A fluid filled cavity which is mesodermal in origin but absent in nematodes is:

Explanation

The coelom is a true body cavity that is entirely lined by mesodermal tissue, providing space for the development and suspension of organs. Nematodes, however, are classified as pseudocoelomates, meaning their body cavity is not completely lined by mesoderm. This distinguishes them from coelomates, which have a fully mesoderm-lined cavity. Other options like the gastrocoel and spongocoel refer to different types of cavities unrelated to the mesodermal origin, while the pseudocoelom, although a body cavity, lacks a complete mesodermal lining and is present in nematodes.