'Tusk-shells, Mail-shells, and Limpets 189 



plates. There is reason to believe that the primitive 

 Limpet was not endowed with this extensive frill, 

 which is a more recent development to fit the species 

 for changed habits. In the Tortoiseshell Limpets the 

 breathing organ is a single plume-like gill attached to 

 the right side of the neck, and protruded when the 

 creature goes a-walking. It is believed that the 

 original Limpet had a pair of these plumes, one on 

 each side ; and if the modern Limpet be taken from its 

 shell, and the mantle cut away so as to expose the 

 upper side of the neck, a little yellow body will be 

 found on each side of it. These are the remains of 

 the true gills, which have gradually dwindled away 

 to useless vestiges as the new and — from the func- 

 tional point of view — more important frill has 

 developed. It is probable that, like Acr}uea, the 

 primitive Limpet lived constantly in the water, and 

 consequently the work of the two small gills ever 

 exposed to the water was sufficient to keep the blood 

 ox3^genated ; but now the Limpet spends the hours 

 between the fall and rise of the tide entirely out 

 of water, and can only manage to keep its gills 

 sufficiently moist to carry on the process slowly. It 

 therefore seems probable that, in order to keep the 

 blood pure, the slower respiration must be maintained 

 over a much larger surface. The change of habitat 

 and the development of these pseudo-gills might go 

 on ixiri 2:)assii\ early Limpets with merely the 

 beginnings of the frill restricting themselves to those 

 lower rocks that are only uncovered for a short time. 

 But this is not the only change produced in the. 

 Limpet by its altered habits. As it has learned to 

 endure fresh water by its exposure to rains and to 



