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.

Photo credited to Robert Nordsieck.  Pictured is a snail crawling over the blade of a knife.  This experiment helped scientists understand the statocysts and chemoreceptors of snails.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.