Evolution of Coelom and Metamrism

 What is Coelom?

The coelom is the fluid filled body cavity present between alimentary canal and the body wall.

Coelom is formed during embryo development from three germinal layers. Coelom is lined by mesodermal epithelium cells.

Occurrence: Annelida, Arthropods, Mollusca, Echinodermata, Hemichordata, chordata.

Important features of Coelom: 

according to the developmental theory coelom arises as split in the mesoderm which becomes bifurcated into two layers, a  somatic layer lines to the epidermis and the other layer is located around the endoderm.

Coelom becomes bounded on all sides by coelomic epithelium which secrets coelomic fluid.

Greater part the coelom form perivisceral cavity or Spanchnocoel.

Types of Coelom: There are three types of Coelom name as

1. Acoelom.

2. Pseudo coelom.

3. Coelom.

Acoelom: It means without coelom or fluid.  filled cavity is absent.

The space between gut and body wall  is filled by a kind of densed packed  connective tissue derived from both ectoderm and endesoderm called parenchyma.

Animals are without body cavity in  triploblastic animals called acoelomates and the group is known as acoelomata.

Example: Porifera, Ctenophora, Platyhelminthes.

Pseudocoelom: 

That means false cavity. The fluid filled cavity lying between the gut and other body wall musculature and generally formed by persistence of embryonic blastocoel is called pseudocoel. The body cavity is bounded externally by the fibrous process of the  longitudinal muscle cells (mesoderm ) and internally by the intestine  (endoderm).

Pseudocoelom  fluid act as hydrostatic skeleton to maintain body shape and circulate nutrients.

Animals that contains pseudocoel are called Pseudocoelomate.

Example: Rotifera, Nematoda and Loricifera.

Coelom or Eucoelom:

It is true coelom lying between the gut and outer body wall masculature and lined by coelomic epithelium derived from the embryonic mesoderm. It is mesodermal origin and opens to exterior through the coelomoducts. The coelomic fluid contains amoeboid or amoebocytes. The animals containing such a body cavity or coelom are called coelomate animal.

Example: Mollusca, Annelida, Arthropoda, Hemichordata, Chordata.

Mode of Coelom formation:

1. Schizocoely: Coelom arises by splitting of mesodermal bands or masses during embryonic development.

Example: Protostomia ( Mollusca, Annelida, Arthropoda) 

2. Enterocoely: The process in which coelom is formed by the evagination from the embryonic archenteron. pouch like structure detached from archenteron and gradually occupy the whole body by enlargement.

Example: Deuterostomia ( Echinodermata, Hemichordata, Chordata )

Origin and evolution of Coelom:

Enterocoel theory: ( Lankester in 1877 )

This theory states that coelom evolved from the gastric pouch of some Cnidarian ancestors such as anthozoans. Gastric pouches separated out from the main gastric cavity to form the coelomic pouches. The theory explain that all bilateral animals are generally coelomate and acoelomate like flatworms are secondarily derived from coelomate ancestors by the loss of cavity. Enterocoelous mode of coelom formation in the embryology of echinoderms, Hemichordates and Chordates is the main  evidence of this theory.

Gonocoel theory: ( Hatschek, 1877 )

Coelom is originated from the cavity of an expanded gonad and its origin is based on the common association between gonad and the coelomic epithelium.  For example, the gonads of tricladid flatworms are arranged in  a linear order and the segmental coelom of Annelida may have developed from this tricladid.

Nephrocoel and Schizocoel Theory:

Nephrocoel: Coelom originated from the  expanded nephridia of flatworms. Chief objection of this theory is that the protonephridia heve not recorded in all Coelomates, even the Echinoderms do not have excretory organ.

Schizocoel: ( Clark, 1964) 

The main statement of this theory is that the Coelom is originated by the splitting of mesoderm.

Significance of Coelom:

1.It serves transport of gases, nutrients, and waste products around the body.

2. It allows storage of sperm and eggs during maturation and act as a reservior for Waste.

3. The Coelom filled with coelomic fluid act as a hydrostatic skeleton helps in locomotion locomotion.

4. Coelomocytes helps in the course of immune system.

Metamerism: 

Metamerism is an architectural body plan in some animals in which the similar body segments and organ system are serially repeated one after another. The similar segments are referred as metamere or Somite.

Animals which exhibits such characteristic feature in their body called metamerically segmented.

Occurrence:  

Atfirst metamerism is seen in phylum annelida. Each segment generally the adds appendages muscles nerves blood vessels excretory organs and a pair of silver micset. Metamerism also seen in arthropods and most chordates.

Important features of metamerism:

1. Metamerism is always confined to the intermediate segments ( trunk ) except anterior Acton ( head region ) and a posterior Pygidium ( Telson ) region.

2. Each metamere or somite represents a mirror image each to other.

3. All segments of body work in coordination.

4. They are integrated to perform a single functional unit.

5. The segments are structurally interdependent on each other.

6. Each segment contains blood vessel, excretory organ, muscle, nerves.

Types of metamerism in different groups.

Homonomus Metamerism:

The condition in which the segments or somites of the animal are all alike. The segmentation is called homonomus metamerism. It is  seen in any Annelida. This is a primitive type of segmentation and is not occur in any existing animal because a few anterior segments are specialised to form head region which is known as Cephalization.

Example: Annelida.

Heteronomous Metamerism: 

In case of Arthropods  and Chordates the segments of the body are totally dissimilar in different body regions in single animals called Heteronomous Metamerism.

Example: 

Arthropods and Chordates.

Complete metamerism:

While the segmentation is occur practically in all system of the body is known as complete metamerism.

Example: Annelida.

Incomplete metamerism:

Intestine is not seen in all the organs of the body is called incomplete metamerism.

Example: Arthropods and chordates.

True metamerism:

When segmentation of the body developed by the segmentation of the mesoderm.

Example: Annelids, Arthropods and most chordates. 

The body of animals consists of a number of segments and the number remain constant in a  particular species except in certain cases of sexual mode of reproduction. New segments are not added to the body after become maturation.

Pseudo- metamerism or Strobilization:

When the segmentation of the body takes place by the segmentation of the ectoderm. The body consists of a number of segments or Proglottids which varies different individuals of the same species.

Cyclomerism theory: 

This theory was proposed by Sedgwick in 1884 and supported by Ramene in 1950.

 This theory states that coelom originated in  some ancestral  Anthozoan Coelenterates, through separation of gastric pouch from the central digestive cavity. Initial two pouches resulted in three pairs of cavities such as Protocoel, Mesocoel, Metacoel. Then loss of protocoel and Mesocoel led to unsegmented coelomate like Molluscs. Next time the sub division of Metacoel produced primary segments, leading to arising of segmented Annelids.

Locomotion Theory:

 The theory was proposed by R.B  clark in 1964 to explain origin of metamerism.

This theory assumed that Metamerism evolved in phylum Annelida due to adaptation to the peristaltic locomotion and for burrowing in chordates. Metamerism is involved with strong undulatory serpentine swimming. In Annelids peristaltic movement involves shortening and lenghting of the body by circular and longitudinal muscle which act opposite each to other.