Question

1)- What reasons might there be for well-defined substrate preferences? Would all snails show the same preferences? What other factors might be involved in the apparent choice of substrates?

2)Snails and slugs will display a single pneumatophore, usually on the right side of the body. In snails, this will only be visible when the head and foot are fully extended (see picture). Try to observe the pneumatophore in one of the lab snails. What is its function?

3) Does the earthworm have a sense of smell? What is the value of an olfactory sense to an earthworm?

Answer 1

1) One of the reasons for well defined substrate preferences for snail is the availability of food like detritus,algae etc. Physical characteristics of the soil,water are also parameters like salinity,presence of inorganic ions,frequency of spates etc. Yes all the snails show various substrate preferences.

Others factors also might be involved like light intensity or degree of humidity.Certain intertidal slugs display strong preference for darkness.Some of them also display positive phototropism when under the water and negative phototropism when out of the water.They may show preference for oxygen-rich stagnant or flowing water or plant rich small water bodies or variable degrees of domestic sewage input.