There are two main groups of water snails. One breathes with gills; the other has a kind of lung and comes to the surface for air. This lung is not like ours, but it has the same work to do. A snail also has blood which carries the oxygen through its body.

A gillbearing snail has an operculum. This operculum is like a door which the snail can close over the opening of his shell after he has drawn himself inside.

Not all snail shells are coiled in the same direction. You will probably find snails with spiral shells, coiled so that the opening is on the right when you hold the shell with the tip up and the opening facing toward you. These are sometimes referred to as right-handed shells. You will also find lefthanded shells.

Among the fresh-water snails with spiral shells which breathe by means of a lung are Limnaea and Physa. Limnaea has a right-handed shell and Physa a left-handed shell, as you can see from the photograph at the beginning of this chapter.

One fresh-water snail which builds its shell in a flat coil is the "wheel snail" or Planorbis. Some caddis fly larvae build their cases in the shape of a wheel snail shell. Like the two snails described above, the wheel snail has one lung.

Perhaps you will find one of the gillbearing fresh-water snails which has an operculum. There are several kinds which are different in size and shape. One of these, Goniobasis, may grow to a length of one and one-quarter inches. Its shell is long and pointed, with the opening on the right. Another, Amnicola, has a shell about one-half inch long, which is usually wider in proportion to its length than that of Goniobasis.

One of the most interesting of gillbearing fresh-water snails is Vivipara. The shell of the adult snail may be as much as two inches long, and is shaped like a top. The eggs of Vivipara develop into young snails inside the parent's shell.

Most fresh-water snails lay their eggs under water in masses of gelatin. The egg-mass is attached to rocks or logs, or to the stems of water plants.