Baer, 1827
Description
Small and stout, transparent worm, often occurring as a chain of several zooids. Five anterior segments fused forming head and containing muscular pharynx. Prostomium absent. Chaetae in ventral bundles only, absent from III-V. Upper tooth of chaetae shorter than lower one, both thin and hooked. In II, 8-20 chaetae per bundle, 72-123 µm long; their number can decrease backward, length being 41-84 µm. In mature individuals, penial chaetae 2-4 per bundle, single-pointed or bifid. Length 0.7-5 mm, segment number in zooids 8-9, in single individuals 11-15. Differing from other congeners in long and strongly curved teeth of chaetae, as well as by the characteristic habitat on molluscs.
Crawling on substratum.
Distribution
Cosmopolitan.
Ecology
The typical form is living as a commensal on the body surface and in the mantle cavity of different freshwater snails, seldom also on clams. Feeding on various minute animals and algae. Another, less known subspecies, Chaetogaster limnaei vaghini Gruffydd, 1965, occurs as a parasite in the snail kidney, feeding on the host's tissue. It is somewhat smaller than the nominate form: chaetae of II 4-7 per bundle, 51-68 µm long.
Reproduction
Paratomy (budding). Sexually mature individuals rare. Seasonal sexual reproduction described in both subspecies.
Literature
Baer, 1827: 611-615, Pl. XXX Figs 23-24; Sperber, 1948: 73-75, Figs 3A-B, 7F, Pl. I Fig. 4; Sperber, 1950: 54, Fig. F. ; Brinkhurst and Jamieson, 1971: 312-314, Fig. 7.1O-P; Chekanovskaya, 1981: 262, Fig. 123; Hrabe, 1981: 33, Pl. 2 Fig. 14; Kasprzak, 1981: 95, Figs 191-192. For the subspecies C. limnaei vaghini: Gruffydd, 1965: 175-196; Kasprzak, 1981: Figs 193-194.