Ruth McWherter, age 14, of Phoenix, Ariz., for her question:

HOW DOES THE MARMOSET DIFFER FROM OTHER MONKEYS?

Marmoset is the common name for a number of small, long tailed tropical American monkeys of the Cailithricidae family. They differ from other monkeys in having long, claws rather than nails on all of their toes. In addition, they also have two instead of three molars on each side of the upper jaw and they have non opposable thumbs.

Most family members are about 12 inches in length. They have soft, silky fur and are gentle in disposition. They have tails that are as long as their bodies.

They feed on fruits and insects.

A female marmoset will have a litter containing two or three young.

The best known of the true marmosets is the wistiti, or ouistiti, the white eared marmoset of Brazil. The wistiti, so called from the sound of its chirping, birdlike cry, has a rounded head with a flattened black face, thick white tufts on its ears and long black and white fur.

Other popular marmosets are the silky and the golden marmosets.