Non Chordates:- Unit 1- Introduction To Coelomates

 EVOLUTION OF COELOM

Body cavity - A body cavity can mean any internal space or a series of spaces present inside body. 

Coelom - The coelom is defined as a liquid filled cavity in the mesoderm. It develops in, and is surrounded by the embryonic mesoderm. 

Pseudocoelom - The body cavity is formed by a persisting of the blastocoel. This is simply a space between the digestive tract and that part of the body wall not bounded by mesoderm.

According to the nature of body cavities there are three major groups of triploblastic animals
↳ Acoelomata
↳ Pseudocoelomata
↳ Coelomata or eucoelomata

1) Acoelomata
No body cavity or coelom is present. Embryonic mesoderm remains as a solid layer, space betweeen endoderm (gut wal) and ectoderm (bodywall) is filled with mesenchyme and muscle fibres.

Ex. Porifera, Coelenterata, Ctenophora, Platyhelminthes and Nemertinea 

 

 2) Pseudocoelomata
Body space is a pseudocoelom or false coelom. It is a persistent blastocoel enclosed between outer ectoderm and inner endoderm and not lined by mesoderm.

Ex. Ectoprocta, Aschelminthes 

3) Coelomata or Eucoelomata 

Body space is a true coelom, enclosed by mesoderm on both sides. 

Ex. Phyla of Bilateria (Annelida to Arthropoda)

 Types of Coelom

a) Schizocoelom
Coelom arises by a splitting of endomesodermal bands which originate from blastoporal region of larva and extend betweeen ectoderm and mesoderm. It is a true coelom called schizocoel.

Ex. Prostomia (Annelida, Arthropoda) 

 

b) Mesenchymal coelom
It is seen only in Phoronide in which mesenchymal cells re-arrange to enclose a space or coelom, which is regarded an aberrant schizocoel. 

 

c) Enterocoelomata
Coelom arises in the form of mesodermal pouches from larval archenteron. After seperation from endoderm, the pouches fuse and expand until they touch the gut and bodywall. Since the coelon arises from larval enteron, it is called enterocoel.

Ex. Deuterostomia (Chaetognetha & Echinodermata)
      Branchiopoda

 

Significance of Coelom

•Plays an important role in the progressive development of complex structures 

•Permits the greater size and contributes directly to the development of excretory, reproductive and muscular systems.

Advantages of coelom in truploblastic organisms

•It act as a water jacket.

•Provide flexibility to the body. Provide space and gives the visceral organs freedom of movement and opportunity for enlargement, further differentiation and greater activity. 

•Act as hydraulic skeleton. It serves as circulatory medium for the transport and distribution of nutritive substances and gases.

•The excretory matter is collected into coelomic fluid then passed out of the body through nephridia.

•The gonads arises from its coelomic epithelium and project into coelom. Ova and sperms are
extruded through special gonoducts connecting the coelom with the exterior.

Theories of evolution of coelom

1) Enterocoel theory

First proposed by Lankester (1875)

Later modified by Sedwick (1884)

Recently modified by Hartman & Ramne (1963)

•According to this theory, the bilateral metazoan ancestor of coelentrates has gastric pockets which become seperated from the central digestive carity to form coclomic pouch.

Objections 

1) Gastric pouches only occur in more advanced cniderians not very suitable for ancestral type.

2) Sealing-off of the gastric pouches defeats the object for which they were formed i.e., for increasing the surfacce area for digestion and absorption.

3) Supporters of this theory tend to associate the evolution of coelom with that of metameric segmentation, an association imposing severe restrictions on the theory.

4) Grastric pouches in cridarians are not formed as the evagination of endoderm, but formed differently by ingrowth of body wall-septa.

 2) Gonocoel Theory
                      By Bergh (1885)
This is the most popular theory of origin of coelom. According to the theory, coelom represents a persistent expanded gonadial cavity or gonocoel. 

Objections 
1) Links the origin of coelom with metameric segmentation and not accounted the unsegmented coelomates.
2) It regards as seperate processes the formation of endomesoderm from the inward migration of gonadial cells and the formation of the coelom by cavitation of the gonads after the release of gametes. This is not supported by embryology.

