How useful is the study of the nature of body cavity and coelom in the classification of animals?

(N/A) Coelom is a fluid-filled space between the body wall and the digestive tract. The presence or absence of a body cavity or coelom plays a very important role in the classification of animals.
$1$. Coelomates: Animals that possess a fluid-filled cavity lined by mesoderm between the body wall and the digestive tract are known as coelomates. Examples include Annelida,Mollusca,Arthropoda,Echinodermata,and Chordata.
$2$. Pseudocoelomates: Animals in which the body cavity is not lined by mesoderm,but instead,the mesoderm is present as scattered pouches in between the ectoderm and endoderm,are known as pseudocoelomates. Aschelminthes is an example of this.
$3$. Acoelomates: In certain animals,the body cavity is completely absent. These are known as acoelomates. An example of acoelomates is Platyhelminthes.