One of the characteristics unique to animals is

a. Gastrulation

b. Multicellularity

c. Sexual reproduction

d. Flagellated sperm

The distinction between sponges and other animal phyla is based mainly on the absence versus the presence of

a. A body cavity

b. A complete digestive tract

c. Mesoderm

d. Tissues

Which of the following was probably the least important factor in bringing about the Cambrian explosion?

a. The emergence of predator-prey relationships

b. An increase in the concentration of atmospheric oxygen

c. The movement of animals onto land

d. The origin of Hox genes

Based on the tree in Figure 32.11, which statement is false?

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a. The animal kingdom is monophyletic.

b. Acoelomate flatworms are more closely related to echinoderms than to annelids.

c. Sponges are basal animals.

d. Bilaterians form a clade.