague hope that it would per- 

 forate some sandstone on which I placed it. It pos- 

 sessed the powers of locomotion, but made no attempt 

 to bore. I then cut a piece of wood from the timber in 

 which it had been found, and placed the Pholas in a 

 hole a little more than an inch deep. Its shell being 

 about two inches long, this arrangement left about an 

 inch and three-quarters exposed. After a short time, 

 the animal attached its foot to the bottom of the hole, 

 and commenced swaying itself from side to side, until 

 the hole was sufficiently deep to allow it to proceed in 

 the following manner. It inflated itself with water ap- 

 parently to its fullest extent, raising its shell upwards 

 from the hole ; then, holding by its muscular foot, it 

 drew its shell gradually downwards. This would have 

 produced a perpendicular and very inefficient action, but 

 for a wise provision of nature. The edges of the valves 

 are not joined close together, but are connected by a 

 membrane; and, instead of being joined at the hinge, 

 like ordinary bivalves, they possess an extra plate 

 attached to each valve of the shell, which is necessary 

 for the following part of the operation. In the action 

 of boring, this mollusk, having expanded itself with 

 ■water, draws down its shell within the hole, gradually 

 closing the lower anterior edges, until they almost touch. 

 It then raises its shell upwards, gradually opening the 

 lower anterior edges and closing the upper, thus boring 

 both upwards and downwards. The spines (points) on 

 the shells are placed in rows, like the teeth of a saw ; 

 those toward the lower part being sharp and pointed, 

 vi'hiist those above, being useless, are not renewed. So 



