Final answer:
Noncoelomate invertebrates possessed the basic animal body plan, from which that of all other animals evolved. Invertebrates evolved several important traits before vertebrates appeared, including multicellularity, radial and bilateral symmetry, and a complete digestive system.
Step-by-step explanation:
The appearance of noncoelomate invertebrates was important because these organisms possessed the basic animal body plan, from which that of all other animals evolved. Invertebrates evolved several important traits before vertebrates appeared, including multicellularity, tissues and organs, radial and bilateral symmetry, cephalization, mesoderm, a complete digestive system, coelom, segmented body, and notochord. These traits are now found in just about all animals.