A unique ligamentary system occurs in the polychaete Nephtys. Its fine structure and physico-chemical properties have been investigated.

The ligaments consist of alternating bands of elastic and inelastic elements, and they attach to the body by means of crystalloid attachment nodes. The nodes are probably glycoprotein in nature as they contain both protein and polysaccharide. They are not birefringent, and they are insoluble in comparison with the other components of the ligaments. Their structural stability is due mainly to strong ionic and hydrogen bonding. They resist enzymatic digestion.

The inelastic elements are bundles of thin, birefringent fibrils, 10 to 14 µ in length. The birefringence is positive with respect to the long axis of the fibrils and is primarily intrinsic in nature. The estimated coefficient of birefringence has a value between 10 and 20 x 10-3. The fibrils consist of a protein with an isoelectric point near pH 6.3. The visible structure of the fibrils is due to both hydrogen and ionic bonding, while the birefringence is more dependent on hydrogen bonding. The fibrils resist heating and peptic digestion, but are slowly digested by trypsin. Of all the fibrous proteins, they most resemble collagen.

The elastic elements are extremely fine, granular cross-membranes bearing delicate filaments to which the ends of the fibrils are attached. They are continuous with a bounding membrane surrounding the ligament, which also bears granules and is also elastic. Some of the granules appear to be folds of the membrane itself. The membranes consist of a protein which is quickly digested by trypsin but not digested by pepsin. Its isoelectric point lies near pH 5. Hydrogen bonding is more important than ionic bonding in the membranes, and Van der Waals forces may also contribute. Classification of this protein is not yet possible.

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