Circulatory system is best represented by
(a)Earthworm
(b)Leech
(c)Ascaris
(d)Both A&B

Hint: Earthworms and leeches include Annelids. They can be found in various environments: saltwater, soil and freshwater. Their circulatory system is distinct from each individual organism. Ascaris is a parasite of the small intestine.

Complete answer:
-Most of them have a closed circulatory system where blood is closed in the body inside the vessels, whereas leeches have an open circulatory system where blood is not contained within the vessels. In leeches, a closed blood vascular system such as that of earthworms is absent. Coelomic channels, filled with blood-like fluid, represent the blood vessels. The haemocoelomic system forms these networks.
-In leeches, there is haemoglobin in coelomic fluid. Via these networks, colourless corpuscles are distributed.
-The circulatory system of the earthworm is closed type. It is made up of blood vessels, lungs, capillaries, loops, and blood glands.
-The blood fluid in the 'coelomic cavity' is in contact with the entire inner body in an open circulatory system, in which nutrients, particles, and gases are exchanged.
- Identified as small intestinal roundworms, Ascaris is a genus of parasitic nematode worms. There is no circulatory system in nematodes. They depend upon simple diffusion for their respiration purpose. Nematodes exchange materials as a means of circulation through their skin.
So, the correct answer is ‘Earthworm’.

Note: -In earthworm, the haemoglobin, like the vertebrates, is not present in the corpuscles, but it is found dissolved in the plasma. The plasma also includes several colourless and nucleated corpuscles.