Final answer:
Animals are classified into acoelomates with no body cavity, eucoelomates with a true coelom within the mesoderm, and pseudocoelomates with a body cavity partly derived from mesoderm and endoderm.
Step-by-step explanation:
Animals are classified based on the presence or absence of a body cavity. The three types of body cavities are: acoelomate, without a body cavity; eucoelomate (or coelomate), with a true body cavity called a coelom, derived entirely within the mesoderm; and pseudocoelomate, with a body cavity that is partly derived from mesoderm and partly from endoderm tissue. Examples include flatworms (acoelomates), earthworms and vertebrates (eucoelomates), and roundworms (pseudocoelomates).
The coelom is significant as it not only protects internal organs through the fluid it contains but also provides a structure for muscle attachment, aiding in motility. The presence of a coelom has had a major evolutionary impact, as evidenced by the diversity and complexity of coelomate organisms. Different developmental processes give rise to the coelom, distinguishing protostomes and deuterostomes among the coelomates.