Q.Distinguish between diploblastic and triploblastic animals, giving one example of each.

During early embryonic development an animal builds its body from either two or three primary germ layers, and this single fact predicts a great deal about the adult.

Diploblastic animals have just two layers: an outer ectoderm (becomes the epidermis) and an inner endoderm (becomes the gastrodermis lining the gut), with an undifferentiated mesoglea sandwiched between them. Corals, jellyfish, sea anemones (Cnidaria) and the ctenophores follow this plan. Without a true middle layer, diploblastic animals are capped at tissue-level organisation and radial/biradial symmetry.

Triploblastic animals add a middle mesoderm layer between ectoderm and endoderm. Ectoderm still forms skin, hair, nails, teeth and nervous tissue; endoderm still forms gut lining, lungs and liver; the new mesoderm forms muscle, bone, the heart, and the coelomic lining. This third layer is what makes organ-system organisation, a true body cavity, and bilateral symmetry with cephalisation possible. Every phylum from flatworms to chordates is triploblastic — the diploblastic/triploblastic split is one of the very first branch points on the animal classification tree.