STRUCTURE OF MUSCLES. 129 



3. Suction. The third metliod of fixing themselves, 

 employed by animals, is suction. The sucker, which acts 

 in the same manner as the moistened circular piece of lea- 

 ther, with a cord fixed to its centre, and applied to the sur- 

 face of a stone, known to every school-boy, varies greatly in 

 its form, and even structure. In the limpet, and other gas- 

 teropodous mollusca, its surface is smooth and uniform ; 

 and the adhesion appears to depend on its close application 

 to every part of the opposing surface. In other animals, 

 as the leech and the sea-urchin, the sucker is formed at 

 the extremity of a tube ; the muscular motions of which 

 may serve to pump out any air which may remain, after 

 the organ has been applied to the surface of the body. In 

 a third class, the sucker is more complicated in its structure, 

 consisting of many smaller ones,, so disposed as to act 

 in concert, as on the head of the Remora, or the breast 

 of the lump-fish. Among the vertebral animals, nei- 

 ther quadrupeds nor birds possess any sucker. It is 

 found among a few reptiles and fishes. The extremities of 

 the toes of many insects possess complicated suckers. Among 

 the mollusca and zoophytes, there are few in which suckers 

 in some form do not exist. By means of this organ, 

 whose power of cohesion must depend, not only on the ex- 

 tent of its surface, but the strength of the muscles which 

 produce the vacuum, these animals can remain m the same 

 spot, although acted on by forces to which their owfi weight 

 could offer no adequate resistance. PENNANT states, that 

 he heard of a lamprey which was taken out of the Esk, 

 weighing three pounds, adhering to a stone of twelve pounds 

 weight suspended at its mouth *. 



4. Cementation. The fourth method, termed cementa- 

 tion, employed by animals to preserve themselves stationary, 



* Brit. Zool. iii, p. 78. 

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