What is the relationship between germinal layers and the formation of body cavity in case of coelomate, acoelomates and pseudocoelomates ? 

Explanation

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) Coelomate : There are two germinal layers in coelomates i.e. ectoderm, mesoderm and endoderm. The mesoderm lines the body cavity and thus body cavity is present.
(ii) Acoelomate : There are two germinal layers in acoelomates i.e. ectoderm and endoderm. Bodycavity is not present in coelomate.
(iii) Pseudocoelomate : There are three germinal layers in pseudocoelomate i.e. ectoderm, mesoderm and endoderm. But mesoderm is present as scattered pouches and do not line the cavity hence true coelom is absent.
Examples :
(i) Coelomates : Phylum - Annelida, Mollusca, Arthropoda, Echinodermata, Hemichordata and Chordata. (ii) Acoelomate : e.g. Platyhelminthes
(iii) Pseudoelomate : e.g. Ascaris.