Question

Assertion: In Molluscs, feather-like gills are present in the mantle cavity.
Reason: These gills have respiratory and excretory functions.
A. Both Assertion and Reason are correct and Reason is the correct explanation for the Assertion.
B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
C. Assertion is correct but Reason is incorrect.
D. Both Assertion and Reason are incorrect.

Answer 1

Hint: As we all know, Molluscs are the second largest phylum in the Animal kingdom. Their body is divisible into Head, Visceral hump and Muscular foot. The dorsal part of the hump is thickened to form the Mantle that sometimes encloses a Mantle cavity.

Complete answer:
Molluscs are the second largest phylum in the Animal kingdom after Arthropods. This phylum involves six different classes which are (a) Monoplacophora, (b) Amphineura, (c) Scaphopoda, (d) Gastropoda, (e) Pelecypoda and (f) Cephalopoda. These classes involve different animals like snails, unio, cuttlefish, octopus etc. Their body is unsegmented with a distinct head, visceral hump and a muscular foot.
A soft and spongy layer of skin forms the mantle over the visceral hump and the space between the hump and mantle is called the mantle cavity. This cavity contains the feather-like gills which play an important role in the respiration and excretion.

Hence, the correct answer is option (B) i.e. Both Assertion and Reason are correct but Reason is not the correct explanation for the Assertion.

ADDITIONAL INFORMATION:
Molluscs are Triploblastic, Bilaterally symmetrical having Organ system level of organization. Their mouth has a file-like rasping organ called Radula which is responsible for the feeding mechanism. In Pinctada (Pearl oyster) of class Pelecypoda, if a sand or dust particle enters the mantle cavity then it starts secreting a layer of Nacreous material which results in the formation of a pearl. Pearls of good commercial quality take about 7 years to be formed. Pinctada vulgaris found in India forms the good quality pearls.

Note:
Assertion and Reason statements should be read carefully, the reason does not explain the logic for the presence of mantle cavity but explains the function of feather- like gills.