While the invertebrates constitute almost 95% of the animal kingdom, our knowledge on their extracellular matrices, particularly on collagen, is very scanty. Our group has been studying the collagen polymorphism in some marine invertebrate tissues with focus on structure-function relationships and molecular evolution. We have established methods to purify unique collagen molecules from some rare tissues of crustaceans and molluscs. Some of these include the intramuscular tissues of the crustaceans and the cartilage and cornea of molluscs. The biochemical parameters in these tissues relating to collagen content, solubility and carbohydrate composition have been determined. The chain composition of these collagens were deduced by SDS-PAGE. We have analyzed the amino acid compositions of these collagens and that of isolated single alpha chains. The physicochemical properties and ultrastructural characteristics of some of these collagens were also studied. The results indicate that the principal component of crustacean muscle is a type V like homotrimer and that of molluscan cartilage and cornea is a unique heteromeric collagen resembling vertebrate type V and XI collagens. These collagens were invariably highly crosslinked, stabilized largely by bound carbohydrates and had significantly high denaturation temperatures. While the crustacean type V collagen formed regularly banded fibrils, the V/XI like collagen of molluscan cartilage lacked periodicity in fibril structure. We correlate the significance of our key observations to the possible functional consequence as well as evolutionary significance, based on the available data on other similar collagens.