484 THE AQUARIAN NATURALIST. 



traction of the whole organ ; and in the latter, by 

 muscles bending a joint. In a few rare instances 

 the movement is retrograde : the animal plants the 

 point of its foot against the clay or mud in the imme- 

 diate vicinity of the opening of its shell, and then, by 

 elongating the organ, pushes itself backwards ; in the 

 same way that a sailor shoves off a boat, by leaning 

 against the oar which he has planted in the sand. 



When the bend in the foot is considerable, forming 

 a sort of elbow, as is the case in the Cockle for example, 

 the creature is projected forward by a succession of 

 short leaps ; it stretches out the leg as far as possible, 

 and by a sudden movement, similar to that of a 

 spring let loose, is enabled to skip about with viva- 

 city. In fine, so varied and multifarious are the per- 

 formances of this versatile instrument, that, although 

 a slight shiver creeps over us at the recollection, we 

 still look back with pleasurable reminiscences at our 

 visit to the Bay of Luce in company with the gallant 

 discoverer of the North Magnetic Pole, on which 

 occasion we for the first time had an opportunity of 

 studying them to advantage. 



