Stenostomum hemisphericum Nassonov, 1924

Noreña, Carolina, Damborenea, Cristina & Brusa, Francisco, 2005, A taxonomic revision of South American species of the genus Stenostomum O. Schmidt (Platyhelminthes: Catenulida) based on morphological characters, Zoological Journal of the Linnean Society 144 (1), pp. 37-58 : 46

Stenostomum hemisphericum Nassonov, 1924

( Fig. 2C )

Description: Adult specimens 1–2 mm long. Four- to five-zooid chains, 18 mm long. Body cylindrical. Anterior end blunt. Lateral and reduced ciliated pits. A slight body constriction between the pharynx and the intestine. Rounded posterior end, without intestinelacking region. Abundant semi-rigid cilia in the anterior and posterior regions. Rhabdites grouped in bundles vertically arranged in the epidermis. Colourless in life.

Anterior brain lobes separated into small independent masses (‘metamerics’). Posterior brain lobes enclosing an internal lobe (trilobate brain). A pair of light-refracting bodies, comprising a pear-shaped vesicle (0.01 mm in diameter) and a spherical element, refractory in its inner part (type 2) associated with the posterior lobe.

Oral pore rounded. Pharynx very long and provided with lateral, circular, oblique and longitudinal muscular fibres, especially developed on the anterior dorsal region. Finger-shaped glands surround the oral pore. Conspicuous pharyngeal sphincter. Excretophores regularly arranged in the anterior intestinal region; irregular on the rest of the body. Nephridiopore subterminal.

Distribution: Ukraine, Crimea ( Nassonov, 1924); São Paulo, Brazil, in aquaria ( Marcus, 1945a); margins of the Suriname River, near Paramaribo, Surinam ( Van der Land, 1970).

Discussion: The description given is based on Marcus’s (1945a) description for the Brazilian specimens with some differences with the Russian specimens described by Nassonov (1924): the Brazilian specimens have glands only surrounding the oral pore. This feature contrast with the Nassonov description. Nevertheless, these glands are not clear in the original drawings. The posterior brain lobes are larger in the Brazilian specimens and the light-refracting bodies more concave in the Russian specimens. These differences would not suffice to separate them because of other similarities: the proportions of the pharynx and intestine; the space taken up by the intestine; the ciliated pits; the shape of the oral pore; the musculature and the glands surrounding the oral pore ( Marcus, 1945a).

Figures

Figure 2. A, Stenostomum glandulosum. B, S. grande. C, S. hemisphericum. D, S. leucops. E, S. matarazzoi. Scale bars = 200 Mm.