Silk thread is obtained from the cocoon of Bombyx mori. It mainly contains a water soluble protein called as serecin. Fibrin is a protein that forms a meshwork, trapping erythrocytes, to become blood clot. Its precursor is fibrinogen. . Chitin is a homy substance or nitrogenous polysaccharide, insoluble in water, alcohol, dilute acids and digestive juices of most animals. It forms part of the cuticle of arthropods and is found sparinlgy in certain other invertebrates. Mucin is a group of glycoproteins secreted by certain cells especially those of salivary glands.