Adaptations

 

Millerelix jacksoni has many adaptations that are similar to those of other snails of the Class Gastropoda. These adaptations have made snails more distinguishable from other members of the Phylum Mollusca. One important adaptation was that of the evolution of a shell. During the development of a snail zygote/egg, the shell develops with the snail’s body inside the egg. Once the snail reaches adulthood or sexual maturity, the shell stops growing (Illinois DNR, 2009). The shell is constructed from calcium mineral deposits on the snail's mantle, a covering that surrounds a snail's organs. The evolution of a shell was very important for member of the Class Gastropoda because a shell is a snail's biggest defense against predators (Illinois DNR, 2009). The snail can retract its body back into its shell when it feels threatened or endangered (Hickman et al, 2012). Another adaptation for snails was coiling. Coiling is spiral winding on a snail's shell that helps the snail distribute its weight. All snails descend from a common coiled ancestor that developed from a bilateral shell. These coiled shells tend to sit posterior and upright on the body of a snail (Hickman et al, 2012). 

One of the most evolutionary adaptations of snails was ontogenetic torsion. Torsion is the changing of the positions of a body part from its initial position to another position on the organism. In many organisms the mouth is located on the anterior portion of the body while the anus is posterior (Hickman et al, 2012). In the case of ontogenetic torsion in snails, the anus turns 180 degrees counterclockwise so that it is facing the direction of the snail's shell opening and head (Hickman et al, 2012). Once ontogenetic torsion is complete, the anus and opening of the mantle cavity are located above the snail's head and mouth. Torsion is important in snails for it helps the snail take in water that flows in the direction of movement (Hickman et al, 2012). The image below depicts torsion in an aquatic snail, but is a good general representation of snail torsion. In the first drawing the anus is posterior on the snail. In the second drawing the anus has moved to the right side of the snail's body. The anus continues moving until it reaches the final position above the snail's mouth and head, which is depicted in the third drawing.

Unlike the image above, Millerelix jacksoni does not have a set of gills, but a lung. Many terrestrial snails have developed this lung from their mantle cavity in place of their lost gills. The snail brings air into its lung and breaths by contraction of the floor of the mantle (Hickman et al, 2012). With the development of this lung came the development of a pneumostome. The pneumostome is an opening located on a snail's mantle that aids the snail in bringing in air into the lung (Illinois DNR, 2009).

Another adaptation in snails is the secretion of mucus. This mucus is a fluid produced in glands located on the snail's exterior surface which prevents the snail from losing fluids and aids the snail in movement (Illinois DNR, 2009). The snail's foot slides over this mucus providing movement for the snail (Illinois DNR, 2009).  

One final adaptation that is not found in all snails, but only in those of the Genus Millerelix and its close relatives is the presence of a denticle. A denticle is a U- or V- shaped (Emberton, 1994) small projection that resembles a tooth and projects towards the inside of a snail's shell (Illinois DNR, 2009). This denticle plays a vital role in defense for the terrestrial snails that possess it because the denticle helps keep out insects that may climb inside the snail's shell to eat it (Illinois DNR, 2009). The denticle was a very important adaptation that aided terrestrial snails in protecting themselves from predators.

**For information on a snail's body plan or unknown snail terminology please see the Facts page.