Body Cavities:
It is evident from the concept of classification that is a combination of important characters is used in classification. One of the important diagnostic characteristics in the subdivision of the animal kingdom. It is the existence or non-existence of a body cavity and the type of body cavity that surrounds the gut. The body cavity is thus a cavity within the body which is distinct from the cavity of the gut or any other cavity like that of the heart.
A true body cavity always refers to a coelom or coelomic cavity which develops from the mesoderm. It can be defined as a secondary body cavity lined by mesoderm from both sides. The mesoderm outside the body cavity is known as Somatic Mesoderm and that inside the body cavity is known as splanchnic mesoderm. The somatic mesoderm lies in association with ectoderm and together is known as somatopleure. Similarly, the inner mesoderm lies in association with the endoderm and is together known as splanchnopleure.