Gastropods

Gastropods
By Barbara Ducote
Fall 97

The class gastropod is the largest class of mollusks. It contains over 75,00 existing species and over 15,000 fossilized forms. Their first appearance can be traced back to Precambrian time. There have been three major evolutionary changes in the gastropods.

Development of the head
Conversion of the shell from a shield to protective retreat
Torsion

These changes have lead to their success as a class.

Gastropods have retained a flat foot. This foot allows them to creep along today as in ancient times. Most gastropods are very mobile and active. The head of the gastropod bears a pair of tentacles with an eye at each tentacle base. A retractor muscle withdraws the head and foot. The bottom of the foot bears a horny disc called the operculum. The operculum fills the aperture of the shell and acts as a protective door or lid.

The shell is an asymmetrical spiral and functions as a portable retreat when danger approaches. The spiraling of the shell allows the organism to exploit crevices and holes for food. The typical gastropod shell is a conical spire composed of tubular whorls, which contains the mass of the organism. The apex of the shell contains the smallest and oldest whorls. The whorls get successively larger and younger. They are located around a central axis (columella) and terminate at the aperture from which the head and foot protrude. A shell may be spiraled clockwise or spiraled counter clockwise. Most are spiraled clockwise with a few spiraled counter clockwise. Shells displaced an infinite variety of colors, patterns and shapes.

Torsion, which is a twisting of the body, is thought to be a separate evolutionary process from the spiraling of the shell. There is not a widely accepted reason for torsion to have taken place. Some scientist believes it occurred in the larval stage. Other think torsion is an adult adaptation. It is thought the changes in torsion may have to do with head protections and the utilization of the anterior water stream. This design would allow for the withdrawal of the developed head when the organism retracts.

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