Answer:
All adult multicellular
organisms typically possess a concentric arrangement of tissues in the body.
These tissues are derived from the three embryonic cell layers called germinal layers.
(i) The outer layer
is the ectoderm, the middle layer is the mesoderm and the inner most layer is
the endoderm.
(ii) Endoderm
layer is associated with the formation of the stomach, colon, liver, pancreas,
urinary bladder and other vital organs in an organism.
(iii) Mesoderm
forms the main structural components of the body like the skeletal muscles, the
skeleton, the dermis of the skin connective tissue, etc.
(iv) Ectoderm
is associated with the formation of CNS, eye lens, ganglia, nerves and glands.
(v) The body
cavity that is lined by mesoderm is called coelom, and the animals possessing
coelom are called as coelomates. e.g., phylum-Annelida, Mollusca, Arthropoda ,
Echinodermata, Hemichordata and Chordata.
(vi) In some
organisms, body cavity is not lined by mesoderm, instead mesoderm is present in
the form of scattered pouches in between ectoderm and endoderm, Such body
cavity is called pseudocoelom and animals possessing there stusturs are refered
to as pseudocoelomates e.g.,Ascaris.
(viii) The
animals in which there is complete absence of body cavity are called
acoelomates.
e.g.,
Platyhelminthes.
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