Mary McKenzie, age 12, of Wellington, Kansas, for her question:
Is it true that snails are the same as slues?
Most ordinary folk would dispute this idea. A snail wears a shell; a slug does not, and this difference between them is obvious. But zoologists base their classifications on more basic biological features. They do not agree that the mere ownership of a shell qualifies an animal for membership in a club of his own.
Since mussels, clams, and oysters are mollusks we tend to assume that all mollusks have shells. It comes as a surprise to learn that the shell less octopus also is a qualified member of the animal Phylum Mollusca. Zoologists classify more than soft bodies. Some mollusks have external shells, some have thin slivers of shell type material inside their bodies, and some have no shells at all. Most members of the group have gills and live in the salty sea. About 5,000 mollusks breathe air and live on land or in fresh water.
Zoologists subdivide this large phylum into five separate classes. By now you suspect that snails and slugs both are qualified mollusks. Correct. Perhaps you suspect that the snails are separated from the slugs in different classes. Not at all. All the snails and slugs belong in the Class Gastropoda. This name means stomach foot. As you know, both snails and slugs crawl along on a special foot on the underside of the tummy. So do periwinkles, whelks, sea slugs, and a host of other marine gastropods with or without shells.
The zoological sorting continues as the gastropod class is divided into three subclasses. Perhaps the snails and slugs are separated into two different subclasses. Not at all. The largest subclass has some 20,000 members with or without shells. All have gills and most of them live in the salty sea. The second subclass includes some 2,300 marine mollusks. It includes assorted sea slugs, others with small outer shells, and a few with small slivers of shell below the skin. This leaves only one subclass for all the everyday creatures we are likely to meet as land or fresh water snails and slugs.
The name of this subclass is Pulmonata, a term coined from an older word for lung. All members of this group are air breathing gastropods. It includes all the snails and slugs that live either on land or in fresh water. Zoologists, it seems, do not rate the shell of the snail as such an outstanding feature. In general, they regard the snail as a slug with a shell and the ug as a snail without a shell.
Most kinfolk of the snails and slugs are marine mollusks. Every mollusk must have a flaplike mantle of loose skin around his soft body. Every gastropod must have a head, a stomach foot, two eyes, and one or two pairs of tentacles. He may have gills or he may use a pocket under his mantle as an air breathing lung. The wearing of a shell on his back is optional, whether he enjoys life on the land, in a fresh water pond, or in the salty sea.