What structure does phylum Cnidaria have that no other phylum does?

Explanation:

Cnidarians are radially symmetrical (i.e., similar parts are arranged symmetrically around a central axis). They lack cephalization (concentration of sensory organs in a head), their bodies have two cell layers rather than the three of so-called higher animals, and the saclike coelenteron has one opening (the mouth).

a. by using fur or clothes to trap heat energy near the body

answer; the tunica intima also called tunica interna, consists of elastic fibers,connective tissue and smooth muscle cells;