Gastropod Foot and Branchial Cavity. 515 



re-attaches itself by means o£ the back end of the foot, the 

 contact of which with the substratum starts the wave-system. 

 This might possibly be correlated with the presence in the 

 foot of Haliotis of longitudinal ganglionated pedal cords, 

 which are absent from that of Helix, the nerve-centres in 

 which type take the form of a pair of pedal ganglia in the 

 anterior part of the foot. 



2. Adhesion at Rest. 



When at rest, Patella, Haliotis, Trochus, Cyprcea, Palu- 

 dina, and often Littorina, adhere to the substratum by the 

 creeping-surface of the foot ; the adhesion is between two 

 moist surfaces — the substratum, moist with water or mucus, 

 and the foot-sole, which is covered with mucus secreted by the 

 sub-epithelial gland-cells. The chief muscle-fibres brought 

 into play in adhesion are : those having a direction vertical 

 to the creeping-surface, which compress the foot from above 

 downwards, and the transverse and longitudinal fibres, which 

 by their contraction bring the greatest possible number of 

 perpendicular fibres above a certain area of the substratum, 

 producing firm adhesion. The amount of reduction of the 

 creeping-surface by the contraction of the transverse and 

 longitudinal muscle-fibres varies among the different genera, 

 and is correlated with the shape and relative size of the shell, 

 since it is by means of the shell that the exposed parts of the 

 body must be protected when the animal is at rest. 



Patella has a very broad oval foot, which does not extend 

 beyond the cap-shaped shell during creeping, so that it shows 

 very little contraction of the creeping-surface during adhesion. 

 In the case of Haliotis there is more contraction, but the 

 shape of the outline of the foot is little altered. 



When Trochus is creeping the outline of the foot is long 

 and narrow, but on coming to rest the creeping-surface is 

 reduced and becomes oval in outline (and in Littorina litorea 

 and L. obtusata). 



The foot of Cypra?a europaia is very long and narrow when 

 creeping, and has a straight anterior margin and a somewhat 

 pointed posterior one ; but when adhering, the outline of the 

 creeping-surface is broader and oval in shape and is about 

 half of the length to which it attains when creeping. 



Paludina has a broad oval foot, with a straight anterior 

 margin, and the creeping-surface is little reduced in adhesion. 



The species of Littorina which live high up the shore — 

 L. obtusata, L. rudis, and L.neritoides — show a specialization 

 in their method of adhesion, as an adaptation to their high- 



35* 



