Tamralipta Mahavidyalaya

PG Department of Zoology
UG Semester II

CORE COURSE III

NON-CHORDATES II: COELOMATES
Unit I: Introduction to Coelomates
METAMERISM
By Sougata Kumar Ghosh
Meaning of Metamerism:
Metameric segmentation or metamerism is an architectural body plan in some
animals in which the similar body segments and organ systems are serially
repeated one after another. The similar body segments are called metameres
or somites.

The animals which exhibit such features called metamerically segmented.

Structurally each meta-mere or somite is constructed on the basis of some
fundamental plan and usually possesses a part of almost all the body systems.

In all respects the segments forming an individual are identical in structure

and the organs they contain or in other words, the individual is formed by
rhythmic repetition of segments which are similar both externally and
internally. This repetition is often disturbed by simplification, by coalescence
of segments or by .differentiation between the segments.

Occurrence of Metamerism:
Metamerism is first seen in annelids in animal kingdom. Each segment usually
contains appendages, muscles, nerves, blood vessels, excretory organs and a
pair of coelomic sacs which are repeated in almost all segments. It is also seen
in kinorhynchs, arthropods and most chordates.

A recent view is that the cestodes are metamerically segmented but the
metamerism of these animals is of different type. Most of the scientists believe
that metameric segmentation appears independently at least three times.
These are:
(i) Annelid and arthropod line,

(ii) Chordate line and

(iii) Cestode line.

Characteristic Features of Metamerism:

1. Metamerism is always confined to the intermediate (trunk) segments except
the anterior acron (head) and a posterior pygidium or telson.

2. Each metamere represents a mirror image of the other.

3. Segmental structures are interdependent on each other.

4. They are integrated into a single functional unit.

5. All the segments of body work in coordination.

Types of Metamerism:
The metamerism in different groups is divided into the following
types:
1. True Metamerism:
The true metamerism is one in which the segmentation of the body develops
by the segmentation of the mesoderm. It occurs in annelids (Fig. 17.14),
arthropods and in most chordates. The body of annelids consists of a number
of segments and the number remains constant in a particular species except in
certain cases of asexual reproduction. New segments are not added to the
body after maturation.
That means after the embryonic stage all the segments become the same age.
Moreover, the segmental structures are interdependent and integrated so that
the individuality of the body is preserved. New segments arise at the posterior
end in front of the pygidium. Hence, newer segments occur at the posterior
end and older segments remain just behind the head.

2. Homonomous Metamerism:
If the segments or somites of the animal are all alike, the’ segmentation is
called homonomous metamerism. It is seen in annelids (Fig. 17.2). Each
metamere contains segmental blood -vessels, nerves, nephridia and
coelomoducts. This is a primitive type of segmentation and is not found in any
existing animal because a few anterior segments are specialised to form the
head which is called cephalization.
A well-defined and well-organised head is lacking in annelids. However,
formation of a ‘head’ is suggested in polychaetes by anteriorly placed struc-
tures and their association with parapodial cirri.

In some polychaetes transformation of one or two post-oral segmental

parapodia into protostomial cirri has occurred. This transformation is
accompanied by a shift of these post-oral segments and their ganglia anterior
to mouth (pre-oral), resulting some sort of a brain formation. Cephalization in
true sense is absent in annelids.
3. Heteronomous Metamerism:
In arthropods and chordates the segments of the body are dissimilar in
different body regions and restricted only to certain organs. This type of
metamerism is called heteronomous metamerism (Fig. 17.49).

4. External Metamerism:
In arthropods, the metamerism is external. Internally the segments are not
marked by partitions.

5. Internal Metamerism:
In vertebrates there is internal metamerism, seen in the embrayos and
confined to the muscular, skeletal (vertebrae and ribs) and nervous system.

6. External and internal Metamerism:

In Annelids, the metamerism is clearly visible both externally and internally.
Externally it is marked by the constriction on the skin of the body and
internally it is marked by the partitions (septa).

7. Complete Metamerism:
When the segmentation is seen practically in all systems, the metamerism is
called complete metamerism. It is seen in annelids.

8. Incomplete Metamerism:
When the segmentation is not seen in all the organs, the metamerism is called
incomplete metamerism. It is seen in arthropods and chordates.
9. Pseudo-metamerism or Strobilization:
In contrast to true metamerism, pseudo-metamerism or strobilization is seen
in tapeworms (Platyhelminthes) where segmentation of the body takes place
by the segmentation of the ectoderm. The body consists of a number of
segments or proglottids which varies in different individuals of the same
species.

