A lab observation notes that a bath sponge remains flexible because its skeleton is mainly a network of protein fibers. The observation points to:

Spongin is a fibrous protein that forms a flexible skeleton in some sponges. It differs from hard mineral spicules and gives certain sponges an elastic texture. This MCQ belongs to the Porifera topic in Animal Diversity-I and is written to test concept understanding rather than memorized wording.