a) The early embryo acquires fundamental cell layers by a process called gastrulation. These layers are called germinal layers or germ layers.

b) The outer germinal layer is called ectoderm (formation of ganglia, Central nervous system, glands, etc.), the middle one is called endoderm (involved in the formation of skeleton, skeletal muscles, the dermis ok the skin, etc.) and the innermost is endoderm (associated with the formation of stomach, colon, livers, etc.)

c) Coelomates are the organisms possessing coeloms. Coelom is the body cavity lined by the mesoderm. Examples of coelomates are phylum annelida, mollusca , arthropoda, etc

d) The organisms with absent body cavities are called aceolomates. E.g. Platyhelminthes

e) 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 pseudocoelom are refered to as pseudocoelomates e.g.,Ascaris.