Evolution of the Vascular System
Eukaryotic organisms are divided according to the usual taxonomy of protozoa and metazoans. Metazoans can be classified either as vertebrates and invertebrates or as diploblasts and triploblasts. Vertebrates have an internal cartilage framework and bone, which invertebrates lack. The diploblast/triploblast distinction is made according to the presence of two or three embryonic leaflets from which tissues and organs form. Diploblasts do not possess more than two leaflets (endoderm and ectoderm), whereas triploblasts possess three (endoderm, ectoderm, and mesoderm).
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