They are the higher group in the world of shells, compared to the bivalve mollusks. The connecting link is found in the Tusk-shells, which, beginning life with two shells, mature with one, but with a head.

From around about the head project many delicate filaments which are protruded into the water to catch microscopic types of life.

They are called Tusk-shells, not because they have tusks, but because their shells resemble tusks in outline.

Another intermediate step between the two-shelled and one-shelled mollusks takes us to the Chitons, or Mail shells -- animals in which the shell is in eight hinged segments, with chitin on the upper surface and shell on the under-side of the plates.

The hinged scheme of armor suggests the armadillo and the wood-louse, and the chiton, like those two creatures, rolls itself up when threatened.

From this point we are in the great company of single-shelled gastropods -- the snails, whelks, and so on.

Here we have a soft-bodied animal set in a shell. The body is marked off into head, eyes, feelers, and a projecting hump in which the vital organs are contained.

This part is well divided from the underpart, the great muscular foot, on the upper back part of which appears the operculum, that horny disk with which the animal closes its shell as with a front door.

Not all gastropods have this security device, but everyone that has seen a periwinkle realizes its excellence.

The most remarkable feature of the snail tribe, from the popular point of view, is the tongue, or radula. This is a ribbon of chitin fitted with a row on row of little horny grapnels which play the part of teeth.

These teeth are arranged in rows across the long tongue and vary in number with the specieis. The fewest are found in a sea slug which has less than a score of teeth all told -- while the common snail of the garden has a serried array of teeth in one hundred and thirty-five rows, one behind another, and one hundred and five teeth to each row-- over fourteen thousand teeth in all!

A large pond snail has twelve thousand teeth, the small fresh-water limpet has nine thousand, the amber shell over three thousand, while a Mediterranean mollusk is estimated to have something like seven hundred and fifty thousand teeth.

However, make no mistake -- there is no resemblance between molluscan teeth and the teeth of mammals. They are tiny hooks of horn set in a ribbon of chitin, and are not all used at once.

Indeed, a small part of the tongue comes into play at a time, the remainder being kept in reserve at the back of the mouth to take the place of teeth in front which have become worn.

This formidable tongue is protruded by the animal and licks the vegetation or flesh which forms its food.

A lion's tongue, when it licks, takes flesh and blood with it. However, it is interesting to note that the snail's and whelk's tongue rasps in the same way. A cabbage which has been attacked by a snail or slug looks as if the tissue had been slowly rubbed off, and that is the case. It has been filed aways by the snail's rasp, and the part which we miss has gone down its throat.

We need not wonder at that, for the whelk rasps its way through the shell of the oyster and banquets on the contents.

Many land mollusks commit attacks of this kind on other gastropods. Some of the slugs and snails are mainly flesh-eaters.

Even the vegetarian species may make a change of diet in captivity. In such cases snails, when spring returns with abundant green food, have been known to refuse it, and missing the flesh which was now denied them, have calmly eaten each other!

The great slug Arionater, the monster whose depredations in the garden makes the serious gardener dread, it eats worms alive or dead, and eats other slugs alive. It has been known to eat sand containing organic matter, and it will readily turn from vegetation to devour plant-lice and other insects.

The slug, therefore, is a friend as well as a foe, a scavenger as well as a robber. The same may be said of certain of the land snails.

They are known to eat beetles, other small slugs, fungus, and lichen. All slugs and snails make their own track of slime along which to glide, and it is by their glistening traces that we follow them.

Where food is plentiful they go and return with great regularity, and where slugs and snails have been, slugs and snails will be found year after year, if it be not disturbed.

Snails can seal their shells with a form of mucus which hardens into a covering for the winter. Slugs can envelop themselves in slime. That slime they can convert into cables.

In the water, there are mollusks of this order which spin a thread in the water, attach it to the surface, apparently by a depression in the upper end of the line of slime which admits a bubble of air and so floats with buoyancy enough to hold the animal at the distance below the surface at which it desires to remain. That enables us to better grasp what follows.

In a conservatory slugs played havoc with precious and rare orchids, so the plant pots were placed in bowls of water and the plants themselves were enveloped in wading, over which even the slimiest slug may be supposed incapable of traveling.

Were the slugs defeated? No, they climbed the rafters of the conservatory and let themselves down by threads of slime on the flowers of the orchids.

These gastropods are marvels of endurance. They exist far up in the Arctic Circle. They flourish in the desert, where plant life is sparse and almost non-existent. They hibernate through the bitter winter. They bury themselves and sleep, fasting during the torrid heat of summer.

Moisture brings them out in such myriads that people in the old days imagined that they had arrived on the wings of the wind. If all favors them, they live about five years, so far as observation goes, and they may live much longer.