Castellani (1913–14)[141] has described similar parasites from a case of splenomegaly, with fever of long standing, in a Sinhalese boy. The bodies were found in the spleen and more rarely in the blood (fig. 52). Castellani has named them _Toxoplasma pyrogenes_. Further researches are needed. [141] _Journ. Trop. Med. and Hyg._, xvii, p. 113. THE SPIROCHÆTES. The Spirochætes are long, narrow, wavy, thread-like organisms, with a firm yet flexible outer covering or periplast. There is a diffuse nucleus internally in the form of bars or rodlets of chromatin distributed along the body. In some forms there is a membrane or crista present (fig. 53), which in the past was compared with the undulating membrane of a trypanosome, but the membrane of a spirochæte does not undulate. Progression is very rapid, corkscrew-like and undulatory movements occurring simultaneously. The genus _Spirochæta_ was founded by Ehrenberg in 1833 for an organism which he discovered in stagnant water in Berlin. Ehrenberg named the organism _Spirochæta plicatilis_. According to Zuelzer (1912) _S. plicatilis_ does not possess a membrane or crista, but an axial filament. _S. gigantea_ has been described by Warming from sea-water. Spirochætes occur in the crystalline style and digestive tract of many bivalve molluscs. The first molluscan spirochæte to be studied was that of the oyster, named by Certes (1882) “_Trypanosoma_” _balbianii_ (fig. 53). Similar spirochætes, probably belonging to the same species, occur in various species of _Tapes_ and in _Pecten_ (the scallop). _S. balbianii_ has rounded ends (fig. 53). Other spirochætes occur in freshwater mussels (_Anodonta_ spp). _S. anodontæ_, studied by Keysselitz (1906) and by Fantham (1907), has pointed ends. Gross (1911) suggested the generic name _Cristispira_ for molluscan spirochætes, because they possess a well-marked membrane or “crista,” which appears to be absent from _S. plicatilis_, according to Zuelzer’s researches. [Illustration: FIG. 53.--_Spirochæta balbianii._ _a_, basal granule or polar cap. _b_, chromatin rodlets. _c_, membrane (“crista”). _d_, myonemes in membrane. (After Fantham and Porter.)] Schaudinn in 1905 founded the genus _Treponema_ for the parasite of syphilis (_T. pallidum_), discovered by him and by Hoffmann. According to Schaudinn the Treponemata have no membrane or crista. The pathogenic agent of yaws or frambœsia, discovered by Castellani, is also placed in the genus _Treponema_, as _T. pertenue_.