How are animals divided into groups based on the morphological trait of the number of tissue layers during development?
a.) two groups: those that have three tissue layers in their embryos and those that have two
b.) three groups: those that have four, three, or two tissue layers in their embryos
c.) three groups: those that have three, two, or one tissue layers in their embryos
d.) two groups: those that have two tissue layers in their embryos and those that have one