The purpose of this investigation was to study the ultrastructure of the mature cysticercoid of gymenolepis diminuta with special emphasis on the parenchyma. Cysticercoids were dissected from Tribolium confusum, fixed in glutaraldehyde- osmium tetroxide, dehydrated in ethanol and embedded in epon. Thin sections between 250-300 A in thickness were stained with uranyl acetate and lead citrate and observed on a Philips lOOC electron microscope. The photomicrographs revealed that the parenchyma is composed largely of macrocells with very distinct membranes, nuclei and nucleoli. Microcells were also present in the parenchyma. These cells were connected with the cuticle by protoplasmic extensions. The scolex contained f ibroua structures that made up most of the suckers. The suckers showed muscle strands as rays extending outward toward the sucker matrix. The study further showed that circular and longitudinal muscles are found in the scolex and periphery of the cysticercoid. It was observed that the cysticercoids are covered with projections called microtriches. The proximal portion of the microthrix was medullated but not as distinct as in the adult worm and is continuous with the cuticular matrix. The distal portion oi the microthrix was an unmedullated, tapered structure. Mitochondria were present in the cuticle, subcuticle and in the macrocell periphery. Pore canals were also present throughout the parenchyma and strobila.

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