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Skeleton made up of spongin fibres occurs in
A. Calcarea
B. Demospongiae
C. Hexactinellida
D. Both A and B

Last updated date: 20th Jun 2024
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Hint: The spongin is a form of collagen protein which helps or creates a fibrous skeleton in the bodies of the organisms mostly of the genre Phylum Porifera. The flexibility of porifera which is mostly noticed is for the presence of spongins in their bodies. Specific Sponge cells as named spongocytes secret this Spongin.

Complete Answer:
1. Calcarea is a class under the Phylum Porifera. They also consist of a variety of sponges but their main feature is the presence of spicules which are made of calcium carbonate in the form of aragonite or calcite. They are basically sedentary filler feeders.
2. Hexactinellida are also a variety of sponges. Their skeleton is made of siliceous spicules and they are also called ‘glass sponges’. That is, due to the presence of silica, it looks like the glass. Although they are the parts of phylum porifera, due to their certain specific characteristics. They are regarded under a separate sub-phylum which is called Symplasma.
3. The class of Demospongiae also falls under the phylum porifera. In general, it shares all of those characteristics which can be seen in Calcarea. Added to this, they are regarded as having had their unique leuconoid structures. However, they also consist of the fibres of protein which is seen inside the spongin.

According to this logic, we can say that the skeleton made of spongin fibres is found in the classes of Demospongiae under the Phylum Porifera.

Additional Information: It is told that the first demosponge was that ancestral presence which can be counted during Precambrian deposits by the end of Cryogenian period.

Note: Here we should note that spongin is found in both of the classes Calcarea and Demospongiae. But spongin with all prevalent characteristics is found only in Demospongiae. In Demospongiae, the spongin fibres and siliceous spicules both are present.