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UPSC MainsZOOLOGY-PAPER-I202010 Marks150 Words
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Q2.

Skeleton in sponges

How to Approach

This question requires a detailed understanding of the skeletal systems, or rather the lack thereof, in sponges (Phylum Porifera). The answer should focus on the spicules as the primary skeletal element, their composition (siliceous or calcareous), types, and their role in providing structural support. Mentioning the role of collagen and spongin fibers is also crucial. A clear explanation of how these elements contribute to the sponge's body structure is expected. The answer should be concise, fitting within the 150-word limit.

Model Answer

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Introduction

Sponges, belonging to the phylum Porifera, are the simplest multicellular animals and are characterized by a lack of true tissues and organs. Unlike most animals, sponges do not possess a rigid skeleton composed of bone or cartilage. Instead, they rely on an internal skeletal framework provided by structural elements called spicules and, in some species, spongin fibers. These elements provide support, maintain shape, and offer protection against predators. Understanding the nature of this 'skeleton' is fundamental to understanding sponge biology.

Spicules: The Primary Skeletal Element

Spicules are microscopic, needle-like structures made of either calcium carbonate (calcareous sponges) or silicon dioxide (siliceous sponges). They are secreted by specialized cells called sclerocytes.

Types of Spicules

  • Monaxon Spicules: Have a single axis (e.g., styles, rays, needles).
  • Triaxon Spicules: Have three axes (e.g., tetractines).
  • Polaxon Spicules: Have multiple axes.

Spongin Fibers

In addition to spicules, some sponges (e.g., bath sponges) possess spongin, a tough, flexible protein fiber. Spongin provides support and contributes to the sponge's elasticity.

Collagen

Collagenous fibers are also present in the mesohyl (gelatinous matrix) of sponges, contributing to structural integrity. These fibers are secreted by amoebocytes.

The arrangement of spicules and spongin varies among different sponge species, influencing their shape and rigidity. For example, encrusting sponges have horizontally oriented spicules, while vase-shaped sponges have vertically oriented spicules.

Conclusion

In conclusion, sponges lack a true skeleton but maintain their structure through a unique skeletal system comprised of spicules (calcareous or siliceous), spongin fibers, and collagen. The type and arrangement of these elements determine the sponge’s shape and provide essential support and protection. This simple yet effective skeletal system is a defining characteristic of the phylum Porifera and reflects their evolutionary position as early metazoans.

Answer Length

This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.

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Topics Covered

ZoologyInvertebratesPoriferaSkeletal SystemSpicules
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