Stenostomum grande Child, 1902

(Fig. 2B)

Stenostomum oesophagium Kepner & Carter, 1931

Description: Single specimens 1–2 mm long, two- to six-zooid chains up to 6 mm long. Body cylindrical, anteriorly rounded. Dorso-lateral ciliated pits longitudinally elongated. Constriction at the level of the oral pore. Posterior end with an intestine-lacking region. Heterogeneous ciliated epithelium, with scarce semirigid sensory cilia. Dorsal epidermis with rhabdites arranged perpendicularly in rosettes. The rhabdites are parallel to the surface in the ventral epidermis, where three longitudinal lines without rhabdites can be seen. Colour in life whitish. From the apical end to the brain, the epidermis bears compact rhabdites giving it a darker colour.

Anterior brain lobes deeply dentate. A pair of lightrefracting bodies type 1 associated with the posterior lobes.

Dilatable, semicircular oral pore. Pharynx occupying 1/5–1/6 of body length. Pharyngeal glands (type a) in the anterior half. Transition between pharynx and intestine with a narrow sphincter. Excretophores irregularly distributed along the intestine. Nephridiopore proximal to the terminal end.

Distribution: Chicago (Child, 1902), Illinois and New York (Graff, 1913), Pennsylvania (Kepner & Carter, 1931), and Virginia (Nuttycombe & Waters, 1938; Kolasa, 1991), USA; southern Finland (Nassonov, 1924); Kola Peninsula (Nassonov, 1925); St Petersburg (Leningrad) (Nassonov, 1926); São Paulo and Paraná State, Brazil (Marcus, 1945b); near Onverwacht, Surinam (Van der Land, 1970); Konin Lakes, Poland (Kolasa, 1977). Los Talas, Berisso, Buenos Aires, Argentina, October 1998 to April 1999 and March 2000. This species has not been previously recorded in Argentina.

Stored material: Ten sagittally sectioned specimens in the CHMLP. No. 5304.

Discussion: Marcus (1945a) indicates two forms: ‘typica’ which corresponds to Child’s (1902) description and ‘megista’ for the specimens coming from Tieté River, São Paulo, Brazil. The difference between these forms is based on body size, brain development, arrangement of the rhabdites and colouring of the pharynx. The specimens from Berisso, Buenos Aires, correspond to the form ‘typica’.

Mature forms of this species have been recognized. Nuttycombe & Waters (1938) and Marcus (1945b) observed specimens with an unpaired testis at the level of the pharynx and Kepner et al. (1933) found mature female gonads.