A thin, compressible, lateral suture and ventral plate overlap permit limited movement of the thick and rigid dorsal and ventral plates of Fuscouropoda agitans. Seven pairs of large dermal glands debouch onto the surface. Trochanteral rotation permits defensive leg adpresion and an insectan type of ambulation. The complex hypopharynx-pedipalpal-coxae has a buccal and cheliceral cavity separated by an atriculated epipharynx. The pharynx is Y-shaped in cross section. Extensive paired salivary glands lie above the very long and dexterous 3-segmented chelicerae, and a large pair of coxal glands debouch on coxae 1. From four blunt-ended tracheae, bundles of unbranching tracheoles extend in specific tracts to all organs. The ventriculus is small with three pairs of large caeca; a tightly packed single layer of digestive cells individually enlarged to absorb-phagocytize and digest the food. A typical mesostigmatid excretory tube is present. A typical acarine synganglion is present; mixed nerves have a basal swelling. A postulated neurosecretory organ arises from the pedipalpal nerve. The oocytes enlarge within funicular stalks from the walls of the small median ovary. A large spermatophore is stored in the seminal vesicle; fertilization occurs during oviposition. A tension hinge partially opens both male and female genital plates; closure effected by muscles acting on very long genital plate apodemes. Within sequentially produced spermatogonial cysts of the testes, meiosis is completely synchronous. A large, multilobed male accessory gland produces a large volume of seminal fluid; a mixture of at least four secretions. The origins and insertions of the body wall, genital organ, digestive tract, mouthpart and leg muscles are listed and illustrated. A comparison of anactinotrichid and actinotrichid mites indicates fundamental and consistent morphological differences in aspects of the cuticle, leg articulations, digestive system, excretory system, reproductive system and coxal glands.Based on external morphology it has been suggested that the Order Acari, the mites and ticks, constitute a diphyletic grouping of Actinotrichida a n d Anactinotrichida (Zachvatkin, '52; Hammen, '72). The Suborders Prostigmata, Acaridida, and Oribatida belong to the Actinotrichida; and the Mesostigmata and METHODS AND MATERIALSA viable culture of Fuscouropoda agitans (Banks, '08) permitted selection of specimens by age or sex and the observation of internal and external parts in mounted, living specimens. Culture tubes made of snap-cap vials with an internal base of plaster-of-Paris: charcoal were kept in desiccators at 95% relative humidity. A small square of 200 mesh, stainless steel screen welded over a circular hole in the cap provided aeration without loss of specimens (Rockett and Woodring, '72). Though related uropodids are probably at least in part predaceous on other mites (Radinovsky, '65; Rohde, ' 591, F. agitans was maintained solely on ground grain and the fungus that grew thereon. Entire specimens and dissected parts we...