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MODEL PAPER 3 — BIOLOGY STD 11 Science — Question

Rajasthan BoardEnglish MediumSTD 11 ScienceBIOLOGYMODEL PAPER 33 Marks
Question
Write an account on skeleton of sponges.
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Answer

The body of sponges contains a supporting exoskeleton. It is made up of either spicules or spongin fibres or both. Spongin fibres are protein fibres. They are either highly branched or show anastomosis. Spicules are calcareous or siliceous with hard spine-like structures. Depending on the branching the spicules are
i. monoaxons (single axis).
ii. triaxons (three-axis forming six rays of hexactinal spicules).
iii. tetraxons (four rays or tetra actinal).
iv. polyaxons (several rays).

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