Coelom: The body Cavity

Coelom: Any space between body wall and alimentary canal lined by mesodermal cells is called coelom. Or A coelom is a hollow, fluid-filled cavity found in living beings, where it acts as a protective cushion for their internal organs. In some animals, such as worms, the coelom acts as a skeleton. The coelom also allows the internal organs to move and grow independently of the outer layer of the body wall.
Special Features of Coelom:
1.Developmentally coelom arises as a split in
the mesoderm which becomes bifurcated into
two layers, a somatic layer lying next to the epidermis and a splanchnic layer around the endoderm.
2. Coelom becomes bounded on all sides by coelomic epithelium which secretes coelomic fluid.
3. The greater part of the coelom forms the peri-visceral cavity or splanchnocoel.
4. It is a fluid-filled space inside the viscera.
5. Because of this packing the viscera remains independent of the movements of the muscles of body wall.
6. In some higher animals part of the perivisceral
cavity is kept separate to form restricted cavities
whose coelomic nature can and only be realized if their developmental history is followed.
7. In all probabilities the ancestral coelomate animals had segmentally arranged mesodermal pouches.
8. From these pouches gametes were formed by the process of proliferation of the epithelial lining.
9. Later on, these pouches became modified in structure and function.
10. The evolution of mesodermal pouches is evident in present day coelomates.
Types of Coelom:
There are three types of coelom
(i) Acoelom: (Gr. A-absent, coelom means coelom) It means without a coelom or body cavity is absent. The space between the gut and body wall is filled by a kind of densely packed connective tissue called parenchyme or mesenchyme. Triploblastic animals without a body cavity are called as acoelomates and the group is referred to as acoelomata. Examples: Platyhelminthes and Nemertea, Gastrotricha, Kinorhyncha.
(ii) Pseudocoelom: It means false cavity.
The fluid-filled body cavity lying between
the gut and body wall musculature, not lined by mesoderm and is generally persistent blastocoel is called pseudocoel.
The term ‘pseudocoelom’ usually refers to the space which does not develop from embryonic mesoderm and not lined by coelomic epithelium derived from the mesoderm.
The body cavity is bounded externally by the fibrous processes of the longitudinal muscle cells (mesoderm) and internally by the intestine (endoderm). Examples: Aschelminthes
(iii) Enterocoelom or Eucoelom:
It is a true coelom lying between the gut and outer body wall musculature and lined by coelomic epithelium derived from the embryonic mesoderm. It is a mesodermal origin and opens to the exterior through the coelomoducts. Examples: Echinodermata, Chaetognatha, Hemichordata and Chordata.
(iii) Enterocoelom or Eucoelom:
It is a true coelom lying between the gut and outer body wall musculature and lined by coelomic epithelium derived from the embryonic mesoderm. It is a mesodermal origin and opens to the exterior through the coelomoducts, e.g., the oviducts and the excretory ducts. The coelomic fluid contains amoeboid cells or amoebocytes. The animals containing such a body cavity or coelom, called coelomates. Examples: Echinodermata, Chaetognatha, Hemichordata and Chordata.
Origin of coelom
1.Enterocoel theory— Sedgwick (1884) suggested that the gastric pouches of anthozoans (Phylum Coelentrata) became separated from the main gastric cavity (gastrovascular cavity) and were transformed into coelomic pouches.
2. Gonocoel theory : The origin of coelom in favour of gonocoel theory is that first coelomic cavities arose from the mesodermally derived expanded gonadal cavities and the cavities persisted after the release of gametes. For example, the gonads of tricladida flatworms are arranged in a linear order and the segmental coelom of annelida may have developed from this tricladid.
3. Nephrocoel theory (Lankester, 1874, Snodgrass, 1938):The theory states that the coelom originated from the expanded nephridia of flatworms. The chief objection of this theory is that the protonephridia have not recorded in all coelomates, even the echinoderms do not have excretory organs. So this theory was discarded by scientists.
4. Schizocoel theory (Clark, 1964):
The theory states that the coelom could have evolved by the splitting of mesodermal plates which in turn made up of 4D or teloblastic cells of gastrula. This theory was rarely accepted by some scientists.

Видео Coelom: The body Cavity канала Dr. B. M. Singh