Sterreria spp.

Raikova, Olga I., Meyer-Wachsmuth, Inga & Jondelius, Ulf, 2016, The plastic nervous system of Nemertodermatida, Organisms Diversity & Evolution (New York, N. Y.) 16 (1), pp. 85-104 : 93

Sterreria spp.

General morphology

Species of the genus Sterreria spp. are small interstitial worms usually up to 5 mm in length, with a length to width ratio of approximately 15:1 ( Fig. 4a). At the anterior end, there is a bilithophorous statocyst with a lithocyte capping the statolith appearing in blisters ( Fig. 4b), one of characteristics of the family Nemertodermatidae ( Sterrer 1998) . Usually, the “blister cap” stays on top of the statoliths even when the animal is turned; only when the animal is squeezed hard, as in Fig. 4c, it becomes visible that the blistered structure is a feature of the cytoplasm of the lithocyte, not of the statolith itself, which is smooth. The statocyst measures 23×16 μm, with the stones 8 μm in diameter. The stones lie well apart from each other and there is a hint of partition between them ( Fig. 4c). Frontal glands described by Faubel (1976), Sterrer (1998) and Lundin (2000) for S. psammicola and S. rubra ( Faubel, 1976) can be observed in the species studied ( Fig. 4b).

The male copulatory organ lies in supraterminal position, just anterior to the caudal adhesive plate ( Fig. 4a). The male opening is lined by numerous rhabdoids ( Fig. 4d). The copulatory organ consists of a false seminal vesicle, as it lacks its own walls, but is composed of tightly packed autosperm with a small penis cone distally ( Fig. 4d). Autosperm and allosperm are quite different morphologically, as is the case in most nemertodermatids ( Sterrer 1998). Cells of autosperm can be observed both in the false seminal vesicle and outside the body, by the male opening ( Fig. 4d). They are longer, have a smooth surface and look rigid as opposed to allosperm cells, which are much shorter, less rigid and display a helical structure on the surface ( Fig. 4c). As seen in Fig. 4c, allosperm cells can be observed in rather incongruous areas, supporting the view that insemination occurs via epidermal injection.