In which phylum of animals, the coelom is filled with blood?
(a)Arthropoda
(b)Annelida
(c)Nematoda
(d)Echinodermata

Hint: These animals possess an open circulatory system where coelom is filled with blood known as ‘hemocoel’. The blood is pumped by the heart and then it moves through the body cavities to reach the internal organs. There are no veins to bring the blood back to the heart.

Complete answer:
Arthropods possess an open circulatory system. Their coelom is filled with blood known as ‘hemocoel’. The term "coelom" means "cavity". The space between the body wall and the alimentary canal is known as a coelom. Their system consists of a dorsal heart and arteries. The blood is pumped by the heart. It then moves through the body cavities to reach the internal organs. There are no veins to bring the blood back to the heart. This is for this reason, that blood for the insects is called "hemolymph". There are three germ layers during embryonic development which are the endoderm, mesoderm, and ectoderm. When the body cavity of the animal is surrounded by mesoderm is known as a coelom. When present in the patches they are known as pseudocoelom.
Additional information:
-Nematodes are the foremost numerous multicellular animals on earth. They lack a discrete circulatory or respiratory system.
-Echinoderms have an open cardiovascular system with cilia circulating the fluids through each arm. It has a system of fluid-filled canals that work in gas exchange, feeding, and in movement.
-An earthworm is one of the animals that are classified in the Annelida phylum. Like many other animals, the earthworm has a closed circulatory system.
So, the correct answer is ‘Arthropoda’.

Note: Rather than a complex and closed system of veins and arteries, organisms with open circulatory systems have a “hemocoel.” This is often a central body coelom present within most invertebrate animals where both digestive and circulatory work is performed.