Class Gastropoda  
     The gastropods are the most numerous and certainly the most well known of the six classes.  They are the univalves such as snails, conchs and whelks.  In other words, they have only one shell.  They received their name from the Latin words gastro meaning stomach and poda meaning foot, because they are, in a sence, stomach-footed.  In examining the anatomy of the gastropod one will discover that they have a muscular foot above which are located their internal organs such as the stomach.  
     This class inhabits salt-waters, fresh-waters and is the only class that contains land dwelling mollusks.  The evolutionary process gave these land gastropods an efficient gliding foot with which to travel across land, eyes with which to see, an aggressive eating mechanism and a pulmonary system with which to extract oxygen from the air.  
     Most gastropods have a well-developed head with tentacles, eyes and a mouth.  Some gastropods even have olfactory and taste receptors.  There is usually a siphon present for taking in water.  The mouth leads into a pharynx, or throat, that most often contains the radula.  In adult gastropods, the internal organs are arranged asymmetrically, and the gills and reproductive and excretory organs are located near the head.  The viscera are arranged asymmetrically owing to the degeneration of most of the organs on one side (typically the left).  This imbalance lends itself to coiling, a common appearance among gastropods.  This coiling occurs during development.  The organs of the right side rotate conterclockwise one hundred and eighty degrees to the left side, making it appear that the organs of the right side are degenerate.  This coiling is considered an adaptation for compactly housing the very long visceral mass of the gastropod.  
     Most gastropods have an operculum or ?trapdoor? that is located on the top of the back part of the foot.  This operculum seals the opening at the mouth of the shell when the creature withdraws in to it.  
     The gastropod's shell is usually made up of a long tube that increases in size as it winds around the central axis or columella (more information on the spiraling of shells).  Each turn, or whorl, is seperated by a suture and the smallest whorl is called the nuclear whorl and is located at the top of the shell.  At the bottom of the shell is the newest and the largest whorl, the body whorl.  The body whorl is where most of the live mollusk?s soft parts lie.  The body whorl ends in an opening: the aperture.  Either the aperture can be on the right or left side determining wether the mollusk is dextral (right -handed) or sinistral (left -handed).  The columella can be solid or hollow and sometimes opens at the base end creating a siphonal canal.  Some gastropods are smooth; others have spiral or axial sculpture, or both (latticed sculptures).



Above is a picture of a beautifully colored aeolid nudibranch.

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