The correct answer is A. Nematoda.
A coelom is a fluid-filled cavity that lies between the digestive tract and the body wall. It is lined with mesoderm, a type of tissue that gives rise to many other organs and tissues. A true coelom is a spacious cavity that is completely lined with mesoderm.
Nematodes are roundworms that lack a true coelom. Instead, they have a pseudocoelom, which is a cavity that is not completely lined with mesoderm. The pseudocoelom is filled with fluid and provides support for the nematode’s organs.
Echinoderms, mollusks, and annelids all have a true coelom. The coelom in these animals is a spacious cavity that is completely lined with mesoderm. The coelom provides support for the animal’s organs and allows for the movement of fluids.
In conclusion, the correct answer is A. Nematoda. Nematodes are roundworms that lack a true coelom. Instead, they have a pseudocoelom, which is a cavity that is not completely lined with mesoderm. The pseudocoelom is filled with fluid and provides support for the nematode’s organs.