How useful is the study of the nature of body cavity and coelom in the classification of animals?
The study of the nature of body cavity and coelom is very useful in the classification of animals. The presence or absence of a cavity between the body wall and the gut wall is very important in classification. The coelom is the body cavity that is lined by mesoderm.
Based on the nature of body cavity, animals are classified into three categories. Animals possessing a coelom are called coelomates, e.g., annelids, molluscs, arthropods, echinoderms, hemichordates and chordates. Animals where the body cavity is not lined by mesoderm, instead the mesoderm is present as scattered pouches between ectoderm and endoderm, are called pseudocoelomates, e.g., aschelminthes. Animals in which the body cavity is absent are called acoelomates, e.g., platyhelminthes.
Thus, the nature of coelom is a fundamental feature that enables us to broadly classify the animal kingdom.
Explanation
The textbook explicitly states that 'Presence or absence of a cavity between the body wall and the gut wall is very important in classification.' The nature of coelom is listed among the fundamental features (level of organisation, symmetry, cell organisation, coelom, segmentation, notochord) that help broadly classify the animal kingdom. The three categories—coelomates, pseudocoelomates, and acoelomates—distinguish major animal phyla from each other.