In the field of human parasitology, snails or gastropods play an important role as intermediate hosts (IHs) of schistosomes, viz. flukes of the liver, intestinal tract and lungs. With respect to the masses of their bodies and shells, snails take part in the turnover of the biomass of a biocoenosis to a considerable extent. However, as the IH of a specific trematode, only a minority of a population of the corresponding snail group, nomally a local species, is generally involved. Therefore, the general reduction of snails in a fresh water lake or river by chemicals does not primarily hit the parasites as a target but instead affects the balance of the biocoenosis. For the control of IH snails, other kinds of intervention into the physiological parasite-host-relationship should therefore be chosen. Instead of killing all snails (target and non-target snails, IH and non-IH species i.e. other species), intervention should interfere with the parasite-snail relationship only. An example is the individual defence reaction of the snail mentioned in the following paragraph. This kind of intervention concerns only target-snails.
The concept of analysing the defence reactions of an IH snail in order to strengthen it, e.g. by genetic manipulation at the population level, has to take into account the above-mentioned minority problem. At the population level, the defence reactions of snails against trematode parasite stages do not form part of their vital interests. The internal defence system (IDS) of gastropods, incorrectly called an invertebrate "innate" immune system, is primarily directed against their microbial and viral pathogens, which can be disseminated quickly by contaminating transmission and threaten the total snail population acutely.
Trematode stages (cercariae) only attack definitive host animals (vertebrates); they do not attack other snails. Therefore the minority of snails involved in the trematode cycle cannot increase by snail to snail infection but only by the introduction of more tremadode eggs releasing miracidia. The minority involved remains constant. The trematode does not threaten the snail population and therefore the snailís genetically based reactive resistance against trematode stages does not create increased evolutionary pressure. On the contrary, the opposite is true: if the minority of snails involved in a trematodeís life cycle do not survive, then the parasiteís survival is threatened. Thus, it is a vital interest of the parasite to spare the life of the infected snail and not to strengthen unduly its metabolism but to handle it with care. Therefore, strong evolutionary pressure probably exists with respect to a physiological balance between the trematode stages (mother sporocyst, many daughter sporocysts, multiple cercariae) and the IH snail.
Different species of trematodes of water fowl may compete for the same snail species, e.g. when a lake that is used by birds of passage silts up. After the miracidium has entered a snail previously infected by another trematode species and becomes ready to develop to the mother sporocyst, an order of precedence decides which species of the trematode stage feeds on the other.
The principle organisation of a gastropod is exemplified by a prosobranchiate, i.e. a gill-carrying snail (fig. 7.1, page 267). The dissection of Biomphalaria glabrata, a planorbid pulmonate snail, is given on the present website (see "Päparation einer Tellerschnecke" with figures).
For the simplified systematics of gastropods, see the present website ("Klassifikation der Gastropoden").