Biology · Ch 4 — Animal Kingdom
Diploblastic and Triploblastic Organisation
Diploblastic and Triploblastic Organisation
Another fundamental character used in classification is the number of germ layers — the embryonic cell layers from which the whole body develops. On this basis animals are either diploblastic or triploblastic.
Diploblastic animals build their bodies from two embryonic layers: an outer layer called the ectoderm and an inner layer called the endoderm. Between these two layers lies an undifferentiated, non-cellular jelly-like layer called the mesoglea. The mesoglea is not a true germ layer and its cells are not organised into tissues; it simply fills the space between the ectoderm and endoderm. Coelenterates are the typical example of diploblastic animals. …
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This pair of sections compares how many cell layers form an animal's body during early development. In the diploblastic plan, the body wall has just two layers: an outer ectoderm and an inner endoderm, with a jelly-like non-cellular mesogloea packed between them. Coelenterates are built this way. In the triploblastic plan, a third true cellular layer, the mesoderm, develops between the ectoderm and endoderm. This mesoderm gives rise to muscles, blood, bone and many internal organs, allowing a far more complex body. All animals from the flatworms (Platyhelminthes) o …