In Limulus striated muscle thick filaments shorten as the sarcomere shortens below rest length. Length tension studies show that this filament shortening is tension generating. Here we propose to study additional invertebrate muscles (barnacle adductor and lobster slow and fast abdominal flexors) to determine whether the Limulus phenomenon is generalized. Further we propose to determine the mechanisms of thick filament shortening. With respect to isolated thick filaments of these muscles we will: 1. determine ATPase activities of long and shortened filaments by pH Stat, 2. determine whether and which filament proteins are phosphorylated by two dimensional gel electrofocusing and autoradiography, 3. determine the mass of long and shortened thick filaments by STEM Biotechnology Resource at Brookhaven National Laboratory, 4. determine the elemental composition of long and shortened thick filaments by proton induced x-ray emission spectroscopy at Brookhaven National Laboratory, 5. determine the surface lattice structure of long and shortened thick filaments by high resolution, low dose stero STEM and ultrasoft x-ray contact microscopy, 6. determine the length-tension relationships as a function of sarcomere length of these muscles using laser diffraction and standard electrophysiological techniques. If thick filament shortening is a component of tension generation in invertebrate striated muscles, knowledge of its mechanisms will have important effects on our understanding of how muscle works in general and perhaps our understanding of how non-muscle cell motility is generated.