i. When the body cavity is absent, the animal is called acoelomate, e.g. platyhelminthes. In some animals; mesoderm is present as scattered pouches between ectoderm and endoderm. Such animals are called pseudocoelomate, e.g. aschelminthes.
ii. The notochord is a flexible rod-shaped body found in embryos of all chordates. It is composed of cells derived from the mesoderm and defines the primitive axis of the embryo. The dorsal nerve cord is a hollow cord dorsal to the notochord. It is formed from a part of the ectoderm that rolls, forming the hollow tube. The notochord is replaced by a vertebral column in higher vertebrates. The nerve cord develops into the central nervous system.
iii. A polyp is a sessile and cylindrical form whereas medusa is umbrella-shaped and free-living. Polyps produce medusae asexually and medusae from the polyps sexually (e.g. Obelia). Examples of polyps are Hydra and Adamasia whereas of medusa is Aurelia.