||This study of the functional morphology of the male reproductive system in astacidean crustaceans has allowed for comparisons of representatives of the superfamilies Parastacoidea, Enoplometopoidea, Nephropoidea, and the Astacoidea with a focus on the crayfishes. Tissues from the testes and (to the extent possible) vasa deferentia were prepared for light and scanning electron microscopy and specimens of the following families were used: Parastacidae -- Parastacoides tasmanicus tasmanicus, Astacopsis franklinii, Parastacus nicoleti; Enoplometopidae -- Enoplometopus occidentalis; Nephropidae -- Homarus americanus; Astacidae -- Pacifastacus leniusculus trowbridgii; Cambaridae -- Cambaroides japonicus, Cambaroides similis, Cambarus (Puncticambarus) acuminatus, C. (Hiaticambarus) longulus, Procambarus (Ortmannicus) fallax, P. (O.) zonangulus, P. (Scapulicambarus) paeninsulanus, and Orconectes (Procericambarus) rusticus. The single organ testis is "H-shaped" in members of the Parastacoidea, Enoplometopoidea, and Nephropoidea and consists of a pair of longitudinal lobes, each composed of an anterior and posterior lobule joined by a transverse commissure or bridge. The derived "Y-shaped" pattern of the testis of the Astacoidea is trilobed and consists of a pair of anterior lobules and a median posterior lobule that in most adult Cambaridae are joined by a trifurcate, constricted stalk, a structure that is lacking in the Cambaroidinae and Astacidae. The sac-like acini lie in the axes of the testicular lobules and produce spermatozoa. As spermatogenesis proceeds, each acinus becomes larger and, with spermiogenesis and the expulsion of spermatozoa into the collecting ducts, undergoes one or two of three fates: (1) acinus regeneration occurs and another cycle of sperm production ensues (adopted exclusively by the Astacidae and Cambaroidinae); (2) secondary acini develop in the wall of existing acini, converting the primary acinus into a passageway to the collecting tubules; or (3) the acinus degenerates and new acini arise from collecting tubules (employed only by the Cambarinae and Cambarellinae). The first and second fates have been adopted by the Parastacoidea, the Enoplometopoidea, and the Nephropoidea. In the Cambarinae and Cambarellinae, the germinal cells are recognizable only in the acinar buds from the collecting tubules and when they assume the role of spermatogonia; they are not evident along the lengths of the tubules nor are they present within an acinus after the onset of spermatogenesis. In all other astacideans examined, the germinal cells seem always to be present in the collecting tubules. Additionally, they appear in the walls of acini by the time the spermatogenic elements are being converted to spermatids, frequently forming clusters, the primordia of secondary acini in the Parastacoidea, the Nephropoidea, and occasionally the Enoplometopoidea. Germinal cells may be disposed in a partial layer or scattered within the walls of an acinus and constitute the initial spermatogonia of a new cycle of sperm production. This is what occurs in the acini of the Astacidae, Cambaroidinae, Parastacoidea, Nephropoidea, and Enoplometopoidea. Introduction -- Parastacoidea -- Enoplometopoidea -- Nephropoidea -- Astacidae -- Cambaridae -- Cambaroidinae -- Summary and Discussion.