Question

Assertion:
The mollusca circulatory system is of closed type.
Reason:
The blood of mollusca contains haemoglobin.

a. If both the assertion and the reason are true and the reason is a correct explanation of the assertion
b. If both the assertion and the reason are true and the reason is not a correct explanation of the assertion
c. If the assertion is true but the reason is false
d. If both the assertion and reason are false

Answer 1

Hint: In a closed circulatory system, the heart pumps blood through vessels that remain separated from the interstitial fluid of the blood. On the other hand, in the open circulatory system, the blood is not enclosed in the blood vessels. Rather the blood is pumped into a cavity. This cavity is called a hemocoel.

Complete answer:
• In Mollusca, the circulatory system is of open type.
• It has a dorsal heart, arteries that open into sinuses and veins.
• Blood is blue coloured. The reason for this colour is the presence of a copper-containing blue respiratory pigment called haemocyanin.
• Cuttlefish happens to be an exception in having a closed blood vascular system.
• Phylum Mollusca is divided into six classes:
- Monoplacophora
- Amphineura
- Scaphopoda
- Gastropoda
- Pelecypoda
- Cephalopoda
• Monoplacophora has a cup-shaped or spoon-shaped shell. Amphineura has a nerve ring that is non ganglionated around the mouth. Scaphopoda’s shell is tubular and open at both ends. Gastropoda has a shell that is made up of one piece. Pelecypoda has a shell that is made up of two halves. Cephalopoda has it’s head and foot regions combined and is modified into a structure that has eyes and tentacles.

Hence, the correct answer is option (D).

Note:
• Phylum mollusca is often referred to as the soft-bodied animals.
• The term mollusca was coined by Johnston.
• This phylum is the second-largest animal phylum. It includes over 60000 species.
• Molluscs are terrestrial or aquatic. They have an organ system of organization.
• They are bilaterally symmetrical, triploblastic and coelomate.
• Calcareous shells cover the body and are unsegmented.
• Mantle cavity is present between the hump and the mantle. It has feather-like gills. They have respiratory and excretory functions.
• Apple snail, pearl oyster, squid, octopus, tusk shell belong to the Phylum Mollusca.