kept in heliculture (in French commonly called the "gros gris"), but this is genetically distinct from large Algerian forms earlier given this name. EcologyCornu aspersum is a primarily a herbivore with a wide range of host plants. It feeds on numerous types of fruit trees, vegetable crops, rose bushes, garden flowers, and cereals. It also is an omnivorous scavenger that feeds on rotting plant material and on occasion will scavenge animal matter, such as crushed snails and worms. In turn it is a food source for many other animals, including small mammals, many bird species, lizards, frogs, centipedes, predatory insects such as glowworms in the family Lampyridae, and predatory terrestrial snails. The species may on occasion be of use as an indicator of environmental pollution, because it deposits heavy metals, such as lead, in its shell. Parasites Parasites of Cornu aspersum include a number of nematodes. Metacercariae of various species of the digenean genus Brachylaima have also been reported, and those have potential for being harmful to people because the adults can infect humans. However, the snails are capable of trapping cercariae (trematode larvae) in their shell, thus possibly reducing the intensity of infestation by parasites. Behavior The snail secretes thixotropic adhesive mucus that permits locomotion by rhythmic waves of contraction passing forward within its muscular foot. Starting from the rear, the contraction of the longitudinal muscle fibres above a small area of the film of mucus causes shear that liquefies the mucus, permitting the tip of the tail to move forward. The contracted muscle relaxes while its immediately anteriad transverse band of longitudinal fibres contract in their turn, repeating the process, which continues forward until it reaches the head. At that point the whole animal has moved forward by the length of the contraction of one of the bands of contraction. However, depending on the length of the animal, several bands of contraction can be in progress simultaneously, so that the resultant speed amounts to the speed imparted by a single wave, multiplied by the number of individual waves passing along simultaneously. A separate type of wave motion that may be visible from the side enables the snail to conserve mucus when moving over a dry surface. It lifts its belly skin clear of the ground in arches, contacting only one to two thirds of the area it passes over. With suitable lighting the lifting may be seen from