In traditional two-kingdom systems, the multicellular animals
were referred to broadly as metazoa to distinguish them from
one-celled "animals," the protozoa. In our classification
scheme, the traditional protozoa belong to Kingdom Protoctista;
in our system, animals may be defined as multicellular,
heterotrophic, diploid organisms that develop anisogamously—from
two different haploid gametes, a large egg and a smaller sperm.
The product of fertilization of the egg by the sperm is a
diploid zygote that develops by a sequence of mitotic cell
divisions. These mitoses result in first a solid ball of cells
and then a hollow ball of cells called a blastula. The defining
characteristic of all animals is their development from a
blastula. In most animals, the blastula invaginates, folds
inward, at a point to form a gastrula, a hollow sac having an
opening at one end. Further growth and movement of cells produce
a hollow digestive system called an enteron if it is open at
only one end, and a gut or intestine if it ha