204 



THE SEA. 



the latter that of the edges of successive growths of the shell. These points have the 

 most accurate resemblance to those set on a steel rasp in a blacksmith's shop. It is 

 interesting to know that the shell is preserved from being itself permanently worn away 



by the fact that it is com- 

 posed of arragonite, a sub- 

 stance much harder than those 

 in which the Pholas burrows. 

 Yet we see by comparing 

 specimens one with another, 

 that such a destructive action 

 does in time take place, for 

 some have the rasping points 

 much more worn than others, 

 many of the older ones being 

 nearly smooth. 



The animal turns in its 

 burrow from side to side when 

 at woi'k, adhering to the in- 

 terior by the foot, and there- 

 fore only partially rotating 

 to and fro. The substance 

 is abraded in the form of fine 

 powder, which is periodically 

 ejected from the mouth of the 



hole by the contraction of the branchial siphon, a good deal of the more unpalpable portions 

 being deposited by the current as it proceeds, and lodging as a soft mud between the valves 

 and the stone. Mr. Hudson, who watched some Pholades at work in a tide-pool in the 

 chalk, observed the periodic ejection of the cloud of chalk powder, and noticed the heaps 

 of the same material deposited about the mouth of each burrow. The discharges were 

 made with no regularity as to time. Mrs. Merrifield records a curious fact : " A ladv 

 watching the operations of some Pholades which were at work in a basin of sea-water, 

 perceived that two of them were boring at such an angle that their 

 tunnels would meet. Curious to ascertain what they would do in 

 this case, she continued her observations, and found that the larger 

 and stronger Pholas bored straight through the weaker one, as if it 

 had been merely a piece of chalk rock." 



" What," says Mr. Gosse, " is that object that lies on yonder 

 stretch of sand, over which the shallow water ripples, washing the 

 sand around it and presently leaving it dry ? It looks like a stone ; 



but there is a fine scarlet knob on it, which all of a sudden has disappeared. Let 

 us watch the movement of the receding wave, and run out to it. It is a fine example 

 of the great spinous cockle (Cardinm rusticnm] for which all these sandy beaches that 

 form the bottom of the great sea-bed of Torbay are celebrated. Indeed, the species 



PHOLAPES IX A BLOCK OP GXEISS. 



SPixors COCKLE. 

 (Cardivm edule.) 