New segments are added to the body throughout life. The proglottids or
segments differ in the degree of development. The segments or proglottids are
functionally independent or self-contained units and new segments are always
formed and there is no cooperation between the segments. The new segments
are formed at the anterior end, just behind the scolex.

Origin of Metamerism:
There are several conflicting views regarding the origin of metamerism.

Such as:
1. Pseudo-metamerism Theory:
This theory postulates that the metamerism evolved secondarily as a result of
repetition of body parts whose ancestor was acoelomate and un-segmented,
and contained the various systems or organs which had serially spread out
along the entire length of the body (pseudo-segmentation).

This is supported by the fact that Turbellarians contained testes, yolk glands,
transverse connectives of two nerve cords which are serially repeated along
the length of the body, and these organs were separated by the development of
septa producing metamerism. The metameric segmentation was linked with
the evolution of coelom.

Table 17.4: Distinguish between pseudo-metamerism and

metamerism:
Pseudo-metamerism:
1. Segmentation of the body is related to the segmentation of the ectoderm.

2. New segments are formed at the anterior part of the body (behind the
scolex).
3. Segments work as an independent unit.

4. No co-ordination among the segments e.g., tapeworms (Platyhelminthes).

Metamerism:
1. Segmentation of the body is related to the segmentation of the mesoderm.

2. New segments are formed at the posterior end of the body (in front of the
anal segment).

3. Segments work as different units.

4. Segments work in co-ordination with all other segments e.g., Annelida,

Arthropoda, Chordata.

2. Cyclomerism Theory:
This theory was proposed by Sedgwick in 1884 and supported by Remane in
1950 and 1963. This theory is the corollary of the enterocoelous theory of
coelom origin and is associated with the origin of metameric segmentation.

It is assumed that the origin of coelom took place from the gastric pouches of
some ancestral anthozoan coelenterates. The gastric pouches are separated
from the main gastric cavity and arranged in linear fashion. These pouches are
transformed into coelomic pouches in the protocoelomates.

First four gastric pouches are developed in ancestral medusoid coelenterates.

Further division of two pouches resulted into three pairs of coelomic cavities,
viz., protocoel, mesocoel and metacoel in the protocoelomates.

Loss of protocoel (anterior pouch) and mesocoel (lateral two pouches) leads to
the formation of un-segmented coelomates, such as molluscs and sipunculans.
Later subdivision of metacoel (posterior pouch) produces segmented annelids
(Fig. 17.50A-G).
3. Corm or Fission Theory:
According to this theory, the metameric segmentation resulted due to
incomplete separation following the repeated transverse divisions of a non-
segmented ancestor or by asexual reproduction producing a chain of sub-
individuals or zooids. These zooids are united end to end. Such events occur in
scyphozoan strobilae and in platyhelminthes.

Remarks:
The chief objection of this theory is the sequence of zooid formation in
platyhelminthes and scyphozoans is never serial with terminal fission. An-
other objection is that the reproduction by fission is usually confined to sessile
animals where the ancestors were probably free-swimming (Fig. 17.51).
4. Locomotion theory:
R. B. Clark (1964) proposed the locomotion theory to explain the origin of
metamerism. According to this theory, metamerism evolved in annelids as an
adaptation to the peristaltic locomotion and for burrowing, and in chordates
metamerism is associated with the strong undulatory, serpentine swimming.

In annelids, peristaltic locomotion involves shortening and lengthening of

body by circular and longitudinal muscles which act against each other. As
coelom is filled with coelomic fluid which acts as hydrostatic skeleton to
facilitate the locomotion, but peristaltic movement is not possible until it is
compartmented by the development of septa.

With the development of compartmented coelom the fluid pressure main

confined to a particular region of the body and it does not affect the whole
body. The septa and metameric segmentation together allow the part of the
body to contract and other parts in the longitudinal axis relax.
This enables a strong peristaltic wave to propagate down the body. For
burrowing the animals need a hard skeleton but they lack such structure and
the coelomic fluid, and inter-segmental septa act as hydraulic skeleton.

In chordates the metamerism evolved independently for locomotion.

Metamerism allowed the tail muscles to be arranged segmentally for the
undulatory movement of the body.

Significance of Metamerism:
1. It helps in locomotion, not only in burrowing but in all other types of
locomotion.

2. Metamerism offers division of labour.