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 innermost 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 stuctures are refered to as pseudocoelomates e.g., Ascaris.
(vii) The animals in which there is complete absence of body cavity are called acoelomates. e.g., Platyhelminthes.
