Orchibdella diagutta

External morphology

The form is typical of the Cyclobdellidae, of uniform width from the genital region to the posterior somites. The acetabulum features thick lips and is formed by somites I through V. The sucker is of the same width as the body at the level of somite XXVII—nearly half the maximum body width. Five pairs of eyes located on annuli 2, 3, 4, 5, and 7—that is, on II, III, IV, V (a1 + a2), and VI (a1 + a2). The male gonopore is situated on XI b6, and the female gonopore on XII/XIII; alternatively, the former is on XII b5—meaning they are separated by 1/2 + 5, or by 1/2 + 4 + 1/2 annuli. The anus is located on XXV/XXVI. Setae are arranged in four dorsal pairs and three ventral pairs; they are very small and barely visible. Somites I, II, III, and IV are uniannulate; V and VI are biannulate; on VII, a2 is slightly larger than a1 (b5 + b6); on VIII, (b1 + b2) is larger than a2 + b5 + b6; somites IX through XXV are quinquannulate (5-annulate) or complete; XXVI is biannulate, with (a1 + a2) larger than a3; and XXVII is uniannulate.

Internal morphology

The oral chamber lacks any separating velum between itself and the pharynx; the latter is narrow, extends to the posterior limit of somite XIII, and features twelve longitudinal muscular folds that are only slightly prominent. The crop is entirely sacculated, comprising seven distinct chambers. Two pairs of testes are present in each interganglionic space, extending from the posterior half of somite XIV to the anterior half of somite XXIV, for a total of twenty pairs. The vasa deferentia ascend to somite XI a2, turn toward the midline, and descend a very short distance—reaching XII/XIII—where they continue into their respective white spermatic ducts; these ducts follow a tortuous path and function as epididymides. These ducts are not at all compact; in somite XIII, each continues into a short ejaculatory duct that is folded into the shape of a recumbent "U." Each duct then leads into an ejaculatory bulb: the right-hand one is notably broad and thick, ...which opens into the right flank of the prostatic region of the atrium; the left one is less thick, fusiform, and terminates at the apical end of that same prostatic region. The atrium is megalomorphic, extending from XI/XII to XIV a2, with the penial sac slightly undulate; the distal third—slightly less than one-third of the total length—constitutes the prostatic region. Two ovaries, U-shaped with unequal arms, are situated in segments XIV and XV, touching the distal end of the atrium; each ovary continues into segment XIV via its respective oviduct, which joins that of the opposite side.

Habitat & range

Habitat

Freshwater

Region

South America

Recorded from

Argentina