Walk in the garden on summer evenings and you'll hear crunch, crunch as snail shells crush under your, hopefully not, bare feet. Snails and slugs are closely related molluscs that move about on a muscular foot lubricated with mucus from glands on the underside of the foot. Several pairs of tentacles with eyes on the end help these animals make sense of the world.
Note slime on plants at the hind end of this leopard slug.
Snails have made these slime trails across a path.
Note snail's black eyes on the end of is tentacles.
They are soft-bodied animals that need a moist atmosphere to live in. Snails have a spiral shell which they retract into in dry weather. They also secrete a mucous plug, which, when dry, stoppers the entrance to their shell which slows further drying. They will also huddle in a group, under flower pots and behind bricks to slow down evaporation from their bodies. Slugs are more vulnerable to drying out as they have no external shell to retreat into but they exude mucus around themselves which dries and protects them. Both "hibernate" in dry conditions and slow down their bodily activity. Another way to slow down water loss from the body is to be nocturnal.
Snails and slugs are the bane of the home gardener as they are notorious grazers of green garden plants. However, they have some good habits and also feed on dead plant and animal material and fungi and I can often find them in my compost heap feeding on the decaying organic matter inside. Some are carnivorous such as the Leopard slug pictured below. It feeds on snails. Some snails are predatory and feed on earthworms and other snails.
Predatory leopard slug.
Mucous? Yuk! Mucus is nasty and slimy but it's great stuff for snails and slugs. Apart from lubricating the foot in its passage across even quite sharp objects, it slows down water loss from the body when dry, forming plugs in shell openings. Carnivorous slugs use it to entangle their prey. Slugs also produce great gouts of it during mating. I was puzzled by large slimy spots on my courtyard walls until I went out one night to find two Leopard slugs closely entwined, hanging off the bricks on long, wet ropes of mucus.
Leopard slug meets ant: When the ant in the centre photo came up to this slug, the slug's defensive reaction was to pull in all its tentacles to protect its eyes, and close its mouth and its respiratory (or breathing) pore in the saddle. When the ant went away, the slug elongated its tentacles, opened its mouth and respiratory pore again and went about its business.
Defensive reaction of slug to a disturbance