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Siphonoglyph is
A. Kind of nematocyst
B. Medusa form
C. Flagellated groove in the gullet of Metridium
D. A flagellated cavity in the coelenteron of all cnidarians.

Last updated date: 13th Jun 2024
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Hint: Cnidaria is a phylum under the kingdom Animalia which has over 11,000 species of freshwater and marine water aquatic animals. Mostly having marine animals they include corals, hydrae, jellyfish, sea anemones, sea pens, sea whips and sea fans.

Complete answer:
The two classes of the phylum Cnidaria i.e. Scyphozoa and Anthozoa have a flagellated groove in the gullet of Metridium which is longitudinal. It is called siphonoglyph which is a ciliated groove on one or both ends of the mouth. It extends in the pharynx and is used to create currents in the water which is important for respiration and maintenance of internal pressure. In some species, they are introduced in bilateral symmetry.
i. Nematocyst is a giant explosive cell which contains a giant secretory organelle that can deliver a sting to other organisms. The nematocyst is called Cnidocytes. They have a most distinguishing feature i.e. presence of cnidocytes which they used for capturing their prey, these are specialised cells in the phylum Cnidaria.
ii. There are two basic body forms in Cnidarians which are Medusae/Medusa and polyps. The medusa form is a swimming form and polyps are sessile. Both of these bears tentacles surrounding their mouth having cnidocytes. Mostly they produce colonies which contain single organisms made up of polyp or medusa like zooids.
iii. A flagellated cavity in the coelenteron of all cnidarians: Coelenteron is seen in the Cnidarians which carries extracellular digestion. They have flagella or radial projections that increase the surface area for digestion.
So, the correct answer is Option (C).

Note: Most of the Cnidarians prey on organisms that range in size from plankton to animals which are larger than themselves several times, but much of them obtain their nutrition from dinoflagellates and a very few are parasites.