What is Metamerism? Types, Theories and Significances.

 

What is Metamerism? Types, Theories and Significances.

Metamerism: Meta- after + meros- part.

Repetition of internal body parts is called metamerism. Each segment is called metamere or somite e.g. annelids, arthropods, molluscs and chordates.

Metamerism in Animals: Diagram

In bilateral animals like annelids, the longitudinal division of the body trunk into a linear series of similar sections or parts is termed metameric segmentation or metamerism. Each part or section is called a segment, somite or metamere.

In annelids, the body is externally divided by annuli and internally by septa. The external division corresponds to the internal division and is called metameric segmentation.

In adult chordates, internal segmentation is exhibited by repetition of ribs, vertebral nerve, muscles and blood vessels. In arthropods, only external metamerism is present in the form of pseudo-metamerism or false segmentation because segmentation is not of embryonic origin.

* Metamerism was recognized for the first time in annelids.

* A group of mollusca, monoplacophora also exhibit metamerism.

* The term metamerism is applied only when organs of mesodermal origin are so arranged. 

Primarily the segmentation is in the bodywall musculature and sometimes the coelom. 

This then imposes metamerism on the corresponding associated supply systems (nerve, blood vessels and excretory organs). 

Longitudinal structures such as the gut,  principal blood vessels and nerves extend entirely throughout the length of the body. The structures such as gonads are repeated in a few or each segment. 

Metamerism is always limited to the trunk region of the body. 

The head (or acorn)  is represented by the prostomium bearing the brain and sense organs and the pygidium is represented by the terminal part of the body which carries the anus. It does not show metameres. 

New segments arise in front of the pygidium such that the oldest segments lie just behind the head.

Types of Metamerism:

• Depending on the mode of repetition, and 
• which part of the body is repeated, metamerism is of the following types,

External and Internal Metamerism:

Metamerism is visible in most of the annelids both externally and internally. e.g. earthworm. 

Its body consists of a great number of similar segments and all the body organs such as musculature, setae of locomotion, blood vessels, nerves, ganglia, excretory organs and gonads are repeated segmentally.

The digestive tract remains unaffected, although it extends through every segment.

In arthropods, metamerism is chiefly external.

 In humans and other vertebrates, internal metamerism of body muscles, nerves, certain blood vessels, vertebrae and ribs are seen.

Complete and Incomplete Metamerism:

In annelid worms, metamerism is complete involving all the systems. The metameres are alike or homonomous, each having blood vessels, nerves, nephridia and coelomoducts. This condition is called homonomous metamerism.

In higher animals like arthropods and vertebrates, the metamerism is of incomplete type because the segments or metameres of different regions of the body are dissimilar. This type of metamerism is called heteronomous metamerism

Also, in these animals, the metamerism is more clear and complete in larval and embryonic stages. However in adults, due to specialisation or modification, the segments no longer remain similar. This can happen,

• by fusion of segments, 
• by the differentiation between segments, 
• by the disappearance of organs or 
• by the development of other structures such as limbs.

Heteronomous conditions always appear at the anterior end and progress posteriorly.

Theories of Metamerism: 

Various hypothesis have been proposed to explain the origin of metamerism but none is acceptable in the absence of convincing evidence.

Theories mostly emphasize either on repetition of organs or Mesodermal segmentation.

1. Pseudo metamerism theory:

According to this theory, metamerism developed as a result of the repetition of body parts such as muscles, nerves, nephridia, coelom, blood vessels etc in a single individual.

Serial repetition of organs like testes,  yolk glands and transverse connectives of two nerve cords, were seen in elongated turbellarians and nemerteans. This then changed to segmented condition by the formation of partitions between them resulting in segments having repeated parts of each system. This process still occurs in some annelids.

This type of segmentation must have been developed as an adaptation for an undulatory mode of swimming. (but this is also witnessed in ribbon-like animals, whether segmented or not).

This theory was supported by Hymen (1951) and Goodrich.

2. Cyclomerism theory:

Originally proposed by Sedgwick (1884) and supported by Remane.

This theory assumes that coelom originated in some ancestral radiate actinozoan coelenterate, through the separation of four gastric or enterocoelic pouches from the central digestive cavity or gut.

• The division of two pouches formed three pairs of coelomic cavities i.e. the protocoel, the mesocoel and the metacoel, in the protocoelomates or ancestral coelomates.
• Loss of protocoel and mesocoel led to unsegmented coelomates such as molluscs and sipunculids.
• Later on, the metacoel was subdivided producing primary segments which led to the segmented form of annelids.

  According to this theory, all bilateral metazoans were originally segmented and coelomate and the unsegmented groups like flatworms and nemerteans have lost these characteristics secondarily.

3. Corm or fission theory: 

According to this theory, metameric segmentation might be a result of the unsuccessful separation of a chain of sub-individuals or zooids which were attached end to end, after division by transverse fission repeatedly or by asexual budding.

This individual with time developed into a complex individual.

Thus, according to this theory, a segmented animal is a chain of completely coordinated sub-individuals.

Perrier ( 1882), supported the theory but the main objections were,

•  The lack of gradation of age of the chain of zooids ( such as in annelids)
• In the case of Platyhelminthes Scyphozoanzoan strobilae, the sequence of the zooid formation is never serial (fission can occur anywhere in the middle).
• In cestodes, the proglottids are serially arranged but in a reversed order.
• Also,  reproduction by fission occurred in sessile animals,  but they were probably free swimming.

4. Embryological theory:

According to this theory, metamere metameric segmentation is an embryological accident.

Segmental repetition must have resulted in the adults, due to fragmentation of the mesodermal derivatives in the elongating embryo or larva because of some mechanical stress.

