CLASS: Gastropoda As a class they are long lived, some appeared in the Cambrian and at the present day they are the most abundant molluscs. They occupied a number of life modes, most lived in water: typically shallow marine areas but they can also live in fresh water and some forms survive on dry land.

Modern day forms include: Marine: Limpets winkles. Dry land: Snails slugs (shell less).

Copy diagrams in Black page 53.

The most striking feature is the coiled shell, forms because the internal organs are twisted. The soft body parts are lined throughout the shell and sections could be extended at will: Head, foot and siphons. The HEAD extended out of the ANTERIOR END and a FOOT extended out of the majority of the shell APERTURE. The head consists of TENTACLES for sensing and below this is the MOUTH.

The foot is a muscular organ, which allows movement. Terrestrial forms secrete mucus to aid their movement. Other soft body parts are concentrated in the anterior end where the shell is often at it's widest. The ANUS is also at the anterior end. Water living forms contain GILLS and some have a SIPHON that can be extended.

It has one valve = UNIVALVE and is coiled vertically and usually spirals to the right (dextral). The shell is mostly made of calcite or aragonite. In simple terms the shell is a conical tube closed at the pointed end (apex). This end is also called the POSTERIOR. The shell is secreted by the MANTLE and grows along the aperture.

Each completed coil is called a WHORL. The line along which the whorls meet is called the SUTURE. The LAST WHORL is called just that the rest are the SPIRE. The spire may be high, pointed with many whorls or short with a few whorls. Sometimes the shell can be flattened giving a planispiral form (similar to ammonites).

The size of the last whorl varies sometimes being slightly bigger than the previous one but sometimes it is much larger. A SIPHONAL CANAL may extend the aperture at the anterior end. This varies in length and is used to support the siphon (that takes in water). Sometimes the INNER LIP of the aperture can appear thickened and almost folded back as it grew = CALUS.

In some genera a type of lid mechanism can be used to shut off the aperture when the soft parts were withdrawn called the OPERCULUM. The SHELL ORNAMENT varies: smooth, fine, coarse ribs, tubercles or sometimes spines. Because of the coiling muscle scars are not usually visible. Most genera coil right handed (dextral) and therefore the aperture is on the right. Only a very few are sinistral.

See reef video from Discovery. Generally the marine gastropods living in the shallowest water had the thickest shells (littoral zone) e.g. the cap shaped limpets. Gastropods with an aperture with siphonal canal were usually carnivores and lived on soft sediment (e.g. ate bivalves and other gastropods). The gastropods without a siphonal canal were generally herbivores.

They did not burrow and therefore lived on hard surfaces. They crawled when necessary. The freshwater forms generally had thinner shells.