Which of the following is true of members of the phylum Cnidaria?
a. They are not capable of locomotion because they lack true muscle tissue.
b. They are primarily filter feeders.
c. They may use a gastrovascular cavity as a hydrostatic skeleton.
d. They are the simplest organisms with a complete alimentary canal (two openings).
Answer: c. They may use a gastrovascular cavity as a hydrostatic skeleton.