Chemically, snail mucus is a watery gel containing a small proportion less than 10 per cent of glycoprotein polymers — large, complex molecules that link together to give the slime its distinctive properties. Snail slime is secreted by glands located all over the body, though the largest, and that responsible for the silvery trails, is at the front of the foot. Yet this limitation doesn't appear to hold back snails and slugs.
A snail uses its single long, muscular foot to crawl on a layer of mucus-like slime that it secretes. This mucus has unusual physical properties, and scientists assumed that these sticky properties were essential for snail movement. But in a recent study, Stanford graduate student Janice Lai used high-resolution videos and lasers to study moving snails and slugs, and discovered that they don't really need their slime to be special. It was already known that snails and slugs propel themselves by generating a series of muscular pulses on their feet.
These waves of muscle contraction and relaxation travel along the central portion of the foot from tail to head. The waves move much faster than the snail itself, and generate enough force to push the snail forward.
Lai found out that these muscular waves are sufficient to propel the snail forward on a flat surface, without needing the special mucus to provide more traction. The mucus does help the snail stick to surfaces, however, and comes in handy when traveling up a wall or across a ceiling, upside down. Lai shot high-resolution videos of crawling snails and used a laser to measure how the muscle waves moved back and forth and up and down on a snail's foot.
She also measured the forces that a snail generated when crawling on a gel. Her detailed measurements of how snails move could help other research groups that work on snail-like robots. Researchers discovered nearly 30 years ago that snail mucus has some unusual properties. It allows the animal to stick to a surface while moving, with the mucus changing its characteristics according to how firmly the snail presses on it.
The slime initially acts like glue, sticking the snail to the surface. Slugs and snails rely on their slime chiefly for climbing vertical surfaces. While it would initially seem that they should be confined to level surfaces, their slime is powerful and sticky enough to allow them to crawl up inverted, sheer surfaces like walls and windows.
The slime's stickiness helps slugs and snails stick to uneven surfaces, like the leaves of plants. This gives them better access to food and shelter, as they can reach otherwise-impossible-to-access areas. These slimy gastropods don't choose how to climb with the slime -- it's all in the mucus's innate properties. The mucus excreted by slugs and snails has the consistency of a liquid gel, which is solid at rest and turns to liquid under pressure.
As one of these creatures moves along, parts of his underside are at rest while others are pushing down, into the mucus -- compare it to the motion of your foot when you walk. The muscles along his underside relaxing and contracting creates a pulse that pushes him forward. It is unclear which species are capable of that, but quite a few can and clearly Cornu aspersum the type in your photo and Helix pomatia observed by me are able to do so. The motion of the snail is basically a wave travelling down the foot muscle: some parts hold firmly to the ground and some push forwards and have loose contact with the ground.
The picture shows a snail approaching a cliff edge and braking hard. Its anti-lock braking system has kicked in. Well, I am surprised at all these answers.
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