Prosthiostomum siphunculus (Delle Chiaje 1822)

Mature, elongated specimens 10–18 mm long, occasionally as long as 30 mm, and 4–6 mm wide. Body shape long and narrow, sometimes with a little fan-like expansion at the anterior end, straight margins, rounded anterior and tapered posterior end. Without tentacles. Dorsal surface smooth. Dorsal colouration beige to yellow, without spots or bands (Figure 6A, B). Ventral side smooth and pale. Sucker in the middle of the body or slightly anterior. Cerebral eyes arranged in two slightly curved rows; marginal eyes along the anterior edge; young individuals have an eye-free medial area. Well-developed plicate tubular pharynx located near the anterior end, oriented frontally; oral pore closely behind the brain. Male and female genital pores separated and located posterior to pharynx (Figure 6C). Male system consists of a short penis papilla armed with a conical, Arabian dagger-like stylet an anterior orientated seminal vesicle and two spherical prostatic vesicles with a smooth glandular epithelium covered with a thick muscle layer. Vasa deferentia open separately into a seminal vesicle. Ejaculatory duct and prostatic ducts very long and spiral-shaped; prostatic ducts meet at proximal end of small penis papilla. Male atrium elongated and conical (Figure 6D). The female system lies posterior to male pore and is characterized by a short, spindle-like female atrium, a cement gland pouch, followed by a backwards orientated vagina, and backwards directed uteri.

One specimen was found in mussel samples collected from the floats of the third dock of the Yacht Club Ribeira at a depth of 1 metres (42°33.7700N, 008°59.3260W). The other animal was collected from under a rock in the area “A Ameixida” at a depth of 6 metres (42°32.2490N, 008°59.1640W). Prosthiostomum siphunculus is known from the western European Atlantic coasts, the Mediterranean and the Tyrrhenian Sea, and also from North and South Africa, Somalia and Vietnam (Prudhoe 1985).