Animals can also be grouped by the number of embryonic germ layers. Diploblastic animals, such as coelenterates, have an external ectoderm and an internal endoderm, with an undifferentiated mesoglea in between. Triploblastic animals, from platyhelminthes to chordates, have a third layer, the mesoderm, between ectoderm and endoderm. The presence or absence of a cavity between body wall and gut wall is very important. A body cavity lined by mesoderm is a coelom, and animals possessing it are coelomates, for example annelids, molluscs, arthropods, echinoderms, hemichordates and chordates. In some animals the cavity is not lined by mesoderm; instead mesoderm occurs as scattered pouches between ectoderm and endoderm. This is a pseudocoelom, found in pseudocoelomates such as aschelminthes. Animals with no body cavity, such as platyhelminthes, are acoelomates. Another feature is segmentation, where the body is externally and internally divided into segments with serial repetition of at least some organs. This is metameric segmentation or metamerism, as in the earthworm. Finally, the notochord is a mesodermally derived rod-like structure formed on the dorsal side during embryonic development in some animals. Animals with a notochord are chordates; those without it are non-chordates, such as Porifera to Echinodermata.