Zoology · Ch 2 — Kingdom Animalia
Diploblastic and Triploblastic Organisation
Diploblastic and Triploblastic Organisation
During early embryonic development, an animal's tissues and organs are built from either two or three primary cell layers, called germ layers, and this single developmental fact predicts a great deal about the adult body.
Diploblastic animals build their body from just two layers: an outer ectoderm, which becomes the epidermis (outer body covering), and an inner endoderm, which becomes the gastrodermis lining the gut cavity. Between the two lies an undifferentiated, largely non-cellular layer called the mesoglea. Corals, jellyfish and sea anemones — the cnidarians — along with ctenophores, follow this plan. Because there is no true middle layer to build muscle and organs from, diploblastic animals are capped at tissue-level organisation and radial or biradial symmetry. …
What this figure shows. A side-by-side cross-section comparing a diploblastic body wall (outer ectoderm forming the epidermis, inner endoderm forming the gut-lining gastrodermis, with an undifferentiated jelly-like mesoglea sandwiched between them — the plan seen in corals, jellyfish, and sea anemones) against a triploblastic body wall, which adds a true middle mesoderm layer between ectoderm and endoderm. The ectoderm gives rise to skin, hair, nails and neurons; the mesoderm to muscle, bone and heart; and the endoderm to the gut lining, lungs and …