PHYIDAE), AN AVIAN TREMATODE WITH METACERCARIA RESTRICTED TO THE CONUS ARTERIOSUS OF THE FISH, MOLLIENESIA LATIPINNA LE SUEUR

A search for the natural host was without success, and as a result, the descrip- tion of Ascocotyle leighi is based on adults recovered from day-old unfed chicks which were infected with metacercariae. Infections of 28 chicks were attempted, 16 of which were successful. The adults undergo a transient development in the (All measurements in millimeters; 10 specimens, 72 hours or older, were measured) Diagnosis: with characters of the genus. Small distome. Body pyriform, concave ven- trally, 0.283 to 0.402 (av. 0.339) long by 0.084 to 0.122 (av. 0.099) wide at level of acetabulum. Anterior end 0.038 to 0.059 (av. 0.050) wide at level of lateral expansions of oral sucker and exhibiting an anterior lip-like projection. Cuticula entirely scaly-spinose. Unevenly distrib- uted pigment granules, probably remnants of cercarial eyespots, persisting dorsally in regions lateral to pharynx. Oral aperture terminal and surrounded by double row of 48 to 52 spines (24 to 26 in each row) directed posteriorly, those of anterior row 0.012 to 0.014 (av. 0.013) long and those of posterior row 0.009 to 0.011 (av. 0.010) long. Oral sucker elongate and conical, with posterior muscular projection which is often contracted into an "S" shape; acetabulum 0.031 to 0.042 (av. 0.036) in diameter, slightly post-equatorial, and retractable into ventro-genital sinus. Prepharynx 0.035 to 0.105 (av. 0.073) long, slender, ventral to posterior projection of oral sucker; pharynx 0.028 to 0.035 (av. 0.032) long by 0.021 to 0.028 (av. 0.023) wide; esophagus very short; intestinal ceca short and expanded, terminating just anterior to acetabulum. Ventro-genital sinus median, with single preacetabular gonotyl lying to left of median line. Seminal vesicle large and located left, slightly curved, extending transversely on level with anterior margin of ovary and tapering toward ovarian end into ejaculatory duct; ejaculatory duct turns abruptly left and arcs around left side of acetabulum to point near gonotyl, where it joins with metraterm to form common duct opening into ventro-genital sinus through genital pore between gonotyl and acetabulum. Testes spherical to ovoid, side by side at posterior end of body, 0.036 to 0.049 (av. 0.042) in diameter, and usually in contact with each other. Ovary spherical, 0.021 to 0.028 (av. 0.026) in diameter, anterior to, and in prox- imity of, right testis. Seminal receptacle spherical and median, although sometimes displaced slightly left, on level with posterior margin of ovary. Vitellaria lateral, extending from nlear posterior end of body to level of aperture of acetabulum. Uterus extending into posttesticular region, joining with ejaculatory duct dorsal to gonotyl to form common genital duct. Eggs 0.016 to 0.018 (av. 0.017) long by 0.009 to 0.011 (av. 0.010) wide.