Question

Bilateral symmetry does not occur in
(a)Frog
(b)Octopus
(c)Mammal
(d)Obelia

Answer 1

Hint:-Obelia is radially symmetrical. It belongs to the phylum cnidaria (Coelenterata). They are aquatic, mostly marine, sessile, and radially symmetrical animals. The other three belong to bilateral symmetry.

Complete step-by-step solution:-
Obelia shows Radial symmetry. It does not exhibit bilateral symmetry. Because an imaginary axis will
divide their body into two halves.
Important characters :
- Coelenterata are all aquatic, most of them are marine and few are freshwater forms.
- Skeleton when present in external, calcareous, and massive.
- Respiration is by simple diffusion. Digestion is both extracellular and intracellular.
- The body enclosed a single cavity called the gastrovascular cavity. The anus is absent.
- Mouth is situated on an elevated portion called the hypostome. The mouth is usually
surrounded by a circle of tentacles which helps in capturing the prey.
- Tentacles bear nematocysts cells in them. The nematocysts bear toxic fluid in them which
paralyzes the prey during food capture.
- These organisms have two germ layers, outer ectoderm, and inner endoderm and thus they are
diploblastic.
They exhibit forms like polymorphism (Obelia).
- Frog: It belongs to class Amphibia and exhibits bilateral symmetry.
- Octopus: It belongs to the phylum Mollusca and exhibits bilateral symmetry.
- Mammal: It belongs to class Mammalia and exhibits bilateral symmetry.
So, the correct option is (D).

Note:- Note- Any plane that passes through the center does not divide them into equal halves. When any plane passing through the central axis of the body divides the organism into two identical halves is called radial symmetry. Examples for radial symmetry are Obelia, Adams, Gorgonia, etc. The body can be divided into identical left and right halves in only one plane is called bilateral symmetry. Annelids, Arthropods are examples of bilateral symmetry.