Faustula gangetica ( Srivastava, 1935 ) Yamaguti, 1958

Faustula gangetica ( Srivastava, 1935) Yamaguti, 1958 View in CoL

Figs. 3 View FIGURE 3 , 4 View FIGURE 4

Material examined: Adult specimens from intestine of Hilsa shad : one hologenophore examined by molecular and morphology techniques; two paragenophores examined only by molecular techniques; eight paragenophores examined ( IEE RAS 14367) only by morphology techniques.

Sequences deposited: PV248946 and PV248946 ( 28S rRNA gene); PV253915– PV253917 (ITS2 region).

Description (measurements as min-max and mean in parentheses). Body oval with truncated anterior extremity and conical posterior end, with distinct shallow depression in median area of second quarter of ventral side; length 743–1052 (854), maximum width 351–502 (418) in middle part of body. Tegument spined. КопИЯ_1Anterior or antero-dorsal side of ventral depression with 5–6 rows of cross-oriented ridges of large subtegumental muscle fibres; muscle fibres in compression state form distinct ridges on surface of ventral depression. Forebody 36.9– 39.1% (37.7%) of body length. Oral sucker trapezoid, 74–102 (86) × 94–111 (104), mouth slightly subterminal. Ventral sucker rounded to oval, 94–124 (104) × 100–125 (113), enclosed in ventral depression. Sucker-width ratio 1: 1.06–1.13 (1: 1.08). Prepharynx absent. Pharynx 76–95 (85) × 62–87 (68), surrounded by large muscle fibres extending to outer surface of oral sucker. Oesophagus length 67–90 (76). Intestinal bifurcation within posterior quarter of forebody. Caeca terminate in anterior half of hindbody; post-caecal space 36.2–42.4% (39.4%) of body length.

Testes two, smooth or slightly indented, oval, occasionally approximately rectangular, opposite, equally overlapped by ventral sucker region and anterior aspect of hindbody or more protruded beyond posterior margin of ventral sucker, up to being located immediately posterior to ventral sucker; right testis 109–144 (134) × 75–105 (92); left testis 113–160 (131) × 83–109 (93). Post-testicular space 34.9–39.8% (36.9%) of body length. Cirrus sac pyriform, median, surpasses ventral sucker short distance into hindbody, 141–217 (197) × 104–131 (123), containing short unarmed thick-walled cirrus, very short ejaculatory duct, vesicular pars prostatica with field of numerous prostatic cells and long, tubular, sinuous or convoluted seminal vesicle; communicating with common genital atrium. Seminal vesicle with small sphincter on distal end; pars prostatica approximately 1/3–1/4 of cirrus sac length; field of prostatic cells extensive, surrounding pars prostatica and seminal vesicle. Common genital atrium tubular, with sphincter on distal end. Genital pore median, enclosed in ventral depression, post-bifurcal, just posterior to zone of transversely oriented muscle fibers.

Ovary 8–9-lobed, located partially posterior to testes, slightly sinistro- or dextro-submedian, 152–158 (155) × 134–146 (141). Oötype at level of ovary. Canalicular seminal receptacle small, but discernible. Laurer’s canal long, opens median in posterior quarter of hindbody. Vitellarium follicular; follicles in two lateral fields, extending from level of intestinal bifurcation to ovary. Uterus extensive; metraterm thin, poorly differentiated, opens into common genital atrium, sinistro-distally to male pore. Eggs oval, 18–20 (19) × 11–12 (12). Excretory vesicle Y-shaped with short stem surrounded by distinct sphincter; excretory arms extending anteriorly to level of posterior margin of pharynx; excretory pore terminal.

Remarks. The oval body shape, the median position of the common genital pore, the multilobed ovary and its partially post-testicular position, the position of the testes at level of the posterior margin of ventral sucker or immediately posterior to it, as well as the position of blind ends of the caeca at the ovary level, the position of the Laurer’s canal pore near to posterior end of the body, the rather short lateral fields of the vitelline follicles, and the cirrus sac and the excretory vesicle morphology support the identification of our specimens as members of the genus Faustula (compare with Bray 2008; Dronen et al. 2021). The five currently known species of Faustula can be differentiated by position of the common genital pore relative to the bifurcation of the intestine and the position of the cirrus sac relative to the ventral sucker ( Dronen et al. 2021). Our specimens correspond to the description of F. gangetica sensu lato in all these characters, as well as mean and maximum morphometric values ( Table 2 View TABLE 2 ; Srivastava 1935; Hafeezullah & Siddiqi 1970; Simha 1974; Garner et al. 2019). The concept of F. gangetica sensu lato is based on the idea of conspecificity of F. gangetica sensu stricto (= Orientophorus gangeticus Srivastava, 1935 ) with Orientophoros ilishii Srivastava, 1935 and Faustula mandapamensis Simha, 1974 ( Dronen et al. 2021).

The presence of the shallow depression in ventral side of the body, the discernible muscle fibres between the pharynx and the oral sucker, and the zone of distinctly differentiated transversely oriented muscle fibres located anterior to the common genital pore, are reported here for the first time. None of the existing descriptions mention these three features of this species ( Srivastava 1935; Hafeezullah & Siddiqi 1970; Simha 1974; Garner et al. 2019). Nevertheless, all of them are present in our re-examined vouchers of F. gangetica , deposited in the Smithsonian National Museum of Natural History ( Fig. 4 View FIGURE 4 ).