5. Locomotory theory:

It is a combination of the postulates of pseudo-metamerism and embryological theories implying that metamerism evolved through adaptation to different types of locomotion. Likewise,

• In annelids through burrowing, 
• In chordates from the undulatory, serpent in swimming movement.
• In cestodes, it evolved as a response to reproduction.

R.B. Clark ( 1964 ), explained that the ancestors of annelids must have been elongated coelomate animals which then evolved as compartmented coelom by the development of septa and metameric segmentation which localised the function of hydraulic skeleton.

Locomotory movements can be better controlled if body wall muscle action is localised and metamerism permits it, aiding to evolution of this condition in annelids. The localisation produced a strong peristaltic wave moving down the body, giving the worm its burrowing ability.

The segmentation of coelom and body wall muscles led to the subsequent segmental organisation of the nervous, circulatory and excretory systems.

According to Berrill (1955), In the case of chordates, metamerism evolved as an adaptation for undulating swimming movements, 

where the notochord gave support to the body followed by metameric segmentation of muscles of the body wall. Thus, alternate waves of contraction flowing down the body could enable strong swimming.

The nervous and circulatory systems segmentation was caused by the initial muscle contraction.

In cestodes, metamerism evolved as a response to reproduction. After reproduction the whole reproductive package was expandable and the proglottids carried the complete series of organs.

Conclusion :

 So it was concluded that the evolution of metamerism in a phylum might not be similar to others. It probably arose independently giving a major advantage to the group of animals in concern.

Significance of metamerism:

The biological significance of metamerism is indicated by the fact that the most successful protostomes, the arthropods and the most successful and advanced deuterostomes, the vertebrates, both have metameric body plans.

• Metamerism especially benefits burrowing annelids by subdivisioning the coelom crosswise into many compartments. This makes a part more rigid by localised contractions of segmental muscles against the coelomic fluid, which cannot be moved as it gets trapped in each segment because of Septa ( transverse membranous partitions). This improves the efficiency of the annelidan hydrostatic skeleton which allows terrestrial forms like earthworms to penetrate quite resistant soils.
• Another importance of the metameric body plan is suggested by the appearance of metamerism in planktonic larvae before settlement. The pelagic polychaetes show swimming by serpentine lateral body undulation ( mode of locomotion) that benefits from a segmental anchorage of trunk muscles and a compartmentalized hydrostatic skeleton. The back sweeps of the oar-like parapodia are synchronised with the body undulations during swimming. This connects the effective strokes with the localised trunk rigidity.
• Furthermore, during the development of the metameric body plan, growth and complexity can be achieved by the serial repetition of the basic organisation as a whole set. As in the case of annelids, each such segment includes a pair of nephridia, a paired ganglion giving off segmental nerves, a set of segmental blood vessels, metameric muscles running from the septum to septum, a set of chitinous setae and a pair of parapodia appendages.

• Some annelids reproduce asexually by budding, so the addition of a segment by budding would result in the repetition of such a basic set of genetic instructions, as in the case of,

Example: Bristleworm, Dodecaceria, fragments its central trunk region into individual metameres each of which in turn buds off four young worms before dying.

• In most annelids, metamerism is homonomous, such that successive trunk segments and their appendages are identical. This suggests that the same pattern of gene regulation and expression results in forming a new trunk segment except for the nonmetameric anterior portions of the digestive tube.
• The homonomous metameric condition seems to be necessary for the evolutionary diversification of body segments and appendages as in the case of polychaetes annelids but can be marked to a much greater extent in arthropods.

( let us make it short and in points for a short note on the "Significance of Metamerism".)

1. Metamerism provided an effective locomotory mechanism generating efficient body undulating movement.

2. The development of repeated body organs in segments gave scope for evolution.

3. The study of metameric segmentation can reveal the primitiveness of an animal as metamerism resulted in greater complexity in body organisation.

4. The fluid-filled coelomic compartments help in borrowing movement by providing a hydrostatic skeleton giving rigidity to burrow hard soil and also making swimming more efficient showing advancement over ciliary or creeping movement found in lower invertebrates.

5. Different segments can be specialised for different functions providing regional specification of the body and proper division of labour.

Multiple Choice Questions (MCQs)

1. Metamerism first appeared in,

a. Arthropoda 

b. Mollusca 

c. Annelida 

d. Chordata

Answer: (c) Annelida 

2. Each body segment in an led is called a, 

a. Somite

b. Seta

c. Parapodium

d.Tagma

Answer: (a) Somite 

3. Metamerism in annelids is,

a. External only 

b. Internal only 

c. Both external and internal

d. None

Answer: (c) Both external and internal 

4. True metamerism occurs in,

a. Tapeworm 

b. Ascaris 

c. Earthworm 

d. Hydra

Answer: (c) Earthworm 

5. Body segments in pseudometamerism,

a. Work independently 

b. Are coordinated

c. Fuse together 

d. None

Answer: (a) Work independently 

6. The repeated body segments in annelids are separated by,

a. Mesenteries 

b. Septa

c. Tagma

d.None

7. Homonomous metamerism means,

a. Dissimilar segments 

b. Similar segments 

c. Modified segments 

d. Fused segments 

Answer: (b) Similar segments 

8. The process of fusion of body segments for specialisation is, 

a. Metagenesis

b. Cephalization

c. Tagmatization

d. Metamorphosis 

Answer: (c) Tagmatization 

9. Heteronomous metamerism is exhibited by,

a. Earthworm 

b. Leech

c. Cockroach 

d. Nereis

Answer: (c) Cockroach 

10. Metamerism provides advantage in, 

a. Locomotion and flexibility 

b. Digestion 

c. Reproduction 

d. Respiration 

Answer: Locomotion and flexibility 

Know about Nephridia in Annelids ( an important question being answered )

Nephridia: Types, Function and Physiology of Excretion in Annelids