Please see the image. A) The acoelomate state is probably most basal, followed by the pseudocoelomate, since one or both are found in two of the three clades in the figure. The true coelom seems to have appeared more than once in the history of the animals since something other than a true coelom is found in two of the clades as well as more basal phyla not shown in the figure. B) The presence of a true coelom has become more common, especially since the more successful phyla (the measure of that varies, but Arthropoda and Chordata are often mentioned in this way) have true coeloms.
Work Step by Step
Review the chapter and make notes about which phyla are discussed in detail and which have the different states for the coelom. Help in making the phylogeny can be found in Fig. 32.11 (use the bottom two-thirds of that diagram minus Acoela and Hemichordata).