Germ layers are group of cells behaving as a unit during
early stages of embryonic development. These germ
layers differentiate to give rise to all the tissues/organsof
the fully formed body of individuals.
On the basis of germ layers animals are classified as
follows
1. Diploblastic
Animals in which the cells are arranged in two
embryonic layers—an external ectoderm and an
internal endcxierm, are called diploblastic animals. In
addition, an undifferentiated layer, mesoglea is present
in between the ectoderm and the endoderm, e.g.,
coelenterates.
2. Triploblastic
The animals in which the developing embryo has a third
germinal layer mesoderm, in between the ectoderm and
endoderm are called triploblastic animals, e.g., all animals
from phylum—Platyhelminthes to phylum— Chordata.