Question

Evolutionary perspective and definition of coelom

Answer 1

The word coelom means a cavity.The coelom is the main body cavity in most animals and is positioned inside the body to surround and contain the digestive tract and other organs.

In developed animals, it is lined with a mesodermal epithelium. In other animals, such as molluscs, it remains undifferentiated.

The evolutionary origin of the coelom is uncertain. The oldest known animal to have had a body cavity was the Vernanimalcula.

The evolutionary sequence was from acoelomate to pseudocoelomate to coelomate.

The acoelomate theory states that coelom evolved from an acoelomate ancestor. The enterocoel theory states that coelom evolved from gastric pouches of cnidarian ancestors. This is supported by research on flatworms and small worms recently discovered in marine fauna.

The difference between a pseudocoelomate and a coelomate animal is the absence or presence of an epithelial lining or peritoneum which is present in a coelomate animal and absent in a pseudocoelomate animal.

The embryological origin of a true coelom varies.  If it develops from a split in cells of the mesoderm (the middle layer of cells in an embryo, it is said to be schizocoelous.  If it develops from out-pocketing from the embryonic gut it is said to be enterocoelous.