Coelom:
Coelom 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.
The animal groups which do not possess any coelom or true body cavity are known as acoelomate and the groups which possess coelom or true body cavity are known as coelomate.
Function of Coelom:
During gastrulation three primary germinal layers develop in the embryo. These layers are:
- Ectoderm
- Mesoderm
- Endoderm
Coelom develops from this mesoderm. Mode of coelom formation is of two types –
1. Enterocoelic Coelom Formation:
In Enterocoelic Coelom Formation, mesodermal pouches develop on either side of the primitive gut or archenteron of the embryo. The gut is endodermal in origin. During differentiation of the gut, the mesodermal pouches pinch off from the gut, come side by side, and ultimately fuse. The cavity that lies within the fused mesodermal pouches is the coelom.
As this type of coelom primarily develops from the diverticular of the archenteron, it is known as Enterocoelic coelom. It is found in Amphioxus, a lower chordate and start fish, an echinoderm.
2. Schizocoelic Coelom Formation:
In Schizocoelic Coelom Formation, the mesoderm segregates as a band between the ectoderm and endoderm. This band of mesoderm is known as mesodermal cords. Coelom or Body cavity develops by splitting the mesoderm into an outer somatic layer and an inner splanchnic layer.
This coelomic space is filled up with a fluid that is known as coelomic fluid and coelom is known as schizocoel. Coelom formed in this way is known as schizocoelic coelom formation It is a large cavity found in a number of invertebrate animals like earthworms and in all vertebrates.