Question

Assertion: In gastropods, due to torsion mantle cavity is placed anteriorly behind and above the head.
Reason: During the larval stage, visceral mass, shell, and mantle cavity are twisted up to 180 counterclockwise with respect to the head and foot.
A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
B. Both Assertion and Reason are correct and 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: The class gastropods belong to phylum Mollusca. Members from gastropods are shell- containing animals that include snails and slugs found in water or in the land.

Complete answer: The members of class gastropods have an asymmetrical body with an anterior mantle cavity and have a spirally coiled shell and the posterior visceral mass. They are mostly seen in ocean water, freshwater and land (terrestrial). The shell of Gastropoda has only one opening that has to serve as an entrance as well as an exit for all the visceral organs. Torsion is a special characteristic of gastropods, in which during the larval stage the visceral mass, shell, and mantle cavity are twisted up to 108 degrees counterclockwise with respect to the head and foot i.e. head and foot are fixed. The mantle cavity is highly muscular. It has a function of respiration and contains gills, anus, osphradium, gonophores. The mantle cavity must also open at the anterior side for easy respiration. The visceral mass is the mass of the body embedded in the shell that contains the digestive system, reproductive system, or excretory system. Rearrangement of visceral mass is done so that openings of kidneys, gonads, and anus should migrate to the front side which is the opening of the shell.
Hence, the correct answer is option A.

Note: Torsion is a special characteristic feature that is present only in members of class gastropods. Torsion also allows the migration of small chemoreceptor osphradium to the front side, so that it can chemically analyze the water current entering the mantle cavity.