Which of these is true of echinoderms
a. they have heads.
b. they have radial symmetry.
c. they have shells.
d. they have smooth skin.

yo

the cell is the smallest structural and functional unit of living organisms, which can exist on its own. therefore, it is sometimes called the building block of life. some organisms, such as bacteria or yeast, are unicellular—consisting only of a single cell—while others, for instance, mammalians, are multicellular.