TREMATODOS DE PECES DE AGUAS MEXICANAS DEL PACIFICO. XVIII. UN NUEVO GENERO Y UNA NUEVA ESPECIE DE MONOGENOIDEA BYJOVSKII, 1937

Resumen

Parancylodiscoides chaetodipteri, n. g., n. sp., is described from the gills of a marine fish, Chaetodipterus zonatus (Girard), collected at Salina Cruz, Oaxaca, México (Golfo de Tehuantepec, North Pacific Ocean).
Parancylodiscoides, n. g., is allocated in the family Dactylogyridae, subfamily Ancyrocephalinae, and includes small monogeneid trematodes with a trilobulate prohaptor and three pairs of cephalic glands as organs of adhesion. The opisthaptor is provided with two unequal bars without articulation and two pairs of small unequal hooks articulated with their respective bars and with muscular attachments to the trunk of the body; additionally, there are four pairs of small marginal hooks, three pairs of these being lateral and one pair ventral; a fifth, pair of ventral hooks is located anterior to the bars; two claviform glands extend from the level of articulation of the dorsal hooks with their respective bars to the anterior margin of the opisthaptor. The pharynx is large and pyriform; intestinal ceca, with slight lobulations along the total length of their external margins, are united posteriorly. Two small eyespots are located lateral to the anterior margin of the pharynx. A single testicular mass is localized posterior to the ovary and immediately anterior to the opisthaptor; the vas deferens opens into a large claviform seminal vesicle; the copulatory apparatus consists of a slightly sclerosed ejaculatory bulb which is curved or sausage-shaped; the strongly walled cirrus is winding and widely sinuous; an accessory organ or vesicle is situated parallel to the prostatic vesicle, at the level of the genital pore and
opens into the cirrus.
The small genital pore is median, localized very much posterior to the arch of the intestinal bifurcation. The ovary is submedian and Mehlis' gland is localized between the ovary and the copulatory apparatus; an elongate and well-developed vagina with thick and folded walls, is obliquely and ventrally situated under the right intestinal ceca; and, the sacculate uterus is median and contains few, thinshelled eggs, trapezoidal in form and with a posterior filament. Vitelline glands extend from the level of the pharynx to posterior to the intestinal union, dorsal to the lateral cecal arms; groups of vitelline follicles exlend intercecally at the levels of Mehlis' gland and posterior to the pharynx. 
The proposed new genus resembles the genus Tetrancistrum Goto et Kikuchi, 1917 in possessing one pair of unarticulated bars and two pairs of subequal hooks on the opisthaptor, but it can be differentiated from this genus by the position of the vaginal aperture and by the structural arrangement of the male reproductive apparatus. It resembles Ancylodiscoides Yamaguti, 1937 in general aspects of the male reproductive apparatus and the vaginal aperture, but may be distinguished from this genus by certain structural features of the
male reproductive organs, and most markedly by the structure of the opisthaptor; moreover, species of the genus Ancylodiscoides have been reported only as parasites of freshwater fishes.