Class Gastropoda
(snails and slugs)
Major attributes:
- Has become asymmetrical through torsion.
- Ganglionated nervous system.
- Reproduction varies - external fertilization and hermaphoditism.
- Most species have a foot, visceral mass, mantle and mantle
cavity.
- Radula characteristic organ of Gastropoda.
Description:
The gastropods, which include
the snails, whelks, periwinkles, abalones and slugs, are the largest
group of mollusks. They have either a single shell or no shell.
Gastropods are common in both salt and fresh water and on land.
They have become asymmetrical through an anatomical rearrangement
called torsion (exemplified by snails). The visceral mass has
become coiled and rotated through 180 degrees so that mouth, anus,
and gills all face forward and the head can be withdrawn into
mantle cavity. In cases where the shell has been lost (terrestrial
slugs), the mantle is moved back towards its original position
by detorsion.
Some Interesting Facts:
- Land-dwelling snails do not have gills, but the area in their
mantle cavities once occupied by gills is rich in blood vessels,
and the snail's blood is oxygenated there. Thus, the mantle cavity
has become, in effect, a lung.
- In some Gastropods, the primitive form of reproduction (separate
sexes with external fertilization) is retained. In most Gastropods,
however, fertilization is internal and hermaphroditism has evolved
repeatedly.
- Gastropods have a ganglionated nervous system with as many
as six pairs of ganglia connected by nerve cords. The cerebral
ganglia supply tentacles and eyes; the pleural ganglia, the mantle;
the pedal ganglia, the foot muscle; and the subesophageal, supraesophageal
and visceral ganglia supply the visceral mass.
Pertinent Books Found in Lamson Library:
- Graham, Allistar, 1977. British Prosobranch and other
Operculate Gastropod Mollusks. Keys and Notes for the Identification
of the species. London, NY: Academic Press for the Linnean
Society.
- Hughes, Roger N., 1986. A Functional Biology of Marine
Gastropods. Baltimore, MD: John Hopkins University Press.
(William K. Fox, Fall 1994; edited by B. Miller,
Spring 1995)