Metamerism in Annelida
When segmentation in bilateral animal
such as annelids involves a longitudinal division of body into a linear series
of similar section of parts is termed as metamerism. Or metameric segmentation.
Metamerism of body was encountered for
the first time in annelids. In some animal group such as arthropods and
vertebrate also have body part metaphorically repeated. In some animals
metamerism is internal as well as external. Earthworm is best example in which
both internal and external meamerism is found. In arthropods metamerism or
segmentation is chiefly external. Some higher animals inclueding human shows an
internal metamerism of body muscles, nerves, certain blood vessels, vertebrae
and ribs.
Body of annelids is metamerically
segmented. Internal and external metameism is visible in most of the
annelids. The body of leech is divisible
into many regions as follows.
1. Cephalic
region
2. Pre-clitellar
region
3. Clitellar
region
4. Middle
region
5. Caudal
region
6. Posterior
sucker
Each segment is divided by ring also
called as annuli. The segment is either uni-annulate, bi-annulate, tri-annulate
or Quinqua-annulate. Segment with five annuli is called quinqua -annulate
segments or complete segment. The distribution of annuli in different regions
of body is as follows.
Sr.
No.
|
Body
region
|
Segment
No.
|
Distribution
of annuli
|
Complete
or incomplete
|
1
|
Cephalic region
|
I-V
|
I
and II Uni-annulate
|
Incomplete
|
III
- Biannulate
|
Incomplete
|
|||
IV
and V Tri-annulate
|
Incomplete
|
|||
2
|
Pre-clitelar Region
|
VI-VIII
|
VI- Tri-annulate
|
Incomplete
|
VII
and VIII Quinqua-annulate
|
complete
segment
|
|||
3
|
Clitelar
region
|
IX
I XI
|
Quinqua-annulate
|
complete
segment
|
4
|
Middle
region
|
XII-XXII
|
Quinqua-annulate
|
complete
segment
|
5
|
Caudal region
|
XXIII- XV
|
XXIII Tri-annulate
|
incomplete
segment
|
XXIV
and XXV
Bi-annulate
|
incomplete
segment
|
|||
6
|
Posterior
sucker
|
XXVIII
– XXXIII
|
Uni-
annulate.
|
incomplete
segment
|
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