Page:Natural History, Mollusca.djvu/168

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156
CYCLOBRANCHIATA.—PATELLADÆ.

plain, and appeared varnished with a thin coating of some kind of fucus, without any markings upon its surface. As no patellæ were at first discovered, and the isolated situation of the rock prevented any from reaching it, I was at a loss to explain these appearances; but, after some search, a fissure was found at the north end, where five or six limpets had fixed themselves, each having a direct road leading to their pasturage-ground. By the help of a glass, the markings visible on the rock were discovered to be the remains of the above fucus, which had been eaten through or trodden down by these animals in their excursions, and which retained the indentures of their shells. The edge of the vegetable surface was then examined, and found to be nibbled in a circular manner resembling the anterior margin of the shells." [1]

The force with which a limpet adheres to the rock is very great, especially when it has had warning of assault, and has had time to put out its muscular strength. Réaumur found that a weight of twenty-eight or thirty pounds was required to overcome this adhesive force. His experiments seem to prove, however, that its power is mainly owing, not to muscular energy, nor to the production of a vacuum in the manner of a sucker. If an adhering limpet were cut quite through perpendicularly, shell and animal, the two parts maintained their hold with unabated force, although of course a vacuum, if there had been one, would have been destroyed by the incision. The power is said to reside in a very strong glue, a very viscid secretion, deposited at the will of the animal. " If, having detached a patella," says Dr. Johnston,

  1. Mag. Nat. Hist. iv. 347.