Evolution, During Digestion, of the Bacterial Flora in the Alimentary System of Helix Aspersa (Gastropoda: Pulmonata): A Scanning Electron Microscope Study

The use of scanning electron microscopy (SEM) allowed a study of the distribution of bacteria in the various digestive organs of the snail Helix aspersa Müller. The bacteria are enclosed by mucous secretions (mucous film or mucous grains) and sometimes attached on the cilia of some of the digestive walls. According to the food that was given to the snails, different morphological types appeared, two of which dominated. Adult snails were fasted for 4 days, given a dehydrated artificial food and then sacrificed at different times during digestion. The presence of bacteria may be related to the time of digestion. In fact, bacteria seem to accompany the food mass; they develop mostly in the stomach and in the intestine where they may help digest the food. Fasting or hibernating snails do not possess bacteria in the alimentary lumen or on the digestive walls. However, the residual faeces localized in the distal, intestinal lumen, lodge great quantities of bacteria. From these results, the endogenous or/and exogenous existence of the bacterial flora in alimentary system of Helix aspersa is discussed.