The correct answer is E.

This is the definition of chordates and helps to explain why they have such an advanced nervous system compared to other organisms. Answer choice A is incorrect because coelenterates have a mesoglia rather than a full germ layer of mesoderm. This mesoglia is mostly gelatinous with a few scattered cells. Answer choice B is incorrect because annelids have ventral double solid nerve cords. Answer choice C is incorrect because mollusks are broken down into three parts: the foot, which aids in locomotion, the visceral mass, and the mantle, which typically secretes the shell. This shell is not chitinous, rather chitinous exoskeletons are characteristic of insects. Answer choice D is incorrect because flatworms do not have a respiratory or circulatory system although they do have primitive excretory and reproductive systems.