8 . JANUARY. 



the arrangement of its muscles, it is separated into two 

 portions by a line which runs down the middle ; and 

 that these two sides move alternately. The muscles of 

 the right half, for example, are moved a little onward, 

 and take a fresh hold of the ground, while those of the 

 left remain clinging ; then the right half clings, while 

 the left relaxes and advances a little beyond the right, 

 and again clings, when the right makes its forward 

 move. So that the effect is exactly that of two feet 

 advancing by alternate steps ; and if your own two 

 feet were enclosed in one elastic stocking, your own 

 progress would appear very much like that of the 

 Trochus. Indeed, some shell-fishes not distantly allied 

 to this, as the pretty little Pheasant- shell, 1 whicli I 

 occasionally find among these rocks, really have the 

 foot divided into two distinct and separate halves, in 

 which this alternate motion is, of course, more obvious. 

 Looked at from above, we discern that this foot 

 thickens towards the middle, where it is overlapped 

 by a broad wing-like expansion on each side. This, 

 for manifest reasons, is known by the name of the 

 cloak or mantle. In all cases it performs important 

 offices in the economy of the animal, as I shall pre- 

 sently describe ; and in this instance we see it is 

 adorned along its edges with certain lappets and long 



1 Pliasianella pullus. 



