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An apical complex comparable to that found in the Sporozoa is described from zoospores of Dermocystidium marlnum Mackin, Owen, and Collier, a pathogen of the American oyster (Crassostrea oirginica Gmelin) . The complex consists of a conoid, polar ring, up to 39 subplasmalemmal microtubules, rhoptries, and micronemes. Micropores and a subpellicular membrane equivalent were also found. Acid phosphatase activity was found in cistemae of the endoplasmic reticulum, inclusion bodies, and vesicles within the conoid lumen. No polysaccharides were detected in the rhoptries and micronemes using the Thiery method. Observations indicate that D. marinum is a protozoan in the subphylum Apicomplexa and is most closely related to the coccidian Sporozoasida Leuckart. Since the initial description by Gustafson et al. (1954), nwnerous authors have noted that coccidians, gregarincs, Sarcocystis sp ., Besnoitia sp., and Frenkelia sp. all have, in one or more cell stages iJ1 the li£e cycle, a hollow, cone-like structure at Lhe antedor end of the cell. This organelle, termed a conoid, consists of tubular subunits, spirally arranged to form a truncated hollow cone 0.08 μ.m to 0.4 μ.m in diameter at the anterior end and 0.2 μ.m to 0.53 μ.m at the posterior end ( Scholtyseck, 1973). Anterior to the conoid are 2 (possibly 3: Porchet-Hennere, 1975) preconoidal rings consisting of elecb·on-dense granular material connected to the conoid by a canopy of elecb·on-dense material which has an opening at the extreme anterior end. A ring of electrondense material, the polar ring, encircles the conoid. The ring is an anterior elaboration of the two closely opposed unit membranes (''subpellicular membrane") which lie slightly under the plasmalcmma. Two or more flaskshaped, membrane-bound sacs of electrondense material converge on the conoid, the neck of each "flask" lying within the conoid Ju.men. These sbuctw·es, termed rhoptries or paired organelles, are believed to contain lytic enzymes which aid the organism in host cell penetration. Scattered throughout the anterior 1⁄2 or 11.i of the cell are the micronemes which are cord-like, membrane-bound sacs of electron-dense material. Some workers have suggested the rhopbi es and micronemes are Received for publication 26 January 1976. • Contribution No. 786, Virginia Institute of Maline Science, Gloucester Point, Virg inia 23062. 959 one functional system, possibly the latter giving rise to the former (Vivier and Petitprez, 1972) . Attached to and radiating from the polar ring are generally 22 to 24 microtubules which lie beneath the "subpellicular membrane" and extend most of the distance toward the cell posterior. In most of the organisms with conoids are also found 1 to 9 invaginations of the plasmalemma, the micropores. These specialized stmctures often have a collar of electron-dense mate1ial around the distal part of the invaginatiou and another outer cylindrical collar fonncd by infolding of the "subpellicular membrane." The latter is discontinuous at the proximal end of the invagi.nation. Collectively the conoid, polar ring, subpellicular microtubles, micronemes, and rhoptries have been termed the apical complex (Levine, 1973). The consistency with which the complex has been observed in Protozoa that appear to be related for other reasons, has led to the consb.uction of the subphylum Apicomplexa Levine 1970, which includes the piroplasms, gregarines, and coccidians. The latter group includes sp ecies of Toxoplasma, Sarcocystis, Besnoit ia, Plasmodium, Frenkelia, Eirneria, and Isospora. In this paper it is shown that the oyster pathogen, Dermocystidium marinum, also has the organelles described above and thus has affinities with the Apicomplexa. There are numerous suggestions in the literature as to the affinities of the pathogen, ranging through such diverse groups as the Rhinosporidiaceae 960 THE JOURNAL OF PARASITOLOGY, VOL. 62, NO. 6, DECEMBER 1976 of the Endomycctales (Mackin, 1962), Sy11chytriaceae of the Chytricliales (Mackin and BoswolJ, 1956), Labyrinthulia ( Mackin and Ray, 1966), Ascomycctes (Mackin, 1951), lJaplosporida (Sprague, 1954) , and EntomophtJ1oralcs (F. K. Sparrow, Jr. in Ray, 1954). Mackin and Ray ( 1066) have renamed the organism Labyrinthomyxa marina thus considering it to be allied to the Labyrinthulia. Their decision resulted from observations of gliding cells with labyrinlhulid "tracks," plasmoclia, and vegetative division figures similar to those of Labyrinthomyxa sauvageatii (Dul,osct1 1921). Perkins ( 1974) did not find any laby1inthulid characteristics in the oyster pathogen, except an unusual kinetosome granule, and e»'Presscd the opinion that D. marinu.m could not, at that time, be related to any k11own group

Zoospores of the Oyster Pathogen, Dermocystidium marinum. I. Fine Structure of the Conoid and Other Sporozoan-Like Organelles