Scanning electron microscopy (SEM) of the surface of Hymenolepis diminuta, particularly the scolex, indicates that dense populations of microtriches occur on the rostellum, suckers, and scolex proper. Average microtrichoid densities of 49.2/,U2, 64.8/,U2, and 56.4/j42 were obtained for these areas. Differences in size were not observed. The excellent preservation of microtriches attests the quality of the critical point drying method for preparing cestodes for study by high magnification SEM. Fine structural studies of the tegument of cestodes have demonstrated that the surface cytoplasm is extended as microtriches, consisting of cylindrical cytoplasmic bases capped by dense structures termed "shafts" (Read, 1955; Rothman, 1963; Jha and Smyth, 1969). Functionally, these microtriches have been suggested to: (1) increase the surface area for absorption and secretion; (2) aid in maintaining position in host; and (3) agitate the microhabitat (review by Lee, 1966, 1972; Smyth, 1969, 1972). Such functions are in part supported by observations which suggest that the density of microtriches changes throughout the strobila (Rothman, 1963; Berger and Mettrick, 1971). Although Rothman (1963) reported that in Hymenolepis diminuta "the apical region of the scolex is devoid of microtriches or has very fine ones" several authors have reported their presence on the scolex of other cestodes: Echinococcus granulosus (Jha and Smyth, 1971), E. multilocularis (Sakamoto and Sugimura, 1969, 1970), Diphyllobothrium erinacei (Yamane, 1968), Taenia hydatigena (Featherston, 1971), sparganum of Spirometra erinacei (Kwa, 1972), and Hymenolepis nana (Rosario, 1962). Berger and Mettrick (1971) reported polymorphism in the microtriches from Hymenolepis diminuta, H. microstoma, and H. nana by SEM. They suggested that the microtriches played a role in locomotion of these organisms within the host's gut. The only other SEM of H. diminuta described the surface topography of the eggshell (Voge, 1969). The present study was undertaken in an attempt to determine by scanning electron Received for publication 5 January 1973. microscopy if such microtriches were present on the apical region and suckers of H. diminuta and if the variation in size of these microtriches, as reported by Berger and Mettrick (1971), could be confirmed. MATERIALS AND METHODS The Rice Institute strain of Hymenolepis diminuta was maintained in the laboratory in Holtzman rats. Adult worms were collected from experimentally infected rats. Specimens were fixed in cold 4% paraformaldehyde (Lynn et al., 1966) in 0.1 MN oacodylate buffer for 2 hr, and subsequently sonicated to remove intestinal debris. Tissues were rapidly dehydrated in an ascending series of ethanol solutions, transferred through a series of intermediate fluids (3:1, 2:1, 1:1) of 100% ethanol and amyl acetate to 100% amyl acetate according to the procedures of Anderson (1951, 1966). Specimens were transferred to liquid CO2 in a Denton DCP-1 critical point dryer until the amyl acetate was removed. The temperature was elevated to above the critical point of CO, (31 C, pressure 72.9 atm) and bled off. The dried specimens were positioned on metal specimen stubs, outgassed in a vacuum evaporator, rotarycoated with gold palladium (200 A or less), and examined with an AMR 1000 scanning electron microscope. Values obtained for microtrichoid density were determined by counting microtriches from 1-/u2 areas chosen in a random manner. Values represent minimum values as all microtriches may not be visualized in the micrographs. Diameter values were determined by measuring the diameters of 25 randomly chosen microtriches. Stated values represent the average followed by the range in parentheses.