Adaptation
Many of the specific adaptations of Millerelix peregrina were very difficult to find. The succeeding lines provide an overview of the adaptations of the more general group: the terrestrial pulmonates, or the land inhabiting air-breathing snails.
An adaptation is defined as a structure, process, or behavioral quality that
is affected by natural selection and that gradually changes over
time in order to help the individual stay alive and reproduce (Hickman et
al.,
2012). Let us begin from the top of the snail and work our way down.
Gastropods are known to have a hard calcareous outer covering, called a
shell, that protects their soft bodies from predators and harsh
environmental conditions (Burch, 1962). Connected to the inside surface of
the shell is a slender, sturdy layer of skin called the mantle (Burch, 1962).
The thickest part of the tissue is located at the opening of the shell (Burch,
1962). This tissue produces
calcium carbonate, adding to the outer protective covering of the snail and increasing its size
(Burch, 1962). At the anterior end of the body, a pair of eyes is positioned atop two
retractable tentacles (Burch, 1962). Their eyes have an uncomplicated lens
and lack an iris, meaning that they are incapable of fine focus, but are able
to sense direction fairly accurately (Hickman et al., 2012). The
retractable tentacles themselves also act as sensory organs by identifying
chemical signals within the
environment, allowing them to “feel” their surroundings (Pennsylvania
Land Snails, 2012).
Below the eyes is the mouth, which contains the radula
(Burch, 1962). The radula, identified on the right by the gray ridges, is a rasping organ lined with an abundant amount of chitinous teeth used for scraping food off
surfaces and into the oral
cavity (Burch, 1962).
According to Cleveland P. Hickman, Jr., gastropods undergo what is called torsion, or a 180 ̊ twisting of the portion of the body that contains the organs, which occurs during the developmental stages of their lives (2012). As a result of this rotation, the anus and mantle cavity are brought forward and are situated on top of the head (Hickman et al., 2012). Torsion is thought to be a defensive mechanism; it allows the terrestrial pulmonate to draw into its shell more quickly, thus avoiding predators (Pennington and Fu-Shiang, 1985). Coiling is another developmental adaptation that allows the gastropod’s shell to grow in unison with the body, preventing it from becoming too cumbersome for movement (Hickman, 2012). Moving further down our snail, we come to the foot.
Locomotion is acquired by way of a simple flat foot that is located just under the stomach, hence the name “belly-foots” (Burch, 1962). In order to actually move, this muscular foot produces waves of contractions and expansions that travel from the posterior end to the anterior end (Carlson, 1905).
Wondering what these snails eat? If you click Nutrition, all your questions will be anwered.
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