Answer:
The correct answer is - roundworms.
Step-by-step explanation:
The answer is already mention in the question, however, the detailed answer is as follows:
The characteristics that are given in the question are true tissues, bilateral symmetry, and a pseudocoelom. Worms or helminths are known as primitive form of organization of the Bilaterians. All three group of worms or helmints have a basic bilateral symmetry.
These organisms inaugurated various characteristic that are found and carried by other animals such as true tissues, bilateral symmetry, and a pseudocoelom.
Thus, the correct answer is - roundworms.