Thesis (M.Sc.)--Chulalongkorn University, 2000

Helicarionid semi-slugs from Thailand and some parts of Laos, Vietnam and Sabah, Malaysia were recorded of 1 Family, 2 Subfamlies, 6 genera and 12 species. Seven species are new record for Thailand i.e. cf. Megaustenia sp., cf. Cryptaustenia sp.1, cf. Parmarion sp.2, Durgella sp.1, and Durgella sp.2. Two genera (Cryptaustenia and Muangnua) that reported by Solem in 1966 are absent in the present study. Genital system is an important character that can be combined with shell, soft parts and external morphology for identification. The absent of dart apparatus in cf. Cryptaustenia sp.1 and present small dart apparatus of cf. Cryptaustenia sp.2. Parmarion can be distinguished from Muangnua Solem, 1966 by the present of dart apparatus and suggested that Muangnua may not exist in Thai environment. In Durgella libas, Durgella sp.1 and Durgella sp. 2 have a large dart apparatus, while absent in Durgella sp.3. The present of epiphallic retractor caecum and lime-sac of cf. Cryptaustenia can be used to distinguish from Cryptaustenia. Radula morphology is an assistance information for classification, In cf. Cryptaustenia has tricuspid central tooth, lateral teeth and marginal teeth. While Cryptaustenia has tricuspid central tooth and bicuspid lateral teeth and marginal teeth, in Durgella libas and Durgella sp.1 has an enormous number of teeth, unicuspid and very minute central tooth, with very numerous, bicuspid and serrated edge latero-maginal teeth while in Durgella sp.2 shows totally different.