The heartworms that can accumulate within the hearts of dogs and other mammals have a pseudocoelom, an alimentary canal (i.e., digestive system), an unsegmented body, and an outer covering that is occasionally shed. to which major clade of animals does the heartworm belong?

The correct answer is Nematoda.

Explanation:

This kind of worms belong to the:

The Dirofilaria immitis or also known as Hearthworm is a parasitic roundworm spread from host to host through mosquito bite, they have, as mentioned in the excerpt from the question, a pseudocoelom, an alimentary canal an unsegmented body and an outer covering, therefore this kind of worms is classified the Nematoida clade.