 

3) Nephrocoel Theory
             Proposed by Lankester (1874)

According to this theory,
i) Coelom is originated as an expanded nephridium.
ii) Chief objections
     • Presence of protonepheidia in coelomates
     • Absence of excretory organs in coelomates
        (eg. Echinoderms)

4) Schizocoel Theory
i) According to this theory, coelom is regarded as a new formation from mesoderm.
ii) It is mesenchymal origin, and the formation of coelom is not related to gonads or
endodermal pouches and is not present in lower metazoans.

Conclusion
None of the theories is entirely satisctory for three main reasons:-
1. These theories largely ignore the intermediate stages.
2. Relationship between the evolution of the coelom and the evolution of metameric segmentation have been explained but failed to explains the evolution of unsegmented coelomates.
3. No clear statement given concerning the exact nature of a coelom and explaining which cavities should be regarded as coelomic and which is not, in specific cases.

METAMERISM

1) When the segmentation in bilateral animals, such as annelida, involves a longitudinal division of the body into a linear series of similar sections or parts, it is termed metameric segmentation or metamerism. Each section or part is called a segment, somite or metamere.

2) Each metamere typically repeats some or all of the various or organ

3) The head (or acorn) represented by the prostomium and bearing the brain and sense organs, and the pygidium, represented by the terminal part of the body which carries the anus, are not metemeres.

4) New segments arise just in front of the pygidium; thus the oldest segments lie just behind the head.

5) Metameric segmentation seems to have evolved three times independently in animal kingdom:-
i) in the annelids and arthropods
ii) in the chordates
iii) cestodes 

* Homonomous metamerism   

In annelids, metameres are essentially alike or homonomous, each having segmental blood vessels, nerves, nephridia and coelomoducts. This condition is called homonomous metamerism.

* Heteronomous metamerism

Higher animale such as arthropods and vertebrates, show incomplete metamerism.
Because of division of labour, the segments or metameres of different regions of their body become greatly dissimilar. This is called heteronomous metamerism.

* Obscuring of metamerism

1) In arthropods and vertebrates,  metamerism is more complete and metameres are uniform
and clear in the larval and embryonic stages. But metamerism becomes obscure in the adult due to subsequent specialisation or modification, so that the segments are no longer similar.

2) It may result from simplification, by loss of metameres, by fusion of segments (cephalisation), by differentiation betweeen segments, by disappearance of organs, or by development of other structures, such as limbs.

3) Heteronomous condition always appears first at the anterior end and progresses posteriorly. In segmented animals, varying degrees of specialization are met with some of which are extereme. 

Theories of origin of evolution of metamerism 

1. Pseudometamerism theory
  Supported by Hyman (1951) and Goodrich 

(i) This theory stresses that metamerism developed secondarily as a result of repetition of body parts (muscles, nerves, nephridia, Coelom, blood vessels etc.) in a single individual.

(ii) The serial repetition of organs is seen in some elongated turbellarians and nemerteans. Later, a segmented condition was obtained by the formation of cross-partitions in between them. 

(iii) This process of development of cross-partitions after basic segmentation was probably an adaptation for an undulatory mode of swimming, in larval and adult stages of some annelida. 

(iv) However, all ribbon-like animals swim in this way, whether segmented or not.

2) Cyclomerism theory
  Proposed by Sedgwick (1884) and greatly supported by Remane (1950, 1963)

• This theory assumes that coelom originated in some ancestral actinozoan coelenterate, through the seperation of four gastric or enterocoelic pouches from the gut.

• Division of two pouches resulted into three pairs of coelomic cavities, the protocoel, mesocoel and metacoel, in the protocoelomate or ancestral coelomate.

• Loss of protocoel and mesocoel led the unsegmented coelomates, such as molluscs, and sipunculids. Later subdivision of metacoel produced primarily segments, leading to the segmented annelids.

• The phylogenetic simplication of this theory is that all bilateral metazoans originally segmented and coelomate, the acoelomate unsegmented groups (flatworms, nemerteans) have lost these the characters secondarily.