Question

Long answer.
What is the relationship between germinal layers and the formation of body cavity in case of coelomates, acoelomates and pseudocoelomates?

Answer 1

There are three germinal layers present in coelomate. They are ectoderm, mesoderm and endoderm. Mesoderm is lining the body cavity and true body cavity is present. Acoelomate also has three germinal layers, ectoderm, endoderm and mesoderm. They are triploblastic, but the body cavity is absent. In pseudocoelomates also, three germinal layers are there – ectoderm, endoderm and mesoderm. But mesoderm is present in scattered pouches and they do not line the cavity. Hence, true coelom is absent.

Complete answer:
There is concentric arrangement of tissues present in organisms. The tissues are derived from ectoderm, mesoderm and endoderm only. Absence or presence of body cavity and gut wall is very important in classification. Body cavity that is lined by mesoderm is called a coelom.
In acoelomate, there is no coelom present between ectoderm and endoderm. Only solid mesodermal parenchyma is present between the two layers. Flatworm is an acoelomate.
In pseudocoelomate, Coelomic pouches are present scattering in the mesoderm, but there is absence of continuous cavity. Roundworm is a pseudocoelomate.
There is a mesodermal lined body cavity present between gut wall and body wall. It is formed by splitting of mesoderm. Mollusca is a coelomate.

Body cavity of coelomate, acoelomate and pseudocoelomate

Note:
Early embryos acquire fundamental cell layers through a process called gastrulation. The cell layers are germ layers or germinal layers. Formation of glands, ganglia and central nervous system is due to ectoderm. Formation of skeleton, dermis and skeletal muscles are due to mesoderm. Formation of colon, liver and stomach are because of endoderm, which is the innermost layer. Annelid is an example of coelomate. Platyhelminthes is an example of acoelomate. Nematode is an example of pseudocoelomate.