Question

Biradial symmetry and lack of cnidoblasts are the characteristics of
A) Starfish and sea anemone
B) Ctenoplana and Beroe
C) Aurelia and Paramecium
D) Hydra and starfish

Answer 1

Hint:-Most organisms are symmetrical or have features that show symmetry. In biology, symmetry is observed when any axis (imaginary) divides a feature or an organism into similar parts. In other words, the axis will divide the parts in such a way that one part is a mirror image of the other. Symmetry can be – bilateral, biradial, radial, and spherical. Certain organisms can sting other organisms. The cells that deliver this sting are called cnidoblasts/ nematocysts; these cells are a defining feature of phylum Cnidaria.

Complete Answer:-Complete Answer:
Several imaginary axes pass through the centre of the organism. This divides the body of the organism into several segments that look identical; this symmetry is called radial symmetry. Organisms that are not that mobile or the ones that float in water show this symmetry. Bilateral symmetry is seen when an imaginary axis divides the body into two identical parts. Organisms that move around a lot usually show this type of symmetry.
Organisms that show a combination of both radial and bilateral symmetry have a biradial body plan. Cell organelles called cnidocyst are present in cnidoblasts. These cnidocytes contain a toxin that paralyzes the prey or the attacker. Starfish show radial symmetry and lack cnidoblasts. While sea anemone show radial symmetry and have cnidoblasts. Ctenoplana and Beroe both belong to the phylum Ctenophora. The organisms that belong to Ctenophora are biradial in symmetry and lack cnidoblasts. Instead, most have tentacles called colloblasts to capture their prey.
Aurelia belongs to phylum Cnidaria and they do have cnidoblasts. They show radial symmetry. Paramecium is an organism found in water bodies. They are shaped like a slipper and the organism is asymmetrical. They also lack cnidoblasts. Hydra shows radial symmetry and the presence of cnidoblasts.

The correct option is (B). Biradial symmetry and lack of cnidoblasts are the characteristics of Ctenoplana and Beroe.

Note:- One of the reasons for the relevance of symmetry is that some organisms prefer symmetrical features in their mates. So these animals/organisms have an advantage over the asymmetrical ones. It is believed that to overcome the disadvantages of radial symmetry (being more or less immobile), organisms as they evolved selected bilateral symmetry. Biradial symmetry exhibited by Ctenophora is considered as an intermediate of both symmetries. But, no concrete evidence is available that can prove this point.