Gastrotaenia cygni, a minute cestode lacking external segmentation, occurs beneath the ventricular cuticle of waterfowl. A lateral groove in the parasite maintains an undulating path along the strobila, and sets of reproductive organs follow one another in an undulating path corresponding to that of the groove. Sperm and vaginal ducts are directed toward the base of the lateral groove but do not penetrate the tegument. A redescription of the genus is included. Few recognized orders of cestodes have received less attention than the Aporidea. Representatives of the order are minute, monozoic forms which parasitize various regions of the gastrointestinal tract of waterfowl in the family Anatidae. As the original name suggests, genital pores are considered to be lacking. No life cycle within the order has been resolved. Of the five species currently comprising the order Aporidea, Gastrotaenia cygni Wolffhiigel, 1938 probably has the widest distribution. It has been reported from the proventriculus of swans as well as from beneath the ventricular cuticle of many species of ducks from Eurasia and North and South America. On the other hand, the scanty literature dealing with G. cygni has dealt primarily with reports of occurrence. Studies concerning the nature of the parasite itself number only two (Wolffhiigel, 1938; Heck, 1958), and the results of these have been conflicting. The purpose of this study, therefore, was to elucidate the morphology of the animal and thus resolve conflicts in the existing literature. Received for publication 21 February 1969. * Part of a dissertation submitted to the Graduate School, Colorado State University, in partial fulfillment of the requirements for the degree Doctor of Philosophy. The research was supported by NIH Training Grant 2 TI Al 94-06 and the Institute of Paper Chemistry, Appleton, Wisconsin. t Present address: Department of Biology, Wisconsin State University, Oshkosh. MATERIALS AND METHODS Waterfowl hosts were shot or live-trapped in Larimer and Weld counties, Colorado, from November 1964 to May 1966. In the laboratory, ventriculi and proventriculi were cut open, and the keratohyalin cuticles of the ventriculi were removed with forceps. Glandular surfaces were examined under strong light and with the aid of a magnifying lens for the presence of G. cygni. Proventriculi were also examined. Specimens to be studied by light microscopy were fixed in a number of ways, depending upon the staining technique to be used. Fixatives utilized were 70% ethanol, 10% formalin, AFA, Bouin's, and Zenker's. Stains employed were Mallory's trichrome, Harris' alum hematoxylin and eosin, Ehrlich's acid hematoxylin and eosin, Halmi's azide fuchsin azan, Mayer's acid carmine, Periodic Acid Schiff's, Masson's trichrome, and Movat's pentachrome I. In addition, sectioned material was treated with catechol, fast red salt B (Johri and Smyth, 1956), Roger's AgNO3, and Cajal's AgNO3 formula No. 3 (Davenport, 1960). From specimens embedded in paraffin, cross, frontal, and sagittal sections were made at thicknesses ranging from 4 to 15 A. For the thinner sections, a cryostat was used in which the temperature was maintained at from 10 to 15 C. In addition, whole mounts were stained in Mayer's acid carmine and counterstained with fast green.