Question

In the phylum, Mollusca through there is bilateral symmetry, but it is lost in some forms, due to the phenomenon, called as
A. Asymmetry
B. Torsion
C. Reversion
D. Invagination

Answer 1

After the Arthropoda the Mollusca is the second largest phylum found in invertebrate’s animals. Mollusks and mollusks are members of the phylum family. Very high species proportions undescribed under the Mollusca species and some studies showing very poor results under the taxa record.

Complete answer: The group Gastropoda the sea butterflies and sea hares belongs to the organisms like the snails. Various species of mollusks found in the land or freshwater. It consists of the form of bilaterally symmetrical found in the torsion body. But some organisms have an asymmetrical body and occurred in the larval stage in the torsion. Visceral hump involved in the torsion. The development at different rates of two-foot retractor muscles to be obtained. Over the head region, it provides the anus to the side and brings up the mental cavity. A unique synapomorphy of modern gastropods the torsion can rotate 180 degrees with respect to the head and foot through the visceral mass. In the late veliger stage the development of torsion during the time of gastropods. But during the adult stage, all the torted are gastropods.
Therefore the correct option is (B).

Note: The gut is twisted into a loop, the nerve cords found between the cerebral part and mantle cavity lie dorsally and anteriorly, nerve cords present between the visceral ganglia and cerebral are crossed. In the evolutionary history of Gastropoda, torsion played a very important role.