156 CYCLOBRANCHIATA.—PATELLAD ZA. 
plain, and appeared varnished with a thin coating 
of some kind of fucus, without any markings upon 
its surface. As no patellee 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 lim- 
pets 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 re- 
sembling the anterior margin of the shells.” * 
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 produc- 
tion of a vacuum in the manner of a sucker. If an 
adhering limpet were cut quite through perpen- 
dicularly, 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 secre- 
tion, deposited at the will of the animal. “If, 
having detached a patella,’ says Dr. Johnston, 
* Mag. Nat. Hist. iv. 347. 
