Because they are destitute of skull and vertebral column, for the protection of the brain and the spinal marrow.
Because these appendices may supply the place of bones, and serve as a protection to the viscera, and as supports to the muscles. The blood, in those cases where a circulating fluid can be detected, is usually of a white or grey colour, seldom inclining to red.
Because they are soft, (from molluscus, Latin) and have no skeleton. Their muscles are attached to their skin, and their nervous system is irregular in its form and distribution.
Because they have the body divided into joints or rings, (from annulus, a ring) and, they either possess articulated feet or have certain processes which supply their place.
Because they alternately contract and relax the foot, or expand their muscles, which serve as suckers, and make their motion analogous to that of serpents.
Because they make serpentine undulations of the foot and body, or exert tentacula, or expanded portions of the integuments. Many species rise or sink in the water by aid of an organ somewhat resembling the
* Arranged in Six Orders: - 1. Intestina - 2. Mollusca - 3. Testacea - 4. Crustacea - 5. Corallia - 6. Zoopuyta.
air-bladders of fishes. Others keep, or shift their position by a sudden jerk, produced by shifting the valves of the shell rapidly. The common scallop and the river mussel have the latter properties.
Because they may fix themselves more securely; the sucker acting in the same manner as the moistened circular piece of leather, with a cord fixed to its centre, and applied to the surface of a stone. In the limpet, 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 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 breast of the lump-fish. Neither 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 extent of its surface, but the strength of the muscles which produce the vacuum, these animals can remain in the same spot, although acted on by forces to which their own 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. - Fleming.
Because they have not a heart,or system of vessels.