Final answer:
Animal classification cannot be based only on the presence of a coelom because this feature alone does not reflect the full complexity and diversity of the animal kingdom, which also includes body plan, development, and genetic relationships.
Step-by-step explanation:
We cannot base animal classification solely on whether an animal has a coelom because this criterion alone is too simplistic to account for the vast diversity within the animal kingdom. Animals are classified based on a range of morphological, developmental, and genetic characteristics. The presence or absence of a coelom is just one feature that distinguishes triploblastic animals (those with three germ layers) into different groups: acoelomates (no coelom), pseudocoelomates (partially lined coelom), and eucoelomates or coelomates (fully lined coelom).
While a coelom provides several functional advantages like protecting internal organs, allowing organ movement, and facilitating the diffusion of nutrients and gases, it is not the only feature that defines an animal's complexity or its evolutionary relationships. Flatworms (acoelomates), roundworms (pseudocoelomates), and higher organisms like annelids and vertebrates (coelomates) all have differing body plans and development processes, extending beyond their coelomic status. Furthermore, classifications such as protostomes and deuterostomes are based on embryological development patterns and offer a more nuanced view of animal diversity.