In microbiology|planktology|lang=en terms the difference between theca and epitheca
is that
theca is (microbiology|planktology) the membrane complex enveloping the cells of certain plankton including diatoms and dinoflagellates while
epitheca is (microbiology|planktology) the upper half of the theca of a thecate protist such as a diatom or dinoflagellate.
As nouns the difference between theca and epitheca
is that
theca is (biology) any external case or sheath while
epitheca is (microbiology|planktology) the upper half of the theca of a thecate protist such as a diatom or dinoflagellate.