Stenostomum bicaudatum Kennel, 1889

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 : 43

Stenostomum bicaudatum Kennel, 1889

( Fig. 1C )

Stenostoma bicaudatum Kennel, 1889

Description: Solitary individuals, 1–1.5 mm long. Chains of usually two zooids, 2.5 mm long. Body digitiform, ventrally flattened. Anterior end rounded, with a short mid-dorsal groove. Posterior end with intestine-lacking region, and forming a tail with two appendages (one dorsal, short and thick and one ventral thin, and longer). Front lateral semi-spherical ciliated pits. Homogeneous ciliated epithelium with refractive inclusions. Rhabdites abundant, uniformly distributed, except for two dorsal tracts lacking rhabdites on both sides of the dorsal mid-line.

Anterior brain lobes deeply dentate. Three pairs of light-refracting bodies (type 2) anterior and overlapping the sensory cells of the ciliated pits. The two first pairs are formed by a single sphere and the third pair by two or three spheres (rarely four).

Oral pore rounded. The pharynx is long, occupying almost half the body. It bears longitudinal creases giving it the appearance of two ‘oblong sacs’. Welldeveloped circular, dorsal and diagonal musculature. The claw-organ is found on the dorsal wall of the pharynx, behind the oral pore. It comprises six tentacles covered with an adhesive substance and associated with unicellular glands, and is characteristic of this species. The pharyngeal glands (type a) open onto the posterior portion of the pharynx. Presence of a muscular sphincter between the pharynx and the intestine. No excretophores present. The intestine narrows caudally. The dorsal excretory pore opens anteriorly to the dorsal appendage.

Distribution: Trinidad ( Kennel, 1889); near Paramaribo, Brazil ( Marcus, 1945b); Surinam ( Van der Land, 1970); Paraná River, Argentina ( Noreña-Janssen, 1995).

Discussion: There are several differences between the specimens described by Nuttycombe & Waters (1938) and the South American specimens. These authors mention the presence of chains of up to nine zooids 7 mm long, while in South America only two specimens were found with zooid chains. They also mention the absence of pharyngeal glands. However, Marcus (1945b) mentions small pharyngeal glands associated with the posterior region of this organ.

Marcus (1945b) describes the body as dorsally flattened and ventrally convex and he states that animals swim with their ventral surface upwards, in disparity with the descriptions of Nuttycombe & Waters (1938) and Noreña-Janssen (1995). Marcus (1945b) does not mention the heterogeneous arrangement pattern of the rhabdites described for the specimens in North America (Nuttycombe & Water, 1938).

Figures

Figure 1. A, Stenostomum amphotum. B, S. arevaloi. C, S. bicaudatum. D, S. ciliatum. E, S. cryptops. Scale bars = 200 Mm.