THE GILLS. 51 



area of rock, which we may readily do, notwithstanding 

 the strength of his cement, if we take him at unawares, 

 and give him a smart sudden horizontal rap with a 

 piece of wood, or a moderated blow with a hammer, 

 we shall obtain a view of a structure well worth looking 

 at. The animal is essentially like a Trochus or a Purple 

 inhabiting a conical shell ; only in this case the cone 

 is low and simple, whereas in the others it is tall and 

 slender, and rolled into a spire. One of the most 

 curious peculiarities in the Limpet is its gill or breathing 

 organ. This, we perceive, completely encircles the 

 animal, forming a ring interrupted only at one point, 

 It lies in the fold between the mantle and the foot, 

 commencing on the left side of the neck, and passing 

 quite round the body, parallel with the edge of the 

 shell, in front of the head, till it terminates close to the 

 point where it began. It is a long cord closely beset 

 with tiny leaflets, and thus forming a continual plume. 

 Each leaflet, conical in outline, is permeated with blood- 

 vessels, and clothed with minute cilia, whose constant 

 vibrations cause the circumambient water ever to play 

 over the surface of these organs in ceaseless currents, 

 bringing fresh supplies of oxygen to be respired ; and 

 this is absorbed by the blood through the thin membrane 

 by which they are protected. 



There is a very pretty little shell, not uncommon in 



