232 HISTORY OP THE OKMEE. 



must go through the centre of gravity, for otherwise they could 

 not adequately cope with the opposite force due to the animal's 

 weight which acts at that point. We therefore find that in the 

 past history of the ormer variations have been selected and 

 have accumulated, with the result that the formerly right shell 

 muscle has shifted to a central position and has become much 

 enlarged. It is the right muscle that has so moved, because 

 the left one was already being reduced, as was explained 

 further above. It is to be noted that the very much reduced 

 left shell muscle has acquired a new and different use ; by its 

 contraction it helps to close the front opening of the gill-cavity, 

 an operation already described as of great importance to the 

 animal. 



It often happens that the ormer, in creeping about 

 over loose seaweeds and irregularities of the rock surface, loses 

 its hold and falls. The weight of the shell, and the fact that 

 this is usually downwards when it is creeping, make the animal 

 fall with the shell underneath, and the foot, therefore, in the 

 air. A limpet under such circumstances is comparatively help- 

 less, so specialised has it become to the attachment habit. The 

 ormer, however, is not so specialised, and has a characteristic 

 method of righting itself. The common Gastropod ancestor 

 had a long creeping foot (like that of a Trochus or a snail 

 perhaps) which was pointed behind, making a kind of tail 

 process. The ormer, to some extent, retains this tail, notwith- 

 standing the broadening which has occurred. When now it 

 has fallen and lies shell downwards on the pebbles or grit, it 

 extends this tail process of the foot and moves it about actively, 

 bending it round the shell's edge until it finds a firm surface. 

 Then it affixes the sole of this tail region to that surface, and 

 contracts the muscle fibres which stretch between that and the 

 centre of the shell. In this way the shell is raised a little, and 

 so a larger area of the foot-sole can be fixed to the firm surface. 

 Then these newly-affixed fibres contract, and so the process is 

 repeated till the shell has been raised just past the vertical, and 

 it falls of itself, and then the animal is once more able to creep 

 about as usual. 



In this process of feeling for a firm surface the upper side 

 of the foot must continually rub against the shell edge, and on 

 this rubbed part special gland tissue has developed, whose 

 secretion acts as a lubricant and reduces wear and tear. At 

 least this seems to be a likely explanation of a structure that 

 has long puzzled anatomists. The gland tissue can easily be 

 seen on a fresh animal as a series of brownish lines on either side 

 of the middle of the upper surface of this tail process of the foot. 



