Eli Kirby, age 12, of Decatur, I1., for his question:
ARE THERE MANY KINDS OF GASTROPODS?
Gastropods include all members of the mollusk family, such as snails and slugs. Gastropods form the second largest class in the animal kingdom, outnumbered only by insects. The most recent estimate of the number of known species is 37,500, a revision downward from an earlier estimate of about 80,000.
Gastropods vary considerably in structure and way of life. The smallest species are barely visible, whereas the largest, a sea slug, weights up to 30 pounds. A gastropod is generally characterized by a single shell and an asymmetric body.
The animals are common in most marine and freshwater habitats and the only mollusks to flourish on land.
The gut of a gastropod is a coiled tube with various glands and sometimes a gizzard. The nervous, circulatory and reproductive systems are well developed.
Lower gastropods have separate sexes and reproduce by spawning eggs into the water where they are fertilized with sperm and develop. The young larvae swim about, settle and mature. In advanced gastropods fertilization takes place internally, and coverings are produced that protect the eggs and young, which are sometimes also guarded by the female. At times the whole development process is internal.
In the more modified subclasses, the animals are almost always hermaphroditic, which means they contain both male and female reproductive organs. This allows them to mate with any mature animal of the same species.
The gastropod's mouth is usually equipped with a rasp like tongue called a radula. Generally used in scraping up food, the radula may be considerably modified. In cone shells the teeth on the tongue become dart like and some species can inflict a dangerous wound. In oyster drills, the radula can bore through a shell.
The first gastropods appeared in the early Cambrian period, about 600 million years ago. The most primitive living gastropods are of the subclass Prosobranchia and are mostly marine with a few freshwater and terrestrial species.
The subclass Opisthobranchia is almost wholly marine. The shell tends to be reduced and the gill migrates toward the rear of the body. Eight orders exist which include the bubble shells, sea hares and allies. Two groups of this type swim in plankton.
In the subclass Pulmonata the mantle cavity has become a lung and the operculum (a horny plate that closes the opening of the shell when the animal is retracted) is lost.
Most terrestrial snails and slugs belong to the order Stylommatophora and most freshwater snails belong to Basommatophora.