Helobdella obscura

External morphology

Body elongated, relatively narrow and thick; total length-to-maximum width ratio greater than 4 (4.3–7.9). Without papillae or tubercles; without a nuchal gland or chitinous dorsal plate. Annuli simple, not 63 subdivided. Mouth situated on the anterior lip of the acetabulum, at the level of II/III. Acetabulum small, its diameter slightly less than one-third of the maximum body width; subterminal, with its axis extending the axis of the body. Gonopores separated by a single annulus (XIIa2); the male gonopore is located at XIIa1/a2, and the female at XIIa2/a3. Two eyes present on segment IV. Anus situated between the last two segments, at XXVI/XXVII. Somites I and II are fused into a single annulus, though a faint furrow (I/II) is sometimes present; somite III is uniannulate or indistinctly biannulate; somite IV consists of a subdivided annulus, remaining undivided on the ventral side where it forms the posterior lip of the acetabulum; somite V is biannulate on both surfaces, although a faint furrow (Va1/a2) may be present dorsally; somites VI through XXIV are triannulate (or complete); Segments XXV and XXVI are bi-annulate, though the posterior annulus of XXVI is sometimes barely visible and two to three times smaller than the anterior one (a1 > a2); segment XXVII is uni-annulate, sometimes partially subdivided. The coloration is uniform—devoid of striae or maculae—presenting a violet-gray or dark violet-green hue.

Internal morphology

The pharynx is straight, extending to XII/XIII, with salivary glands composed of small cells scattered throughout segments XI to XIV. The esophagus, located in XIII, is short and wide. The crop, extending from XIV to XIX, is a thick-walled tube divided into six globose chambers by intersegmental constrictions. Each chamber occupies a single somite; there are no lateral caeca, nor even post-caeca. The intestine features the customary four pairs of caeca—which are clavate (club-shaped) and directed anteriorly—originating between segments XX and XXII. There are six pairs of testes, extending from XII/XIII to XVII/XVIII. The vasa deferentia ascend as fine, whitish threads to XII/XIII, then descend along a recurrent path to the ganglion in segment XIV; at this level, they gradually increase in diameter or thickness to form the spermatic ducts. The pre-atrial loop of the spermiducts is very long, extending at least as far as XVII and XVIII, and sometimes even to XX and XXI. The ascending limb is formed by the ejaculatory ducts; throughout its entire course, the tract exhibits curves and intercrossings between the spermiducts of the left and right sides. The ovaries are long, extending as far as XVII and XVIII.

Habitat & range

Habitat

Freshwater

Region

South America

Recorded from

Argentina, Bolivia, Brazil, Peru

Junior synonyms

Gloiobdella obscura

Original description

Ringuelet, Descripciones preliminares de nuevos hirudincos argentinos. (1942). Notas del Museo de La Plata, 7, 217-227

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