Answer;
Phylum Nematoda.
The heartworms belongs to the phylum Nematoda.
Explanation;
-Phylum Nematoda are un-segmented organisms with anterior lateral chemosensory organs (amphids) and with a persistent blastocoel or pseudocoelom; dorsal and ventral nerve cords in epidermis and excretory system of renette cells or tubules.
-The alimentary canal is provided with distinct mouth and anus (complete digestive tract).