Parazoa literally means animals set aside . These animals that are so strange, so unlike all other animal life, that we tuck them away in their own little group. Eumetazoans have a definite symmetry, which sponge animals lack, and share common patterns of embryonic development. The bilaterally symmetric animals can be further divided into three grades. Grade is not a formal taxonomic term.
Grades represent a level of organization.
The group of all animals that fly, for example, could be called a grade. These three grades represent three basic types of body plan found in all animals. These body plans differ mainly in the presence or absence of an internal body cavity, or coelom . In the embryos of all bilaterally symmetric animals, there are three tissue layers distinctly visible in the developing organism. Many primitive animals, like flatworms, have no coelom at all, just a rudimentary internal pouch, like sticking your finger deep into a ball of clay.