What is left, when bath sponges dry up?
a)Spicules
b)Holdfast
c)Tentacles
d)Spongin fibers

Hint Bath sponge is also known by the name of Euspongia. They are called bath sponges due to their texture and appearance which is similar to that of the sponges used for bathing purposes. The bath sponge is made up of porous collagen fibers which give it a sponge-like appearance.

Complete answer:
The bath sponges are made up of a very high porous network of collagen fibers which are called the spongin fibers. These skeletons are obtained by cutting the growing sponges and placing them in water till their flesh gets completely removed. Then we are left with the skeleton which is spongin fibers. The bath sponge is only found attached to the substratum in the shallow water. They do not have spicules present on their body. It has a globular shape. It also has the presence of oscula on the projections or the canals. They have a commercial purpose of the bath sponge used for bathing purposes. They are mostly found in the marine waters. The sponges stay alive only in the saltwater and die when taken out of the saltwater. They also die when kept in the freshwater. If they are exposed to air then their pores get filled with the air and then they eventually die.

So, the answer is ‘spongin fibers’.

Note: The bath sponge has an exoskeleton made up of collagen proteins known as the spongin fibres. When they are exposed to air or are kept in the freshwater then they die and leave their exoskeleton behind them, which can be used for commercial purposes after cleaning and bleaching them.