How did snails adapt to survive on land?

Mesodon mitchellianus adapted in many ways over the years.  One of its adaptations is their teeth which are used for feeding. In order for the snail to feed on vegetation such as leaves, they have adapted mouths which allow them to break down their food easily.

Because M. mitchellianus is a pulmonate, it has a specialized structure made of chitin called a radula (Invertebrate Diversity, 2010).  The radula is used to scrape food and cut through leafy material often consumed by these herbivores (Invertebrate Diversity, 2010). The teeth within the radula allow this scraping and tearing of their diet (Invertebrate Diversity, 2010).

There is much variation among the teeth of snails even within the genus Mesodon. There is little known about the mouth of Mesodon mitchellianus, however we do know that M. mitchellianus has an outer cusp and this is a defining characteristic of its species (Binney, 1878).  This differs from M. thyroidus, which has no side cusp on the outermost lateral teeth (Binney, 1878).  The outermost lateral teeth of M. thyroidus have a cutting point that is long and slender (Binney 1878).  The cutting point is unusual in that it slants downward below the base of their mouth (Binney 1878). The first lateral teeth and central teeth of M. thyroidus lack cutting points, which is different from the outermost lateral teeth, which posses a cutting point (Binney 1878).  This cutting point is used primarily to scrape food and cut through decaying leaves (Binney, 1878).

Many adaptations had to occur in order to allow the snail to be able to live in terrestrial environments.  One significant adaptation is the development of the lung.  M. mitchellianus is a pulmonate meaning that it has a lung and is able to breathe air (Invertebrate Diversity, 2010).  Pulmonate comes from the Greek word pulmanata meaning lung (Invertebrate Diversity, 2010). The gill is not present because it is no longer lives in marine or freshwater environments (Gastropoda, 2002).  The lung is contained in the mantle cavity (Gastropoda, 2002).  The mantle cavity  is modified and has a little opening called the pneumostome and many blood vessels allowing for gas exchange to take place (Gastropoda, 2002). The air enters the shell via the pneumostome when the mantle floor contracts (Gastropoda, 2002).  The pressure within the mantle increases allowing for the oxygen to be easily absorbed by the blood vessels and the pressure is then released and carbon dioxide is pushed out (Gastropoda, 2002)The ability to breath air allows for the snail to survive in terrestrial environments.

The class Gastropoda went through many evolutionary changes involving their shell structure. The shell of the gastropods before the torsion occurred was symmetric and coiled in a flat plane making it difficult for them to move because of the large and heavy shell (Gastropoda, 2002).  Then the gastropods went through torsion, which is an evolutionary change where their body twisted 180 degrees (Gastropoda, 2002).   This then allows the shell to coil along a central axis at a slant creating an asymmetrical shell (Gastropoda, 2002).  This helped to distribute the weight, create more balance, and allow the gastropod to move more easily (Gastropoda, 2002).

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