The top should be securely covered with screen or part of a nylon stocking. Lids which have holes punched in them do not allow adequate ventilation and the sharp edges on the interior are a safety risk . Although the contents of the terrarium can be an elaborate recreation of a garden environment, or something as sparse as only food and water, most will probably be between these two extremes. Water can be supplied in a shallow plastic lid, in a deeper dish which contains a water soaked sponge, or by generously sprinkling lettuce or other fresh leafy food with water. Food can also be put into a shallow plastic lid.
Snails are vegetarians and eat many kinds of plant material.
Cornmeal, oatmeal, and fresh green leaves are all appropriate foods. Chalk, egg shells, or snails should be powdered and added to the food to provide the calcium necessary for healthy shells. Cleaning to remove moldy food, the build up of mucus, and droppings, should be done on a regular basis. Overcrowding should be avoided. Too many snails in too small a space, or inadequate ventilation can cause the humidity to rise to unacceptable levels and the snails to die.