or coelom is important in classifying animals. Most animals possess a body cavity between the body wall and the alimentary canal, and is lined with mesoderm (Figure . ) Animals which do not possess a body cavity are called acoelomates . Since there is no body cavity in these animals their body is solid without a perivisceral cavity, this restricts the free movement of internal organs.
(e.g., Flatworms) In some animals, the body cavity is not fully lined by the mesodermal epithelium, but the mesoderm is formed as scattered pouches between the ectoderm and endoderm. Such a body cavity is called a pseudocoel and is filled with pseudocoelomic fluid. Animals that possess a pseudocoel are called pseudocoelomates e.g., Round worms. The pseudocoelomic fluid in the pseudocoelom acts as a hydrostatic skeleton and allows free movement of the visceral organs and for circulation of nutrients.
Eucoelom or true coelom is a fluid- filled cavity that develops within the mesoderm and is lined by mesodermal epithelium called peritoneum. Such animals with a true body cavity are called coelomates or eucoelomates . Based on the mode of formation of coelom, the eucoelomates are classified into two types, Schizocoelomates – in these animals the Figure. .
Radial and Pentamerous radial symmetry Radial symmetry in sea anemone Pentamerous radial symmetry in starfish XI Std Biology-Zoology Chapter- body cavity is formed by splitting of mesoderm. (e.g., annelids, arthropods, molluscs). In Enterocoelomate animals the body cavity is formed from the mesodermal pouches of archenteron. (e.g., Echinoderms, hemichordates and chordates) (Figure .
). Development of Schizocoelom Ectoderm Gut Endoderm Early mesoderm cells Split in mesoderm Developing coelom Development of Enterocelom Early mesoderm cells Gut Ectoderm Endoderm Separation of pouches from gut Developing coelom Figure . Development of Schizocoelomata and Enterocoelomata found in Annelids in which each unit of the body is very similar to the next one. But in arthropods (cockroach), the segments may look different and has different functions.
Animals which possess notochord at any stage of their development are called chordates. Notochord is a mesodermally derived rod like structure formed on the dorsal