THE MOLLUSCA 203 



I believe, been given, so the following experiment, tried by 

 myself, with the assistance of a friend, may not be without 

 interest. 



Small holes were drilled on each side of the shells of 

 many limpets, at Green Island, Jersey. A steel clip with 

 sliding bar was then attached, and a vertical pull made 

 with a spring balance, the latter having an adjustment to 

 register the maximum reach of the indicator. 



It was found that limpets with the base one inch and a 

 quarter by one inch that is, giving an area of somewhat 

 less than a square inch came off at a pull of seventy pounds \ 

 Larger and smaller examples at the same proportion 

 that is, that the force by which they hold is nearly five 

 times what would be the case if they held by suction 

 only. Whether the rock was smooth (water-worn basalt) 

 or somewhat rough (disintegrating granite) made no 

 difference whatever. 



There are no perceptible grasping arrangements which 

 might cling to minute pores in the stone, so it is a puzzle 

 how they do hold. It is possibly as glue holds to polished 

 glass a point in mechanics which has not yet been defined. 



The young of the limpet emerge, in the form of little 

 disc shaped, ciliated embryos, in the months of December 

 and January. They do not appear to swim freely, but 

 progress on the surface of the rock by the movements of 

 their cilia. 



There are several varieties, but the only well-marked one 

 is variety Athletica, one in which the shell is ridged from 

 apex to base. This is the one shown in the Figure. 



Helcion pellucidum is closely allied to the common 

 limpet, but is of smoother outline, and the shell is more 

 horny-looking, of pale reddish brown colour. It is found 

 among the roots of the oar- weeds (Laminaria) ; it feeds on 

 this plant, eating its way upwards into the stalk. 



In the summer months its young may be seen in numbers 



