1. A microscopical survey of some forty species from twelve invertebrate phyla confirmed the presence of collagen in all groups. 2. A second group of connective tissue fibers, distinguishable from collagen by permanganate/spirit blue staining and which includes invertebrate elastic fibers was found in all groups except the anthozoan coelenterates and the turbellarians. 3. Spirit blue positive fibers were variously found in the dermis, around nerves, amongst muscles, in blood vessels and epithelial basement membranes and traversing the mesoglea of several coelenterate types. It is probable that further work will confirm the physical elasticity of many of these fibers. 4. Anatomically the "spirit blue fibers" are oriented to antagonize muscles in the medusae, accommodate fluid pressures in vascular systems and oppose tissue deformation in many soft-bodied animals. 5. Specifically, in the latter category, radially oriented dermal "spirit blue fibers" oppose the radial distension of the body wall during the simultaneous circular and longitudinal muscle contraction which occurs in the passage of direct peristaltic waves in the burrowing holothurian Leptosynapta tenuis.