Question

The animals with bilateral symmetry in the young stage and radial pentamerous symmetry in the adult stage belong to the phylum
A) Cnidaria
B) Echinodermata
C) Annelida
D) Mollusca

Answer 1

Hint: In biology, symmetry refers to the symmetry found in species such as plants, animals, fungi, and bacteria. In body plans, there are only a few forms of symmetry that are possible. Radial (cylindrical), bilateral, biradial, and spherical symmetry are the four types.

Complete answer:
Organisms with radial symmetry have a repeated pattern around a central axis that can be sliced into multiple identical bits when the central point is cut through, similar to pie slices. Tetramerism, pentamerism, hexamerism, and octomerism are terms used to describe the process of repeating a body part 4, 5, 6, or 8 times around the axis. There are no left or right sides of such species, but they do have a top and bottom surface, or a front and back.

Bilaterally symmetric organisms have a single symmetry plane, the sagittal plane, that divides the organism into two roughly mirror image left and right halves – approximate reflectional symmetry.

Bilaterally symmetric animals are grouped into a broad category known as the bilateria, which includes 99 percent of all animals (comprising over 32 phyla and 1 million described species). Despite the body's external bilateral symmetry, all bilaterians have certain asymmetrical features; for example, the human heart and liver are asymmetrically placed.

Echinoderms are a type of marine invertebrate with some unique characteristics. The majority of the time, their life cycle is complicated. In most echinoderms, the embryo grows into a planktonic larva with bilateral symmetry, which eventually develops into a pentaradial symmetry in adulthood. It has been proposed that a pentamerous arrangement of skeletal parts enhances the animal's skeleton more than, say, a three rayed symmetry.

Radial symmetry is a form of symmetry in which every plane from the central axis can divide an entity into similar halves. Cnidaria is a phylum of animals that live in aquatic and primarily marine environments. They have a radial symmetry.Mollusks have bilateral symmetry, which is a form of symmetry.

Thus the correct answer is option (B) i.e ‘Echinodermata’.


Note: The water vascular system of echinoderms is a special feature. This is a system of canals that runs the length of each body part. The canals of most echinoderms have external projections called tube feet. On the ends of the feet are suckers. Water is forced into the feet by muscle contractions, causing them to stretch outward. As the feet grow longer, the suckers attach to new positions further away from the body.When the feet extend, their suckers connect to new sites, farther away from their previous attachment points. As a consequence, the movement is slow but strong. The suckers are extremely strong. They can also pry open the shells of birds of prey.