Nervous/Sensory System
Gastropods are known for their developed sensory and nervous systems. They have many unique features that enable them to locate food and escape predators.
Instead of
a brain, terrestrial snails have concentrated areas of nerves
known as nerve knots or ganglia (Nordsieck,
2012). There
are different types of ganglia, and they are concentrated at the
front of the body so snails can sense and
react to stimulus in the shortest amount of time (Mckenzie,
2011).
Pulmonates have pedal ganglia that help with movement, pleural ganglia that receive signals from the mantle, and visceral ganglia to control intestines, anus, digestive gland, and heart (Nordsieck, 2012).
Probably one of the first things you notice about a snail are
its long tentacles.
Stylommatophora snails have two sets of tentacles: one short and
one long pair (Nordsieck, 2012).
In fact,
Stylommatophora
means “stalk eyes” (Nordsieck, 2012).
The eyes vary among species; some have eye spots that can only
differentiate light and dark, while others have developed lenses
(Nordsieck, 2012)!
Sight and hearing are not the most important abilities of
terrestrial snails; snails cannot hear (Chase
and Croll, 1981).
Robert Nordsieck describes an important experiment in which the sensory organs of terrestrial snails is examined: a snail is placed on a table and a knife or razor blade placed in front of it (2012). The snail’s head is placed directly in front of the knife so it has to crawl over it (Nordsieck, 2012). The snail is able to crawl over the blade of the knife without being injured (Nordsieck, 2012). In the picture below, the snail is using its tentacles to feel out its environment before going over the blade.
The
snail is able to travel over the blade unharmed because of their
chemoreceptors and statocysts (Nordsieck,
2012).
Since sight
and hearing are not advanced in terrestrial snails, they depend
on their olfactory organs to sense objects (Chase
and Croll, 1981).You might have noticed snails also have a
shorter pair of tentacles where their chemoreceptors are located
(Mckenzie, 2011).
Snails depend on their chemoreceptor to locate food,
detect enemies, and even find mates (Kohn,
1961). When an
enemy is close by, it emits a chemical that the other snail is
able to sense almost immediately and react (Kohn,
1961). Snails
can even differentiate between the chemical stimuli of different
animals (Chase and Croll, 1981).
Snails are able to use their tentacles to smell and their
mouths to taste (Nordsieck, 2012).
Statocysts enable a snail to remain balanced and gives them a sense of their position (Nordsieck, 2012). In order to crawl over the knife blade, the snail has to have a good balance and has to be able to tell where it is relative to the blade.