HISTORY OF THE ORMER. 231 



mantle though it has been modified in the shell. The edge of 

 this mantle along the slit has developed three tentacles which 

 can project through three of the holes in the shell, and so help 

 the animal towards an impression of what is immediately above 

 or outside the shell. When they are irritated the animal with- 

 draws them, and can also bring together the two edges of the 

 slit in the mantle flap, so as to completely close the gill-cavity 

 in this direction. The front edge of the mantle near the slit 

 has grown both in extent and in thickness, and when contracted 

 it can almost close the anterior opening of the gill-cavity, so 

 that the latter can be practically completely protected from 

 unfavourable circumstances. 



We have already seen how the ormer gains an acquaint- 

 ance with what is immediately above its shell, but so important 

 to it is this kind of impression (for avoiding blows and falls 

 and culs-de-sac) that tentacles have developed on a frill which 

 projects all around the shell edge, and grows from the upper 

 part of the foot. Troclius also has this frill, but in its case the 

 tentacles are few in number and comparatively long ; they can 

 easily be seen when the animal is watched creeping under 

 water. Besides those tentacles which project through the holes 

 in the shell and those just described there are, as in all Gastro- 

 pods, the long head tentacles, and, behind them, the eyes which 

 have grown out on stalks, so that the animal can project them 

 beyond the shell edge. The ormer is therefore particularly 

 well provided with sensory arrangements for giving it an 

 impression of its immediate surroundings. 



It has already been said that the limpet relies for safety 

 on being able, at any moment, to secure intimate contact 

 between the edge of the shell and the surface on which it is 

 creeping or resting, and it has also been explained that the 

 muscle fibres making up the mass of its foot go up to be 

 inserted against the shell in the well-known marginal 

 horseshoe area. By contracting these the animal holds the 

 shell down much more tightly than would be possible with, 

 for example, a centrally placed muscle. 



The ormer on the other hand creeps a great deal about the 

 under surface of rocks, and protects itself by holding on to such 

 firm surface as may be available. It holds on by intimate con- 

 tact between the foot-sole and the rock surface, and as often 

 only a portion of the foot-sole is in such contact, so must every 

 fibre in the foot go as straight as possible into the shell, and be 

 fixed into the latter as near as possible to the animal's centre 

 of gravity, to reduce the detaching influence of the animal's 

 weight. In other, and perhaps more correct terms, these fibres 



