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.

Figures

Fig. 4 Light microscopic differential interference contrast micrographs of Sterreria sp. a Overview of the living animal, showing its elongated body shape. The animal is slightly twisted and the posterior end viewed from the left lateral side. Note dorsal supraterminal male copulatory organ and adhesive plate. b Anterior end of the body showing frontal glands and brain cells. c Statocyst of a hard squeezed specimen with two statoliths