Which of the following was probably the least important factor in bringing about the Cambrian explosion?a. the emergence of predator-prey relationshipsb. an increase in the concentration of atmospheric oxygenc. the movement of animals onto landd. the origin of Hox genes

The distinction between sponges and other animal phyla is based mainly on the absence versus the presence ofa. a body cavity.b. a complete digestive tract.c. mesoderm.d. tissues.

Identify the pattern of embryonic development shown in each drawing below and name the phylum (or phyla) that exhibit this pattern.

In a tube-within-a-tube body plan, what is the interior tube derived from?a. ectodermb. mesodermc. the coelomd. endoderm

Which of the following was probably an important factor in bringing about the Cambrian explosion? (A) the movement of animals onto land (B) an increase in the concentration of atmospheric nitrogen (C) the emergence of predator-prey relationships (D) the origin of bilaterian animals

Bilateral symmetry in animals is best correlated witha. an ability to see equally in all directions.b. the presence of a skeleton.c. motility and active predation and escape.d. adaptation to terrestrial environments.

True or false? New major animal body plans have continued to evolve since the Cambrian about 550 million years ago.

Which of the following combinations of phylum and description is correct? (A) Echinodermata—radial symmetry as a larva, coelom (B) Nematoda—roundworms, internal skeleton (C) Platyhelminthes—flatworms, gastrovascular cavity, no body cavity (D) Porifera—gastrovascular cavity, coelom

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