Zoology 10th Edition by Stephen Miller,John Harley

Exercise 14

Recent molecular studies suggest that the Ctenophora, not the Porifera, may be the basal animal phylum, and the Ctenophora form a sister group to the Porifera, Cnidaria, and the Bilateria. The Porifera and the Cnidaria arose soon after the Ctenophora, and ancient anthozoans were closely related to the cnidarian ancestor.
What probably explains the evolution of increased complexity in sponge body forms?