400 THE KEYHOLE LIMPET. 



colour to paper, and may be conveniently used as a blue ink ; several memoranda and 

 pages of my journal, written with this fluid, have, after a lapse of more than five years, 

 retained their original appearance both in colour and intenseness. For this use, however, 

 it must be employed from the recent animal, as it will not keep in any quantity, but 

 becomes thin and discoloured. 



It is believed that this fluid is analogous in use to the black secretion which the 

 cuttle-fish pours forth to obscure the water and elude the pursuit of its enemies ; but 

 this opinion must be received with some qualification. The living examples of Janthina 

 which I have irritated when they have been confined in a vessel containing sea-water 

 have not emitted any of the coloured fluid ; when taken in hand, they would sometimes 

 allow a little to exude ; but the entire quantity obtained from one animal by artificial 

 means was never sufficient to cloud or obscure, although it would stain about half a pint 

 of pure water." 



When attached to the living animal, the shell is covered with a delicate and rather 

 slippery membrane, and the violet hue possesses a peculiar liveliness, which soon vanishes 

 after the death of the inhabitant. The raft is delicate white. Its length is about one 

 inch, and the shell is very similar in shape and dimensions to that of our common 

 garden-snail. 



THE well-known univalves, so familiar under the name of Limpets, are divided into 

 several families, on account of certain variations in the structure of the shell. The first 

 family is termed Fissurellidse, on account of the fissure which appears either at the apex 

 cr in the front edge of the shell. 



All the Limpets are strongly adhesive to rocks, as is well known by every one who 

 has tried to remove one of these molluscs from the stony surface to which they cling. 

 The means by which the animal is able to attach itself with such firmness is analogous 

 to the mode in which the suckers of the cuttle-fish adh^e to the objects which they 

 seize, the formation of a vacuum, and the consequent pressure of the atmosphere, being 

 the means employed. The foot cf the Limpet is rounded, broad, thick, and powerful ; and 

 when the animal wishes to cling tightly to any substance, it presses the foot firmly upon 

 the surface, and retracts its centre, while its edges remain affixed to the rock. A partial 

 vacuum is therefore formed, and the creature becomes as strongly attached to the rock as 

 a boy's leathern sucker to the stone on which he has pressed it. 



The Limpets, although inactive and comparatively stationary, are able to glide slowly 

 by means of the foot, and leave very curious tracks in their progress. In the course 

 of long adhesion to a single spot, the animal works a slight hollow, into which the edges 

 of the shell sink when the creature is alarmed and presses itself to the rock, and thus 

 protects itself very effectually from being dislodged by the insertion of an animate or 

 inanimate lever under the shell. Several sea birds, and one or two of the land birds, are 

 m the habit of feeding on Limpets, which they dislodge by the simple process of driving 

 their sharp beak between the shell and the rock before the mollusc takes the alarm and 

 presses itself against the stone to be moved. When the Limpet is quite at ease, it has a 

 custom of relaxing the foot and raising the shell from the rock, and at such times it may 

 be detached even by a quick stroke with the finger. 



Most, if not all, of the Limpets are edible, and can be eaten either cooked or in a fresh 

 state as they come off the rocks. Shipwrecked mariners have frequently saved themselves 

 from perishing with hunger by gathering the Limpets from the rocks on which the vessel 

 was cast away ; and I have made many a luncheon on these creatures when engaged in 

 searching the shores, and when time has been too precious to be wasted in leaving the 

 spot for refreshment. An empty shell of the mussel forms an admirable spoon for the 

 extemporized banquet. 



The KEYHOLE LIMPET is so called on account of the aperture at the top of the shell, 

 which serves as a passage through which is expelled the water that has passed over the 

 gills. This aperture is found in all the species of the genus Fissurrella, but varies greatly 

 in form and comparative dimensions, being, in some cases, a mere rounded, hole in the 



