Question

Open circulatory system is present in
A. Annelids and chordates
B. Annelids and arthropods
C. Arthropods and chordates
D. Arthropods and Mollusca

Answer 1

Hint: Circulatory system allows the body fluid to circulate and transport nutrients, gases, and bio-molecules to the organs and cells of the body. They are of two types, open and closed. In an open circulatory system, the body fluid is not enclosed in the vessels. Instead, the body fluid is pumped into the cavity called hemocoel and the organs are bathed in it.

Complete answer:
Option A: Annelids have a closed circulatory system. In the closed circulatory system, blood is confined within the blood vessels and the heart pumps blood through these vessels to different parts of the body. Annelids have blood vessels along with one or a series of hearts to pump the blood across the body. Their hearts are not well developed, instead, they are specialized blood vessel structures. Eg: earthworm

Chordates have a closed circulatory system with a network of blood vessels throughout the body. Most of the chordates have well-defined hearts, but not all. Therefore, this is the incorrect option.
Option B: Annelids are characterised with the closed circulatory system.eg: humans.Therefore, this is the incorrect option.
Arthropods are characterized by an open circulatory system where the body fluid is not confined within the blood vessels. Instead, it is present in the hemocoel and bathes the organs and tissues directly. There is no distinction between blood and interstitial fluid and hence the body fluid is called hemolymph.eg: ants, bees
Option C: Arthropods are characterized by an open circulatory system. But chordates have a closed circulatory system. Therefore, this is the incorrect option.
Option D: Arthropods are characterized by the open circulatory system.
Mollusks have an open circulatory system, which does not involve blood vessels. In mollusks, blood is stored in the gills. The heart pumps blood from the gills to the organs and blood is then swept back to the gills. eg: freshwater mussel. Therefore this is the correct answer.

Hence, the correct answer is option (D)

Note: Although most of the molluscs have an open circulatory system, some complex molluscs like squid and octopus have a closed circulatory system. They belong to the class sepia. They have one or two hearts and an arterial-like vessel to push blood through the body.