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 studyof the distribution of bacteria in the various digestive organsof the snail Helix aspersa Müller. The bacteria are enclosedby mucous secretions (mucous film or mucous grains) and sometimesattached on the cilia of some of the digestive walls. Accordingto the food that was given to the snails, different morphologicaltypes appeared, two of which dominated. Adult snails were fasted for 4 days, given a dehydrated artificialfood 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 developmostly in the stomach and in the intestine where they may helpdigest the food. Fasting or hibernating snails do not possess bacteria in thealimentary lumen or on the digestive walls. However, the residualfaeces localized in the distal, intestinal lumen, lodge greatquantities of bacteria. From these results, the endogenous or/and exogenous existenceof the bacterial flora in alimentary system of Helix aspersais discussed. (Received 26 June 1989; accepted 16 October 1989)