Prostoma graecense (Böhmig, 1892)

(Figures 1–2)

Description of the Argentine specimens

Worms of slightly flattened, round body section, about 0.5 to 2.5 cm long, with four to six turbellarian­like cup ocelli, five being a usual figure; all late embryos and newly hatched specimens bred in the laboratory showed four eyes. The smallest animals were pale yellow whereas larger ones were orange­brown with small darker spots. A cupshaped frontal organ, dorsal to the rhynchopore, was easily noticeable by the active ciliar movement within it (Figure 1).

Body wall with two muscular layers, outer circular and inner longitudinal, as typical for monostiliferous Hoplonemertini; no diagonal fibres observed.

Nervous system consisting of a pair of dorsal and a pair of ventral cerebral ganglia connected by commissures forming the neural ring around the rhynchodaeum. The cerebral organ connected with two external, oblique cephalic grooves. A pair of lateral nerve cords arose from the ventral ganglion, with no accessory nerve. The highly basophilous cephalic glands connecting to the frontal organ extend backwards up to, at least, the cerebral dorsal commissure in all the specimens sectioned (Figure 2).

Rhynchocoel about two thirds the body length, with two muscular layers (outer circular, inner longitudinal). Rhynchodaeum with a thin layer of longitudinal fibres, noticed only in a few specimens. Proboscis differentiated into three regions (anterior, middle bulb, and posterior), with three muscular layers (outer longitudinal, middle circular, inner longitudinal). The principal stylet—nearly as long as its base—laid in the basis of the middle bulb region; the two lateral pouches containing accessory stylets were surrounded by some unicellular glands. Late embryos and newly hatched worms already possess the main stylet.

Oesophagus non­rhynchodaeal, short, ciliated (Figure 2); its cilia were not always visible due to lumen collapse. Stomach with a strongly ciliated epithelium.

Ovotestes located between the intestinal diverticula. A large ovum and a few remaining spermatic cells formed each ovotestis in mature specimens. One of the smallest specimens had only testes. Spherical eggs laid in the laboratory were 0.032 mm in diameter.