common descent means the evolutionary in biology and how they have something linked with their common ancestors.
Answer:
The answer is ectoderm. The mesoderm is the middle layer of lining. The endoderm is the innermost layer. You can remember it by:
Endo/ sounds like inner
Meso/ M for middle
Ecto/ sounds like exo, bugs have exoskeletons that are on the outside.
Explanation:
Cholera is caused by the bacterium, Vibrio cholerae and produces CTX (cholera toxin, choleragen)...
It contracts and is pulled downwards