Final answer:
Valves in lymphatic vessels prevent the backflow of lymph by allowing it to move forward under pressure and closing to stop it from moving backward when the pressure decreases.
Step-by-step explanation:
The structural feature of lymphatic vessels responsible for preventing lymph from flowing backward is Valves. These one-way valves are located fairly close to each other, helping to ensure that lymph flows in only one direction, towards the heart. The valves function similarly to those found in veins, allowing lymph to move forward when pressure is applied (such as via the contraction of muscles around the vessels) but preventing backflow when the pressure is less.