Metamerism in Annelids

Definition :-

Metamerism is a type of body segmentation in bilateral animals(such as Annelids) which involves a longitudinal division of the body into a Linear series of similar section of parts.

Each section or part is called a segment or somile or metamere. In annelids each segment is demarcated externally by an anterior and a posterior groove.

Metamerism in Animal Kingdom :- 

(i) Metamerism encountered for the first time in annelids.

(ii) Apart from this it is also found in Arthropode and vertebrate and one group of mollusca (monoplacophora).

(iii) Tape worms show pseudometamerism or strobilization, which is not true metameric segmentation.

Characteristics of Metamerism :-

(i) Each metamere repeats were some all of the various organ unit.

(ii) The organs of mesodermal origin are so arranged.

(iii) The primary segmental divisions are the body wall musculature and sometimes the coelom.

(iv) This in turn imposed a corresponding metamerism, the nerves, blood vessels, excretory organ etc.

(v) Longitudinal structures such as gut, principle blood vessels and nerves extend the entire length of the body passing the successive segments while other structures such as gonads are repeated in each or few segments.

(vi) Metamerism is always limited to the trunk region. The head or acron represented by prostomium (bearing the brain and sense organs) & the pygidium (bearing the anus ) are not metameres.

(vii) Metameres in annelids are essentially alike & complete externally & internally.

(1) Pseudometamerism Theory(Hyrnan,1951 Gardrich) :- 

(i) Firstly repetition of body parts-muscle, nerves, nephridia, coelom blood vessels occured.

(ii) Secondly cross partition developed between them and a segmented condition obtained.

(iii) Each segment received a repeated part of each system.

Evidence :- 

(i) Serial repetion of organs (testes, yolk glands, transverse nerve connectives ) are seen in elongated turbellarians.

(ii) During the formation of somiles in larval and adult stage of some annelids, cross partition develops later.

(2) Cyclomerism Theory : ( Sedgwick,1884; Remane, 1950 ) 

(i) Four gastric or enterocoelic or enterocoelic pouches are separated from central gut.

(ii) Three pairs of coelomic cavities the protocoel, mesocoel & metacoel originate by division of 2 pouches.

(iii) Gradually protocoel & mesocoel are lost, only metacoel remain. 

Example :- Mollusca

(iv) Later division of metacoel produces primary segments.

Evidence :- 

All bilateral matazoans were originally segmented and coelomate and the acoelomate unsegmented group ( flatworm, nemerteans ) have lost these characters secondarily.

(3) Fission Theory ( Perrier, 1882) :-

(i) The ancestor animal was unsegmented.

(ii) By transverse fission during asexual reproduction produced a chain of sub individuals, which remained united end. so end due to incomplete separation.

(iii) Later those sub individuals become integrated morphologically and physiologically into one complex individuality.

Evidence :- 

Occurs in some platyhelminthys and annelids today.

Opposition :-

(i) In some platyhelminthys and scyphozoans fission occurs always somewhere in the middle of the chain.

(ii) Reproduction by fission is usually confined to sessile animals, but the ancestor were probably free swimming.

Significance :- 

(i) It has provided effective locomotory mechanism coordinated contraction along body generates efficient body undulating movement.

(ii) Fluid filled coelomic compartments provide hydro static skeletons for burrowing. Different turger pressure  

are effected by flow of coelomic fluid from one part of the body to the other.

(iii) Different segments can be specialized for different functions- which leads to the development of organization. It is not clearly marked in annelids, but well developed in arthropods. example- Spermatheca, clitellium are involved with reproduction, thus regional specification of the body with proper division of labour.