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: They both belong to phylum- Ctenophora. Also called comb-bearers and comb-jellies. Their body’s external surface bears eight median comb plates.

Complete answer:
In order to answer this question we need to have knowledge about symmetry and its types which are- spherical, radial, biradial, and bilateral.
In addition, the knowledge of cnidoblast will help us answer this easily.

Symmetry is the arrangement/repetition of the body parts in animals or plants in an orderly manner. It is spherical, radial, biradial or bilateral type.

> Spherical symmetry: In protozoan groups Radiolaria and Heliozoia, the body has the shape of a sphere and the parts are arranged concentrically around or radiate from the centre of the sphere. Animals possessing this type of symmetry have no sides, and any plane passing through the centre divides the body into equivalent halves. It is common only in minute animals with simple internal construction.

> Radial symmetry: The body of an animal that has radial symmetry has a general form of a short/long cylinder or bowl, with a central axis from which the body parts are arranged in a regular manner. The main axis is heteropolar, one of which bears the mouth and called anterior end, while the other is posterior end, forming the rear end of the animal and may bear the anus and hence the body will be divided into symmetrical halves. Radial symmetry is found in coelenterates and echinoderms.

> Biradial symmetry: Many animals have two other axes at right angles to the antero-posterior axis. They are known as sagittal and transverse. The body components of such animals are arranged in a manner that all the similar parts are located to either side of a central axis and each side of the body is identical to the opposite one but different from the adjacent side. Biradial symmetry occurs in the Jellyfish, Sea Anemone.

> Bilateral symmetry: Animals possessing this type of symmetry have a dorsal side, a ventral side and distinct left-right sides. They also have an anterior side known as head, and a posterior side called tail region. They also exhibit cephalization which can be defined as the clustering of sensory organs at the anterior side. A vast majority of insects, fishes, amphibians, reptiles, birds, mammals, and most crustaceans have the characteristic feature of bilateral symmetry. Example: Bivalve Mollusks

A cnidoblast (also known as nematocysts) is an explosive cell (a coiled thread ) containing one giant secretory organelle called a cnidocyst which is capable of delivering a sting to other organisms. This is a characteristic feature of phylum Cnidaria consisting of Sea anemones, hydrae, jellyfish, etc. Cnidae are used to capture prey and as a defense against predators.

Therefore, the answer is b, Ctenoplana and Beroe!

Note: The branch of science dealing with plants- botany also uses the term symmetry for flowers. A flower is symmetrical when each whorl consists of an equal number of parts or when the parts of any one whorl are multiples of that preceding it.