If the animal has no legs, then it is one of the following:

If it has a hard shell it is a WATER SNAIL.

If it has a rounded body and a tail, but no fins, it is a TADPOLE, which is a young FROG.

If it has a sucker on each end of its body it is a LEECH.

If it has a flat body and glides along it is a FLATWORM.