METAMERISM (With reference to phylum annelida)
Metamerism is the division of body into
Anterior-posterior rows forming similar segments called somites/metamere. Each
metamere typically has repetition of body organs. The term metamerism is only
applicable when organ of mesodermal origin are arranged so. Organs like
nephridia, blood vessels, reproductive bodies etc.
Types of Metamerism
a) Homonomous metamerism: When every
segments of the body have similar organ arrangement. This type of metamerism
does not exist in any animals.
b)
Heteronomous
metamerism: When each
segments of the body have different organ arrangement.
THEORIES OF METAMERISM EVOLUTION
CORM
AND FISSION THEORY-
According to this theory the existence of segmented animals
occurred when the non segmented ancestors underwent asexual transverse fission.
A chain of zooids are formed which remain united from end to end due to
incomplete separation. This occurs in some platyhelminths and annelids even
today. In the passage of time these zooids got integrated both morphologically
are physiologically to form a segmented individual. This theory was proposed by
Perrier (1882). But had certain drawbacks.
A) No gradation of
age was found.
B)
Asexual transverse fission occurred in sessile animals but the
ancestors of annelids were probably free swimmers.
Pseudometamerism:
This theory was
proposed by Hyman (1951). This theory suggests that metamerism evolved
secondarily due to the repetition of body organs like blood vessels, nephridia,
reproductive bodies and nerve cord etc.
Because
of this repetition of organs it resulted in segmentation of body by the
formation of cross-partition between them, thus each segments will receive a
part of the repeating organ. This process is observed in somite formation of
larvae and in some adult annelids even today, this type of metamerism was
thought to be an adaptation to swimming mode of life. But all ribbon like
animals can swim whether they are segmented or not.
C
YCLOMERISM:
This
theory was originally proposed by Sedgwick (1884) and later it was supported by
Remane (1950, 63). According to this theory 4 gastric or enterocoelic pouches arise
from the digestive cavity or gut. Out of the 4 pouches two of them got divided
to form 3 pairs of coelomic cavities – Protocoel, Mesocoel, and Metacoel. The loss of protocoel and mesocoel lead to the
existence of non segmented coelomic animals such as mollusca.
Later
the metacoel got segmented primarily giving rise to segmented annelids. The
phylogenetic implication of this theory is that the bilateral metazoans were originally
segmented and coelomate, but the acoelomate and non segmented groups (flat
worms) lost these characters primarily.
Embryological theory:
According
to this theory the existence of segmented animals was an embryological
accident. Due to the elongation and growth the of larvae/embryo the mesoderm of
it was under a lot of mechanical stress which made it fragmented. Later when
they larvae grew into adult it got segmented.
LOCOMOTION THEORY:
This
theory was proposed by R.B Clark (1964). According to this theory the
segmentation of annelids was an adaptation to locomotion and burrowing. The ancestors
of annelids used to burrow marine sand and mud. Their coelom was filled by
fluids that served as hydrostatic skeleton that helped in burrowing but it
hampered locomotion. So the body was divided into septa to provide flexibility.
One part of the body can contract while other part of the body remains relaxed.
Thus metamerism in annelids initiated from the division of coelom and later the
whole body got segmented.
