HISTOEY OF THE ORMER. 229 



The special difficulty which such an animal would 

 encounter on the intertidal shore is its liability to be knocked 

 off the rock over which it creeps by waves and tide wash, by 

 stones, or by enemies. 



Among the limpet -like descendants of this ancestor, the 

 shell rim has undergone increased downward growth, so that it 

 has come to cover the whole animal. When the limpet feels 

 any shock, it therefore pulls its shell down, by means of the 

 muscle already spoken of, until the shell rim is pressed against 

 the surface on which the animal is living, and it then " holds 

 on " with its body thus completely sheltered. The muscles 

 which pull the shell down have extended all around the animal 

 except, naturally, above the head, and so has come into exist- 

 ence the well-known horse-shoe. The spiral curl of the shell 

 has broken off and disappeared among these forms, except at a 

 very young stage ; and we can easily understand that such a 

 round knob at the top of the shell would have given too much 

 purchase to the waves. 



Among he Top-shells ( Trochus), the spirally curled shell 

 has also grown, but this time mainly in length, so that the 

 animal can pull itself back completely. When thus pulled 

 back, the posterior portion of the foot remains nearest the shell 

 mouth, and on its exposed surface the skin has developed a 

 horny protection — the operculum. So retracted and protected 

 the animal can be buffeted about by the waves without running 

 great risks. 



The slit and the keyhole limpets (yFissurella, &c.) and the 

 ormer seem on the other hand to have become confined to nooks 

 beneath boulders and chinks in the under side of rocks, where 

 this danger of detachment is less felt ; but this habitat has led 

 to the survival of other modifications. The spaces were often 

 narrow and a high hump and shell such as that of the supposed 

 ancestral form would have been a great embarrassment. In 

 the ormer, therefore, variations of its position have survived, 

 so that this spiral has laid itself down on its left side — why 

 the left side will be discussed later. In this way height was 

 reduced and the shell was modified into a covering plate, 

 the remains of the spiral curl surviving at the back. 



Other difficulties of the ancestral Gastropod were con- 

 nected with the breathing process — the outgoing current 

 interfered with the incoming ones on either side, it w<as difficult 

 to breathe when the tide was away, and sand and debris tended 

 to soil and damage the gills. The variations w r hich have 

 accumulated in the different types and helped to overcome 

 these difficulties have been very numerous, and we can only 



