Answer: The correct answer for the fill in the blank is Coelom.
Coelom is a fluid filled body cavity in most of the animals that is present inside the body of the organism and has the digestive tract and different body organs of the system. Roundworms have definite digestive system that runs throughout the length of their body and includes mouth, pharynx, intestine, and anus.
The nutrients that are obtained after digestion of food are absorbed through digestive tract ( mainly from intestine).
Developing a coelom made nutrients more easily absorbed by roundworms. A coelom is a fluid filled cavity developed in the mesoderm and it is the main body of a roundworm. It contains the digestive tract and many other organs.
