Explanation: In the context of Indian General Knowledge (GK), understanding the anatomy and classification of marine invertebrates is crucial. Jellyfish, belonging to the phylum Cnidaria, are fascinating creatures that have no true skeleton. Their bodies are composed of a gelatinous material that provides buoyancy and shape without the need for bones or a rigid internal structure. This unique characteristic sets them apart from other marine animals.
To understand why jellyfish have no skeleton, it is important to first define what a skeleton is. A skeleton is a framework of bones or other rigid structures that supports and protects the body of an animal. In vertebrates, such as fish, reptiles, birds, and mammals, the skeleton is made up of bones. In invertebrates, the skeleton can be either internal (endoskeleton) or external (exoskeleton).
Jellyfish, however, do not have any form of rigid internal or external skeleton. Instead, they have a body structure that is primarily composed of a gelatinous material called mesoglea. This mesoglea is sandwiched between two layers of cells: the outer epidermis and the inner gastrodermis. The mesoglea provides buoyancy and helps maintain the shape of the jellyfish, allowing it to float and move through the water.
It is important to note that while jellyfish do not have a skeleton, they do have other structures that provide some form of support. For example, they have a simple nervous system and a gastrovascular cavity that helps with digestion and circulation. However, these structures do not function as a true skeleton.
In contrast, other options like starfish (sea stars) and silverfish do have some form of skeleton. Starfish have a skeleton made up of calcium carbonate plates embedded in their skin, providing them with a rigid structure. Silverfish, being insects, have an exoskeleton, which is a hard outer covering that provides support and protection for their bodies.
Sponges, while not having a true skeleton, do have a supportive structure made of spicules or spongin fibers. These structures provide some form of support but are not considered a true skeleton.
In summary, among the given options, jellyfish are the only ones that have no true skeleton. Their unique body structure, composed of a gelatinous material, allows them to float and move through the water without the need for bones or a rigid internal structure. This characteristic is a key feature of jellyfish and distinguishes them from other marine animals.